Analog to Digital. Used to refer to the conversion of analog data to
its digital equivalent.
In a system of moist air, the ratio of the mass of water vapor
present to the volume occupied by the mixture; that is, the density of
the water vapor component. Absolute humidity is normally expressed in
grams of water vapor in a cubic meter of air (25 g/m3)
The process in which radiant energy is retained by a substance. A
further process always results from absorption, that is, the
irreversible conversion of the absorbed radiation into some other form
of energy within and according to the nature of the absorbing medium.
The absorbing medium itself may emit radiation, but only after an energy
conversion has occurred.
Acids form when certain atmospheric gases (primarily carbon dioxide,
sulfur dioxide, and nitrogen oxides) come in contact with water in the
atmosphere or on the ground and are chemically converted to acidic
substances. Oxidants play a major role in several of these acid-forming
processes. Carbon dioxide dissolved in rain is converted to a weak acid
(carbonic acid). Other gases, primarily oxides of sulfur and nitrogen,
are converted to strong acids (sulfuric and nitric acids). Although rain
is naturally slightly acidic because of carbon dioxide, natural
emissions of sulfur and nitrogen oxides, and certain organic acids,
human activities can make it much more acidic. Occasional pH readings of
well below 2.4 (the acidity of inegar) have been reported in
industrialized areas. The principal natural phenomena that contribute
acid-producing gases to the atmosphere are emissions from volcanoes and
from biological processes that occur on the land, in wetlands, and in
the oceans. The effects of acidic deposits have been detected in glacial
ice thousands of years old in remote parts of the lobe. Principal human
sources are industrial and power-generating plants and transportation
vehicles. The gases may be carried hundreds of miles in the atmosphere
before they are converted to acids and deposited. Since the industrial
revolution, emissions of sulfur and nitrogen oxides to the atmosphere
have increased. Industrial and energy-generating facilities hat burn
fossil fuels, primarily coal, are the principal sources of increased
sulfur oxides. These sources, plus the transportation sector, are the
major originators of increased nitrogen oxides. The problem of acid rain
not only has increased with population and industrial growth, it has
become more widespread. The use of tall smokestacks to reduce local
pollution has contributed to the spread of acid rain by releasing gases
into regional atmospheric circulation. The same remote glaciers that
provide evidence of natural variability in acidic deposition show, in
their more recently formed layers, the increased deposition caused by
human activity during the past half century.
Acquisition of Signal
The time you begin receiving a signal from a spacecraft. For
polar-orbiting satellites, radio reception of the APT signal can begin
only when the polar-orbiting satellite is above the horizon of a
particular location. This is determined by both the satellite and its
particular path during orbit across the reception range of a ground
Active System (Active Sensor)
A remote-sensing system that transmits its own radiation to detect
an object or area for observation and receives the reflected or
transmitted radiation. Radar is an example of an active system. Compare
with passive system.
Advanced Very High Resoution Radiometer (AVHRR)
A five-channel scanning instrument that quantitatively measures
electromagnetic radiation, flown on NOAA environmental satellites. AVHRR
remotely determines cloud cover and surface temperature. Visible and
infrared detectors observe vegetation, clouds, lakes, shorelines, snow,
and ice. TIROS Automatic Picture Transmissions (APT) are derived from
this nstrument. See TIROS.
Particles of liquid or solid dispersed as a suspension in
The act or process of establishing a forest, especially on land not
See artificial intelligence.
Large body of air, often hundreds or thousands of miles across,
containing air of a similar temperature and humidity. Sometimes the
differences between air masses are hardly noticeable, but if colliding
air masses have very different temperatures and humidity values, storms
can erupt. See front.
The existence in the air of substances in concentrations that are
determined unacceptable to human health and the environment.
Contaminants in the air we breathe come mainly from manufacturing
industries, electric power plants, exhaust from automobiles, buses, and
The weight of the atmosphere over a particular point, also called
barometric pressure. Average air exerts approximately 14.7 pounds (6.8
kg) of force on every square inch (or 101,325 newtons on every square
meter) at sea level.
Airborne Imaging Radar.
Also known as.
The ratio of the outgoing solar radiation reflected by an object to
the incoming solar radiation incident upon it.
Substance capable of neutralizing acid, with a pH greater than 7.0.
A mathematical relation between an observed quantity and a variable
used in a step-by-step mathematical process to calculate a quantity. In
the context of remote sensing, algorithms generally specify how to
determine higherlevel data products from lower-level source data. For
example, algorithms prescribe how atmospheric temperature and moisture
profiles are etermined from a set of radiation observations originally
sensed by satellite sounding instruments.
An active instrument (see active system) used to measure the
altitude of an object above a fixed level. For example, a laser
altimeter can measure height from a spacecraft to an icesheet. That
measurement, coupled with radial orbit knowledge, will enable
determination of the topography.
Height above the Earth's surface.
See amplitude modulation.
Transmission of a continuously variable signal as opposed to a
discretely variable signal. Compare with digital. A system of
transmitting and receiving information in which one value (i.e.,
voltage, current, resistance, or, in the APT system, the volume level of
the video tone) can be compared directly to the information (in the APT
system, the white, black, and gray values) in the image.
Data other than instrument data required to perform an instrument's
data processing. Ancillary data includes such information as orbit
and/or attitude data, time information, spacecraft engineering data, and
Instrument used to measure wind speed, usually measured either from
the rotation of winddriven cups or from wind pressure through a tube
pointed into the wind.
1. The deviation of (usually) temperature or precipitation in a
given region over a specified period from the normal value for the same
region. 2. The angular distance of an Earth satellite (or planet) from
its perigee (or perihelion) as seen from the center of the Earth (sun).
See Keplerian elements for examples of use.
An ordered assembly of elementary antennae spaced apart and fed in
such a manner that the resulting radiation is concentrated in one or
The focused pattern of electromagnetic radiation that is either
received or transmitted by an antenna.
A wire or set of wires used to send and receive electromagnetic
waves. Two primary features must be considered when selecting antennas:
beamwidth, or the "width" of the antenna pattern (wide beamwidth
suggests the ability to receive signals from a number of different
directions), and gain, or the increase in signal level. Generally
beamwidth or gain can be ncreased only at the expense of the other. Gain
can be increased by multiplying the number of antenna elements, although
this adds "directionality" that reduces beamwidth. Important antenna
considerations include the following:
- The physical size of antenna
components is determined by the frequency of the transmissions it will
receive--the higher the requency, the shorter the elements. At high
frequencies, use of a satellite dish will compensate for the reduced
amount of energy intercepted by shortened components.
- The antenna
design should fit the type of radio frequency (RF) signal polarization
it will receive. The orientation of radio waves in space is a function
of the orientation of the elements of the ransmitting antenna. A
circularly polarized wave rotates as it propagates through space.
Antennas can be designed for either right or left-handed circular
polarization. Earth-based communication antennas are either vertical or
horizontal in polarization, and not suited for space communication.
Police and cellular phone transmissions use vertical polarization
because simple vertical whip antenna is the easiest sort of
omnidirectional antenna to mount on a vehicle.
- The antenna needs to
produce sufficient signal gain to produce noise-free reception.
antenna should be clear of conductive objects such as power lines, phone
wires, etc., so height above the ground becomes important. Basic antenna
- driven element--the parts connected to and receiving
power from the receiver/transmitter;
- Parasitic elements--the parts
dependent upon resonance rather than connection to a power source;
- A director or parasitic element that rein forces radiation on a line
pointing to it from the driven element;
- A reflector or parasitic element that rein forces radiation on a line
pointing from it to the driven element. A fundamental form of antenna is
a single wire whose length approximately equals half the transmitting
wavelength. Known as a dipole antenna, it is the unit from which many
more complex forms of antennas are constructed. One of the most common
forms of VHF antenna is the Yagi/beam, named for the Japanese scientist
ho first described the principles of combining a basic dipole (driven
element) and parasitic elements. A common TV antenna is an example of
this type. A Yagi/beam antenna is directional and therefore includes a
rotator to aim (direct) the antenna. See yagi. An omnidirectional
antenna has a wide beamwidth and consequently does not require
"tracking" (aiming the antenna toward the signal source). An example of
an omnidirectional antenna is the turnstile antenna, a variation of the
standard dipole antenna well suited for space communications. The
quadrifilar helix antenna is omnidirectional and an inherently excellent
antenna for ground station use. Quadrifilars are also used on NOAA's
polarorbiting environmental satellites. The parabolic reflector or
satellite dish antenna collects RF signals on a passive dishshaped
surface. A feedhorn antenna--a simple dipole antenna mounted in a
resonant tube structure (cylinder with one open end)--transfers the RF
energy to a transmission line. The bigger the dish, the greater the
amount of RF energy intercepted, and therefore the reater the gain from
A high pressure area where winds blow clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere. See cyclone,
See Acquisition of Signal.
Apogee (aka Apoasis or Apifocus)
On an elliptical orbit path, point a which a satellite is farthest
from the Earth. See perigee diagram
See Automatic Picture Transmission.
Layer of water-bearing permeable rock, sand, or gravel capable of
providing significant amounts of water.
The parallel of latitude that is approximately 66.5 degrees north of
the equator and that circumscribes the northern frigid zone.
French random-access Doppler data collection system. Used on NOAA's
Polar-Orbiting Environmental Satellites (POES), ARGOS receives platform
and buoy transmissions on 401.65 MHz. This data collection system now
monitors more than 4,000 platforms worldwide, outputs data via VHF link,
and stores them on tape for relay to a central processing
Argument of Perigee (aka ARGP or w)
One of the six Keplerian elements, it gives the rotation of the
satellite on the orbit. The argument (argument meaning angle) of
perigee-- perigee is the point on an orbital path when the satellite is
closest to the Earth--is the angle (measured from the center of the
Earth) from the ascending node to perigee. Example: When ARGP = 0
degrees, the perigee occurs at the same place as the ascending node.
That means that the satellite would be closest to Earth just as it rises
up over the equator. When ARGP = 180 degrees, apogee would occur at the
same place as the descending node. This means that the satellite would
be farthest from Earth just as it rises over the equator. See Keplerian
elements for diagram.
Neural networks. The branch of computer science that attempts to
program computers to respond as if they were thinking--capable of
reasoning, adapting to new situations, and learning new skills. Examples
of artificial intelligence programs include those that can locate
minerals underground and understand human speech.
The point in an orbit (longitude) at which a satellite crosses the
equatorial plane from south to north.
The ratio of image width to image height. Weather Facsimile (WEFAX)
images have a 1:1 aspect ratio (square); a conventional TV aspect ratio
is 4:3 (rectangle).
Astronomical Unit (AU)
The distance from the Earth to the sun. On average, the sun is
149,599,000 kilometers from Earth.
ATLAS (Atmospheric Laboratory for Applications and Science)
The focus of ATLAS is to study the chemistry of the Earth's upper
atmosphere (mainly the stratosphere/mesosphere) and the solar radiation
incident on the Earth system (both total solar irradiance and spectrally
resolved radiance, especially ultraviolet). Science operations onboard
ATLAS 1 (March 1992) and ATLAS 2 (March-April, 1993) began a
comprehensive and systematic collection of data that will help establish
benchmarks for atmospheric conditions and the sun's stability.
The air surrounding the Earth, described as a series of shells or
layers of different characteristics. The atmosphere, composed mainly of
nitrogen and oxygen with traces of carbon dioxide, water vapor, and
other gases, acts as a buffer between Earth and the sun. The layers,
troposphere, stratosphere, mesosphere, thermosphere, and the exosphere,
vary around the globe and in response to seasonal changes. Troposphere
stems from the Greek word tropos, which means turning or mixing. The
troposphere is the lowest layer of the Earth's atmosphere, extending to
a height of 8-15 km, depending on latitude. This region, constantly in
motion, is the most dense layer of the atmosphere and the region that
essentially contains all of Earth's weather. Molecules of nitrogen and
oxygen compose the bulk of the troposphere. The tropopause marks the
limit of the troposphere and the beginning of the stratosphere. The
temperature above the tropopause increases slowly with height up to
about 50 km. The stratosphere and stratopause stretch above the
troposphere to a height of 50 km. It is a region of intense interactions
among radiative, dynamical, and chemical processes, in which horizontal
mixing of gaseous components proceeds much more rapidly than vertical
mixing. The stratosphere is warmer than the upper troposphere, primarily
because of a stratospheric ozone layer that absorbs solar ultraviolet
energy. The mesosphere, 50 to 80 km above the Earth, has diminished
ozone concentration and radiative cooling becomes relatively more
important. The temperature begins to decline again (as it does in the
troposphere) with altitude. Temperatures in the upper mesosphere fall to
-70° to -140° Celsius, depending upon latitude and season. Millions of
meteors burn up daily in the mesosphere as a result of collisions with
some of the billions of gas particles contained in that layer. The
collisions create enough heat to burn the falling objects long before
they reach the ground. The stratosphere and mesosphere are referred to
as the middle atmosphere. The mesopause, at an altitude of about 80 km,
separates the mesosphere from the thermosphere--the outermost layer of
the Earth's atmosphere. The thermosphere, from the Greek thermo for
heat, begins about 80 km above the Earth. At these high altitudes, the
residual atmospheric gases sort into strata according to molecular mass.
Thermospheric temperatures increase with altitude due to absorption of
highly energetic solar radiation by the small amount of residual oxygen
still present. Temperatures can rise to 2,000° C. Radiation causes the
scattered air particles in this layer to become charged electrically,
enabling radio waves to bounce off and be received beyond the horizon.
At the exosphere, beginning at 500 to 1,000 km above the Earth's
surface, the atmosphere blends into space. The few particles of gas here
can reach 4,500° F (2,500° C) during the day.
Having to do with the atmosphere. For example, "atmospheric conditions" refer to
conditions in the atmosphere.
Atmospheric Infrared Sounder
Advanced sounding instrument selected to fly on the EOS-PM1 mission
(intermediate-sized, sun-synchronous, morning satellite) in the year
2000. It will retrieve vertical temperature and moisture profiles in the
troposphere and stratosphere. Designed to achieve temperature retrieval
accuracy of 1°C with a 1 km vertical resolution, it will fly with two
operational microwave sounders. The three instruments will constitute an
advanced operational sounding system, relative to the TIROS Operational
Vertical Sounder (TOVS) currently flying on NOAA polar-orbiting
satellites. See Earth Observing System, TIROS-N/NOAA Satellites.
The amount of force exerted over a surface area, caused by the
weight of air molecules above it. As elevation increases, fewer air
molecules are present. Therefore, atmospheric pressure always decreases
with increasing height. A column of air, 1 square inch in cross section,
measured from sea level to the top of the atmosphere would weigh
approximately 14.7 b/in2. The standard value for atmospheric pressure at
sea level is: 29.92 inches or 760 mm of mercury 1013.25 millibars (mb)
or 101,325 pascals (Pa)
Atmospheric Radiation Measurements Program (ARM)
U.S. Department of Energy program for the continual, ground-based
measurements of atmospheric and meteorological parameters over
approximately a ten-year period. The program will study radiative
forcing and feedbacks, particularly the role of clouds. The general
program goal is to improve the performance of climate models,
particularly general circulation models of the atmosphere.
Atmospheric Response Variables
Variables that reflect the response of the atmosphere to external
forcing (e.g., temperature, pressure, circulation, and
The range of wavelengths at which water vapor, carbon dioxide, or
other atmospheric gases only slightly absorb radiation. Atmospheric
windows allow the Earth's radiation to escape into space unless clouds
absorb the radiation. See greenhouse effect.
A coral island consisting of a ring of coral surrounding a central
lagoon. Atolls are common in the Indian and Pacific Oceans.
The decrease in the magnitude of current, voltage, or power of a
signal in transmission between points. Attenuation may be expressed in
decibels, and can be caused by interferences such as rain, clouds, or
radio frequency signals.
Frequencies that the human ear can hear (usually 30 to 20,000 cycles
See solar wind.
Automatic Picture Transmission (APT)
System developed to make real-time reception of satellite images
possible whenever an APTequipped satellite passes within range of an
environmental satellite ground station. Transmission (analog video
format) consists of an amplitude-modulated audible tone that can be
displayed as an image on a computer monitor when received by an
appropriate ground station. APT images are transmitted by polar-orbiting
satellites such as the TIROS-N/NOAA satellites, Russia's METEOR, and the
Chinese Feng Yun, which orbit 500-900 miles above the Earth, and offer
both visible and infrared images. An APT image has thousands of squares
called picture elements or pixels. Each pixel represents a four-km
See Advanced Very High Resolution Radiometer.
The direction, in degrees referenced to true north, that an antenna
must be pointed to receive a satellite signal (compass direction). The
angular distance is measured in a clockwise direction.
1. In radio, a continuous sequence of broadcasting frequencies
within given limits. 2. In radiometry, a relatively narrow region of the
electromagnetic spectrum to which a remote sensor responds; a
multispectral sensor makes measurements in a number of spectral bands.
3. In spectroscopy, spectral regions where atmospheric gases absorb (and
emit) radiation, e.g., the 15 µm carbon dioxide absorption band, the 6.3
µm water vapor absorption band, and the 9.6 µm ozone absorption
The total range of frequency required to pass a specific modulated
signal without distortion or loss of data. The ideal bandwidth allows
the signal to pass under conditions of maximum AM or FM adjustment. (Too
narrow a bandwidth will result in loss of data during modulation peaks.
Too wide a bandwidth will pass excessive noise along with the signal.)
In FM, radio frequency signal bandwidth is determined by the frequency
deviation of the signal.
An instrument used by weather forecasters and scientists to measure air pressure
at any given moment for a particular place. A standard
mercury barometer has a glass column about 30 inches long, closed at one
end, with a mercury-filled reservoir. Air pressure is usually measured in
millibars or in inches of mercury. Mercury in the tube adjusts until
the weight of the mercury column balances the atmospheric force exerted
on the reservoir. High atmospheric pressure forces the mercury higher in
the olumn. Low pressure allows the mercury to drop to a lower level in
the column. An aneroid barometer uses a small, flexible metal box called
an aneroid cell. The box is tightly sealed after some of the air is
removed, so that small changes in external air pressure cause the cell
to expand or contract.
A substance that forms a salt when it reacts with acid. A base is a
substance that removes hydrogen ions (protons) from an acid and combines
with them in a chemical reaction.
See Beginners All-purpose Symbolic Instruction Code.
Unit of signaling speed. The speed in bauds is the number of
discrete conditions or signal events per second. If each signal event
represents only one bit condition, baud is the same as bits per
A wide area of water extending into land from a sea or lake.
The measure of the "width" of an antenna pattern, measured in
degrees of arc. Generally an antenna with low gain has a wide pattern,
receiving signals well from a number of different directions. See
The combination of antenna azimuth and elevation required to point
(aim) an antenna at a spacecraft. The bearing for geostationary (i.e.,
GOES) satellites is constant. The bearing for polar-orbiting satellites
Beginners All-Purpose Symbolic Instruction Code (BASIC)
A most popular and widespread "high level" language for
microcomputers. BASIC uses a sequence of English-like commands and
A numbering system that uses only 1 and 0 (e.g., 1 is one, 10 is
two, 11 is three). In digital integrated circuits, a 0 is indicated by a
logic low and a 1 by a logic high.
A measurement of the effects of a substance on living
Decomposition of material by microorganisms.
Movements through the Earth system of key chemical constituents
essential to life, such as carbon, nitrogen, oxygen, and
The amount of living material in unit area or volume, usually
expressed as mass or weight.
Well-defined terrestrial environment (e.g., desert, tundra, or
tropical forest). The complex of living organisms found in an ecological
Part of the Earth system in which life can exist, between the outer
portion of the geosphere and the inner portion of the
The plant and animal life of a region or area.
The speed at which bits are transmitted, usually expressed in bits
per second. See baud.
A contraction of "binary digit." The basic element of a two-element
(binary) computer language.
A severe weather condition characterized by low temperatures and
strong winds (greater than 35 mph) bearing a great amount of snow,
either falling or blowing. When these conditions persist after snow has
stopped falling, it is called a ground blizzard.
Lines indicating the limits of countries, states, or other political
jurisdictions, or different air masses.
British Thermal Unit (BTU)
The amount of heat needed to raise the temperature of one pound of
water by one degree Fahrenheit. Compare with calorie.
The basic frame of a satellite system that includes the propulsion
and stabilization systems, but not the instruments or data
A unit of eight bits of data or memory in microcomputer
Act of comparing an instrument's measuring accuracy to a known
The amount of heat needed to raise the temperature of one gram of
water at 15° centigrade one degree centigrade. Compare with British
A man-made watercourse designed to carry goods or water.
The layer formed naturally by the leaves and branches of trees and
A large but narrow gorge with deep sides.
Cape (or Point)
A piece of land extending into water.
All parts (reservoirs) and fluxes of carbon. The cycle is usually
thought of as four main reservoirs of carbon interconnected by pathways
of exchange. The reservoirs are the atmosphere, terrestrial biosphere
(usually includes freshwater systems), oceans, and sediments (includes
fossil fuels). The annual movements of carbon, the carbon exchanges
between eservoirs, occur because of various chemical, physical,
geological, and biological processes. The ocean contains the largest
pool of carbon near the surface of the Earth, but most of that pool is
not involved with rapid exchange with the atmosphere. See appendix for
Carbon Dioxide (CO2)
A minor but very important component of the atmosphere, carbon
dioxide traps infrared radiation. Atmospheric CO2 has increased about 25
percent since the early 1800s, with an estimated increase of 10 percent
since 1958 (burning fossil fuels is the leading cause of increased CO2,
deforestation the second major cause). The increased amounts of CO2 in
the tmosphere enhance the greenhouse effect, blocking heat from escaping
into space and contributing to the warming of Earth's lower
Radio frequency capable of being modulated with some type of
information. See modulation.
The steady-state density of a given species that a particular
habitat can support.
A five-digit number assigned to a cataloged orbiting object. This
number may be found in the NASA Satellite Situation Report and on the
NASA Prediction Bulletins.
Cathode Ray Tube (CRT)
A television picture tube for image display.
See Compact Disk–Read Only Memory.
Temperature scale proposed by Swedish astronomer Anders Celsius in
1742. A mixture of ice and water is zero on the scale; boiling water is
designated as 100 degrees. A degree is defined as one hundredth of the
difference between the two reference points, resulting in the term
centigrade (100th part). To convert centigrade to Fahrenheit: multiply
the centigrade emperature by 1.8 and add 32°. (F = 9/5 C + 32) To
convert Fahrenheit to centigrade: subtract 32° from the Fahrenheit
temperature and divide the quantity by 1.8. (C = (F -32) / 1.8).
Central Processing Unit (CPU)
Main part of a computer consisting of an arithmetic logic unit and a
control unit. See microprocessor.
A family of compounds of chlorine, fluorine, and carbon, entirely of
industrial origin. CFCs include refrigerants, propellants for spray cans
(this usage is banned in the U.S., although some other countries permit
it) and for blowing plastic-foam insulation, styrofoam packaging, and
solvents for cleaning electronic circuit boards. The compounds’
lifetimes vary over a wide ange, exceeding 100 years in some cases.
CFCs’ ability to destroy stratospheric ozone through catalytic cycles is
contributing to the depletion of ozone worldwide. Because CFCs are such
stable molecules, they do not react easily with other chemicals in the
lower atmosphere. One of the few forces that can break up CFC molecules
is ultraviolet radiation, however the ozone layer protects the CFCs from
ultraviolet radiation in the lower atmosphere. CFC molecules are then
able to migrate intact into the stratosphere, where the molecules are
bombarded by ultraviolet rays, causing the CFCs to break up and release
their chlorine atoms. The released chlorine atoms participate in ozone
destruction, with a single atom of chlorine able to destroy ozone
molecules over and over again. International attention to CFCs resulted
in a meeting of diplomats from around the world in Montreal in 1987.
They forged a treaty that called for drastic reductions in the
production of CFCs. In 1990, diplomats met in London and voted to
significantly strengthen the Montreal Protocol by calling for a complete
elimination of CFCs by the year 2000. See Montreal Protocal.
The cyclical changes in physiological processes and functions that
are related to the 24-hour diurnal cycle.
The complete path of an electric current; an assemblage of
electronic elements; a means of two-way communication between two
points--comprised of associated "go" and "return" channels.
Circularly Polarized RF
Radio frequency transmissions where the wave energy is divided
equally between a vertically and a horizontally-polarized
A belt 22,245 miles (35,800 kilometers) directly above the equator
where a satellite orbits the Earth at the same speed the Earth is
rotating. Science fiction writer and scientist Arthur C. Clarke wrote
about this belt in 1945, hence the name.
The average weather conditions in an area determined over a period
Science dealing with climate and climate phenomena.
A person or thing very much like another, e.g., a copy of another
Lines or rows of cumuliform clouds.
Any sudden, heavy rain shower.
A visible mass of water vapor suspended in the atmosphere above
Earth's surface. Clouds form in areas where air rises and cools. The
condensing water vapor forms small droplets of water (0.012 mm) that,
when combined with billions of other droplets, form clouds. Clouds can
form along warm and cold fronts, where air flows up the side of the
mountain and cools as it rises higher into the atmosphere, and when warm
air blows over a colder surface, such as a cool body of water. Clouds
fall into two general categories: sheetlike or layer-looking stratus
clouds (stratus means layer) and cumulus clouds (cumulus means piled
up). These two cloud types are divided into four more groups that
describe the cloud's altitude. High clouds form above 20,000 feet in the
cold region of the troposphere, and are denoted by the prefix CIRRO or
CIRRUS. At this altitude water almost always freezes so clouds are
composed of ice crystals. The clouds tend to be wispy, are often
transparent, and include cirrus, cirrocumulus, and cirrostratus. Middle
clouds form between 6,500 and 20,000 feet and are denoted by the prefix
ALTO. They are made of water droplets and include altostratus and
altocumulus. Low clouds are found up to 6,500 feet and include the
stratocumulus and nimbostratus clouds. When stratus clouds contact the
ground they are called fog. Vertical clouds, such as cumulus, rise far
above their bases and can form at many heights. Cumulonimbus clouds, or
thunderheads, can start near the ground and soar up to 75,000
Coastal Zone Color Scanner (CZCS)
The first spacecraft instrument devoted to measurement of ocean
color. Although instruments on other satellites have sensed ocean color,
their spectral bands, spatial resolution, and dynamic range were
optimized for geographical or meteorological use. In the CZCS, every
parameter is optimized for use over water to the exclusion of any other
type of sensing. The CZCS flew on the Nimbus-7spacecraft.
A hollow copper cylinder, or other cylindrical conductor,
surrounding a single-wire conductor having a common axis (hence
coaxial). The space between the cylindrical shell and the inner
conductor is filled with an insulator which may be plastic or mostly
air, with supports separating the shell and the inner conductor every
inch or so. The cable is used to carry radio frequency signals to or
from antennas, etc.
See Common Business Oriented Language.
Band of organized cumuliform clouds that look like a comma from a
satellite's perspective. Comma clouds are indicators of heavy
Common Business Oriented Language (COBOL)
A computer programming language written for business
Compact Disk–Read Only Memory (CD-ROM)
Type of computer memory that reads and uses information, but does
not allow information to be added, changed, or erased. Digital
information is read by laser. CD-ROM does not depend upon any
proprietary hardware or software, making it an accessible vehicle for
Electronic machine capable of performing calculations and other
manipulations of various types of data, under the control of a stored
set of instructions. The machine itself is the hardware; the
instructions are the program or software. Depending upon size, computers
are called mainframes, minicomputers, and microcomputers. Microcomputers
include desk-top and portable personal computers.
Change of a substance to a denser form, such as gas to a liquid. The
opposite of evaporation.
The transfer of heat from one substance to another by direct
contact. Denser substances are better conductors; the transfer is always
from warmer to colder substances.
One of the large, continuous areas of the Earth into which the land
surface is divided. The six geographically defined continents are
politically defined as seven; Africa, Asia, Australia, Europe, North
America, South America, and Antarctica.
See plate tectonics.
Condensation trails. Artificial clouds made by the exhaust of jet
The rising of warm air and the sinking of cool air. Heat mixes and
moves air. When a layer of air receives enough heat from the Earth's
surface, it expands and moves upward. Colder, heavier air flows under it
which is then warmed, expands, and rises. The warm rising air cools as
it reaches higher, cooler regions of the atmosphere and begins to sink.
Convection causes local breezes, winds, and thunderstorms.
Coordinated Universal Timing (UTC)
(aka Greenwich Mean Time [GMT]) Local time at zero degrees longitude
at the Greenwich Observatory, England. UTC uses a 24-hour clock, i.e.,
2:00 a.m. is 0200 hours, 2:00 p.m. is 1400 hours, midnight is 2400 or
0000 hundred hours.
The apparent tendency of a freely moving particle to swing to one
side when its motion is referred to a set of axes that is itself
rotating in space, such as Earth. The acceleration is perpendicular to
the direction of the speed of the article relative to the Earth's
surface and is directed to the right in the northern hemisphere. Winds
are affected by rotation of the Earth so that instead of a wind blowing
in the direction it starts, it turns to the right of that direction in
the northern hemisphere; left in the southern hemisphere.
Two or more processes that affect one another.
See central processing unit.
The schedule of the maturing and harvesting of seasonal
See cathode ray tube.
One of the interrelated components of the Earth's system, the
cryosphere is frozen water in the form of snow, permanently frozen
ground (permafrost), floating ice, and glaciers. Fluctuations in the
volume of the cryosphere cause changes in ocean sea-level, which
directly impact the atmosphere and biosphere.
The point at which a satellite reaches its highest position or
elevation in the sky, relative to an observer (aka the closest point of
An area of low pressure where winds blow counterclockwise in the
Northern Hemisphere and clockwise in the Southern Hemisphere. See
See Coastal Zone Color Scanner.
Data Collection System (DCS)
DCS units are flown on both GOES and NOAA polar-orbiting spacecraft.
They gather and relay data from both mobile and stationary platforms at
various locations. DCS units on NOAA satellites can also determine the
precise location of moving platforms at the time the data were acquired.
See TIROS-N/NOAA satellites.
The amount of information transmitted per unit time.
See period decay.
A tenth of a bel. A unit used to measure the volume of a sound,
equal to ten times the common logarithm of the ratio of the intensity of
the sound to the intensity of an arbitrarily chosen standard sound. The
decibel also is used to measure relative strengths of antenna and
amplified signals and always refers to a ratio or difference between two
The angular distance from the equator to the satellite, measured as
positive north and negative south.
Defense Meteorological Satellite Program (DMSP)
A U.S. Air Force-managed meteorological satellite program with
satellites circling in sun-synchronous orbit. Imagery is collected in
the visible- to near-infrared band (0.4 to 1.1 micrometers) and in the
thermal-infrared band (about 8 to 13 micrometers) at a resolution of
about three kilometers. DMSP data is available directly from the
satellite for local use aboard ships and at military deployment
locations, but is also usually available to civilian users.
A unit of angular measure represented by the symbol °. The
circumference of a circle contains 360 degrees. When applied to the
roughly spherical shape of the Earth for geographic and cartographic
purposes, degrees are each divided into 60 minutes.
The fan-shaped area at the mouth or lower end of a river, formed by
eroded material that has been carried downstream and dropped in
quantities larger than can be carried off by tides or currents.
The process of retrieving information (data) from a modulated
carrier wave, the reverse of modulation.
Department of the Interior (DOI)
Responsible for our nationally-owned public lands and natural
resources, the DOI is chartered to foster the wisest use of our land and
water resources, protect fish and wildlife, preserve the environmental
and cultural values of national parks and historical places, and provide
for the enjoyment of life through outdoor recreation. The department
assesses energy and mineral resources and is responsible for assuring
that their development is in the best interest of all citizens. The U.S.
Geological Survey (USGS) is part of the DOI.
The point in a satellite's orbit at which it crosses the equatorial
plane from north to south. See diagram, Keplerian elements.
A land area so dry that little or no plant or animal life can
The man-made or natural formation of desert from usable
A device in a radiometer that senses the presence and intensity of
radiation. The incoming radiation is usually modified by filters or
other optical components that restrict the radiation to a specific
spectral band. The information can either be transmitted immediately or
recorded for transmittal at a later time.
The temperature to which air must be cooled for saturation to occur,
exclusive of air pressure or moisture content change. At that
temperature dew begins to form, and water vapor condenses into
Atmospheric moisture that condenses after a warm day and appears
during the night on cool surfaces as small drops. The cool surfaces
cause the water vapor in the air to cool to the point where the water
An analog image converted to numerical form so that it can be stored
and used in a computer. The image is divided into a matrix of small
regions called picture elements or pixels. At sub-satellite point each
pixel represents a specific amount of area. For example, in APT each
pixel represents 4.1 kilometers. Each pixel has a numerical value or
data number value, uantifying the radiance of the image at that spot.
The data number value of each pixel usually represents a value between
black and white, i.e., shades of gray. False color can be applied to the
image by assigning a graduated color palette to represent the gray
shades. The color is "false" because it represents an assigned, not
A system in which information is transmitted in a series of pulses.
The source is periodically sampled, analyzed, and converted or coded
into numerical values and transmitted. Digital transmissions typically
use the binary coding used by computers so most data is in appropriate
form, but verbal and visual communication must be converted. Many
satellite transmissions use digital formats because noise will not
interfere with the quality of the end product, producing clear and
The capability to acquire data directly from environmental
satellites via an Earth station. Data can be acquired from NOAA and
other nations’ environmental satellites, which offer weather information
from geostationary and polar-orbiting satellites.
Parasitic element(s) of a VHF antenna located forward of the driven
element. See antenna.
Data and Information System.
The apparent arc described by heavenly bodies from their rising to
Performed in twenty-four hours, such as the diurnal revolution of
Dobson Unit (DU)
The standard way to express ozone amounts in the atmosphere. One DU
is 2.7 x 1016 ozone molecules per square centimeter. One Dobson unit
refers to a layer of ozone that would be 0.001 cm thick under conditions
of standard temperature (0° C) and pressure (the average pressure at the
surface of the Earth). For example, 300 Dobson units of ozone brought
down to the surface of the Earth at 0° C would occupy a layer only 0.3
cm thick in a column. Dobson was a researcher at Oxford University who,
in the 1920s, built the first instrument (now called the Dobson meter)
to measure total ozone from the ground.
Region near the equator characterized by low pressure and light
shifting winds. See wind.
Doppler Effect (aka Doppler Shift)
The apparent change in frequency of sound or light waves, varying
with the relative velocity of the source and the observer. If the source
and observer draw closer together, the frequency is increased. Named for
Christian Doppler, Austrian mathematician and physicist
The weather radar system that uses the Doppler shift of radio waves
to detect air motion that can result in tornadoes and precipitation, as
previously-developed weather radar systems do. It can also measure the
speed and direction of rain and ice, as well as detect the formation of
tornadoes sooner than older radars.
Any radio frequency circuit that converts a higher frequency to a
lower frequency. This enables signal processing by a receiver. A typical
downconverter will feature one or more states of RF preamplification, a
mixer where the frequency conversion occurs, a local oscillator chain,
and often one or more intermediate frequency preamplifiers to minimize
the effect of line losses between the converter and the
Drag (aka N1)
A retarding force caused by the Earth's atmosphere. Thus by
definition, drag will act opposite to the vehicle's instantaneous
velocity vector with respect to the atmosphere. The magnitude of the
drag force is directly proportional to the product of the vehicle's
crosssectional area, its drag coefficient, its velocity, and the
atmospheric density, and inversely proportional to its mass. The effect
of drag is to cause the orbit to decay, or spiral downward. A satellite
of very high mass and very low cross-sectional area, and in a very high
orbit, may be very little affected by drag, whereas a large satellite of
low mass, in a low altitude orbit may be affected very strongly by drag.
Drag is the predominant force affecting satellite lifetime.
The study of the action of forces on bodies and the changes in
motion they produce.
Earth Observing System (EOS)
A series of small- to intermediate-sized spacecraft that is the
centerpiece of NASA's Mission to Planet Earth (MTPE). Planned for launch
beginning in 1998, each of the EOS spacecraft will carry a suite of
instruments designed to study global climate change. MTPE will use
space-, aircraft-, and ground-based measurements to study our
environment as an integrated system. Designing and implementing the MTPE
is, of necessity, an international effort. The MTPE program involves the
cooperation of the U.S., the European Space Agency (ESA), and the
Japanese National Space Development Agency (NASDA). The MTPE program is
part of the U.S. interagency effort, the Global Change Research
Earth Observing System Data & Information System (EOSDIS)
The system that will manage a dataset of Earth science observations
to be collected over a 15-year period. Existing data indicates that the
Earth is changing, and that human activity increasingly contributes to
this change. To monitor these changes, a baseline of "normal"
performance characteristics must be obtained. For the Earth, these
baseline characteristics must cover a global scale and a long enough
period that the variation caused by seasonal changes and other cyclical
or periodic events (e.g., El Niño and the solar cycle) may be included
in the analyses. The baseline characteristics also must enable
scientists to quantify processes that govern the Earth's system.
Functionally, EOSDIS will provide computing and networking facilities
supporting EOS research activities, including data interpretation and
modeling; processing, distribution, and archiving of EOS data; and
command and control of EOS observatories.
Discipline-specific satellites and instruments that will be used by
NASA to obtain observations before the launch of EOS spacecraft.
Generally smaller than the EOS satellites and instruments, Earth Probes
are planned to complement the broad environmental measurements from EOS
with highly focused studies in areas such as tropical rainfall (TRMM),
ocean productivity (SeaWiFS), atmospheric ozone (TOMS), and ocean
surface winds (NSCAT).
Earth Radiation Budget Experiment (ERBE)
An experiment to obtain data to study the average radiation budget
of the Earth and determine the energy transport gradient from the
equator to the poles. Three satellites were flown in different orbits to
obtain the data: the Earth Radiation Budget Satellite, ERBS (launched in
October 1984), NOAA-9 (launched in December 1984), and NOAA-10 (launched
in September 1986). See Television and Infrared Observation Satellite
Earth Station (aka Ground Station)
Hardware necessary to acquire data directly from environmental
satellites. The WEFAX Earth station diagram illustrates a basic ground
station configuration for obtaining direct readout data from
geostationary environmental (weather) satellites.
Earth System Science
An integrated approach to the study of the Earth that stresses
investigations of the interactions among the Earth's components in order
to explain Earth dynamics, evolution, and global change.
The Earth regarded as a unified system of interacting components,
including geosphere (land), atmosphere (air), hydrosphere (water and
ice), and biosphere (life).
(aka ecce or E0 or e) One of six Keplerian elements, it describes
the shape of an orbit. In the Keplerian orbit model, the satellite orbit
is an ellipse, with eccentricity defining the "shape" of the ellipse.
When e=0, the ellipse is a circle. When e is very near 1, the ellipse is
very long and skinny.
Focus-point type of vision loss caused by looking at the sun for too
long a time, which can burn a hole in the retina of the eye.
The partial or total apparent darkening of the sun when the moon
comes between the sun and the Earth (solar eclipse), or the darkening of
the moon when the full moon is in the Earth's shadow (lunar
Science dealing with the interrelationships between living organisms
and their environments.
Any natural unit or entity including living and non-living parts
that interact to produce a stable system through cyclic exchange of
A warming of the surface waters of the eastern equatorial Pacific
that occurs at irregular intervals of 2–7 years, usually lasting 1–2
years. Along the west coast of South America, southerly winds promote
the upwelling of cold, nutrient-rich water that sustains large fish
populations, that sustain abundant sea birds, whose droppings support
the fertilizer industry. Near the end of each calendar year, a warm
current of nutrient-poor tropical water replaces the cold, nutrient-rich
surface water. Because this condition often occurs around Christmas, it
was named El Niño (Spanish for boy child, referring to the Christ
child). In most years the warming lasts only a few weeks or a month,
after which the weather patterns return to normal and fishing improves.
However, when El Niño conditions last for many months, more extensive
ocean warming occurs and economic results can be disastrous. El Niño has
been linked to wetter, colder winters in the United States; drier,
hotter summers in South America and Europe; and drought in Africa. See
Energy propagated as time-varying electric and magnetic fields.
These two fields are inextricably linked as a single entity since
timevarying electric fields produce time-varying magnetic fields and
vice versa. Light and radar are examples of electromagnetic radiation
differing only in their wavelengths (or frequency). Electric and
magnetic fields propagate through space at the speed of light.
The entire range of radiant energies or wave frequencies from the
longest to the shortest wavelengths--the categorization of solar
radiation. Satellite sensors collect this energy, but what the detectors
capture is only a small portion of the entire electromagnetic spectrum.
The spectrum usually is divided into seven sections: radio, microwave,
infrared, visible, ultraviolet, x-ray, and gamma-ray radiation.
The force that can alter the motion of electricity, measured in
terms of the energy per unit charge imparted to electricity passing
through the source of this force. Electromotive force causes current
flow in a circuit.
Producing an electric current through differences in
Element Set (aka Keplerian elements, classical elements,
Specific information used to define and locate a particular
satellite. The set includes the catalog number; epoch year, day, and
fraction of day; period decay rate; argument of perigee; inclination;
eccentricity; right ascension of the ascending node; mean anomaly; mean
motion; revolution number at epoch; and element set number. This data is
contained in the two-line orbital elements provided by NASA in the NASA
Prediction Bulletin. See Keplerian elements.
The angle at which an antenna must be pointed above the horizon for
optimal reception from a spacecraft.
Bodies in space orbit in elliptical rather than circular orbits
because of factors such as gravity and drag. The point where the
orbiting satellite is closest to Earth is the perigee, sometimes called
peri-apsis or perifocus. The point where the satellite is farthest from
Earth is called apogee, apoapsis, or apifocus. A line drawn from perigee
to apogee is the line-of-apsides, sometimes called the major-axis of the
ellipse. It's simply a line drawn through the ellipse the long
Emergency Locator Transmitter. See Search and Rescue.
A quantitative description of the energy exchange for a physical or
ecological system. The budget includes terms for radiation, conduction,
convection, latent heat, and for sources and sinks of energy.
ENSO (El Nino-Southern Oscillation)
Interacting parts of a single global system of climate fluctuations.
ENSO is the most prominent known source of interannual variability in
weather and climate around the world, though not all areas are affected.
The Southern Oscillation (SO) is a global-scale seesaw in atmospheric
pressure between Indonesia /North Australia, and the southeast Pacific.
In major warm events El Niño warming extends over much of the tropical
Pacific and becomes clearly linked to the SO pattern. Many of the
countries most affected by ENSO events are developing countries with
economies that are largely dependent upon their agricultural and fishery
sectors as a major source of food supply, employment, and foreign
exchange. New capabilities to predict the onset of ENSO events can have
a global impact. While ENSO is a natural part of the Earth's climate,
whether its intensity or frequency may change as a result of global
warming is an important concern.
The complex of physical, chemical, and biological factors in which a
living organism or community exists.
See Earth Observing System.
See Earth Observing System Data and Information System.
EPA (Environmental Protection Agency)
U.S. agency that ensures: Federal environmental laws are implemented
and enforced effectively; U.S. policy--both foreign and domestic--fosters
the integration of economic development and environmental protection so
that economic growth can be sustained over the long term; public and
private decisions affecting energy, transportation, agriculture,
industry, international trade, and natural resources fully integrate
considerations of environmental quality; national efforts to reduce
environmental risk are based on the best available scientific
information communicated clearly to the public; everyone in our society
recognizes the value of preventing pollution before it is created;
people have the information and incentives they need to make
environmentally-responsible choices in their daily lives; and schools
and community institutions promote environmental stewardship as a
A tabulation of a series of points that define the position and
motion of a satellite. See Keplerian elements.
Emergency Position Indicating Radio Beacon. See Search and
Epoch (aka Epoch Time or T0)
Epoch specifies the time of a particular description of a satellite
orbit. See Keplerian elements.
An imaginary circle around the Earth that is everywhere equally
distant (90°) from the North Pole and the South Pole. The equator is a
great circle and defines latitude 0°.
See Earth Radiation Budget Experiment.
European Space Agency.
The process whereby a body of water becomes rich in dissolved
nutrients through natural or man-made processes. This often results in a
deficiency of dissolved oxygen, producing an environment that favors
plant over animal life.
Change from a liquid (more dense) to a vapor or gas (less dense)
form. When water is heated it becomes a vapor that increases humidity.
Evaporation is the opposite of condensation.
The uppermost layer of the atmosphere, its lower boundary is
estimated at 500 km to 1000 km above the Earth's surface. It is only
from the exosphere that atmospheric gases can, to any appreciable
extent, escape into outer space.
Influence on the Earth system (or one of its components) by an
external agent such as solar radiation or the impact of extraterrestrial
bodies such as meteorites.
Federal Aviation Administration.
A process by which graphic or photographic information is
transmitted or recorded by electronic means.
Temperature scale designed by the German scientist Gabriel
Fahrenheit in 1709, based upon water freezing at 32 °F and water boiling
at 212 °F under standard atmospheric pressure. Compare with
See digital image.
Electromagnetic radiation, longer than the thermal infrared, with
wavelengths between about 25 and 1000 micrometers. See electromagnetic
A metallic cylinder closed at one end, used to obtain and direct
radio frequency (RF) energy reflected from a satellite dish. It acts as
a wave guide at microwave frequencies. RF energy inside the horn is
picked up by a small probe; once inside the horn, the wavelength
(energy) of the microwave radiation changes to a guided wave.
U.S. Federal Emergency Management Agency.
Chinese geostationary environmental satellite that was destroyed by
an explosion before launch in April 1994. The name Feng Yun, meaning
Wind and Cloud, was originally applied to the Chinese polar-orbiting
environmental satellite launched in September 1991 (Feng Yun 1-2), which
offered direct readout services. The Chinese polar-orbiter program has
since been abandoned.
Field of View
The range of angles that are scanned or sensed by a system or
instrument, measured in degrees of arc.
The set of influences (electricity, magnetism, gravity) that extend
Device that while selectively passing desired frequencies removes
See frequency modulation.
1) In optics, the distance--usually expressed in millimeters--from the
principal point of a lens or concave mirror to its focal point. 2) The
distance, measured from the center of the surface of a parabolic or
spherical reflector (e.g., satellite dish) where RF energy is brought to
essential point focus.
The area where weak signals collected by a satellite dish,
concentrated into a smaller receiving area, converge.
A cloud on the ground.
Any hydrocarbon deposit that can be burned for heat or power, such
as petroleum, coal, and natural gas.
Hardened remains or traces of plant or animal life from a previous
geological period preserved in the Earth's crust.
A single image or picture. A single complete vertical scan of the
cathode ray tube (CRT).
Atomic or molecular species with unpaired electrons or an otherwise
open shell configuration, usually very reactive. Specific to atmospheric
chemistry, free radicals are: short-lived, highly reactive, intermediate
species produced by dissociation of the source molecules by solar
ultraviolet radiation or by reactions with other stratospheric
constituents. Free radicals are the key to intermediate species in many
important stratospheric chain reactions in which an ozone molecule is
destroyed and the radical is regenerated. See ozone.
Number of cycles and parts of cycles completed per second. F=1/T ,
where T is the length of one cycle in seconds.
Frequency Division Multiplexing
The combining of a number of signals to share a medium by dividing
it into different frequency bands for each signal. See signal.
Frequency Modulation (FM)
The instantaneous variation of the frequency of a carrier wave in
response to changes in the amplitude of a modulating signal. As applied
to APT, the radio signal from the satellite is broadcast on an FM
transmitter and received on the ground on an FM radio receiver. See
frequency division multiplexing, signal.
A boundary between two different air masses. The difference between
two air masses sometimes is unnoticeable. But when the colliding air
masses have very different temperatures and amounts of water in them,
turbulent weather can erupt. A cold front occurs when a cold air mass
moves into an area occupied by a warmer air mass. Moving at an average
speed of about 20 mph, the heavier cold air moves in a wedge shape along
the ground. Cold fronts bring lower temperatures and can create narrow
bands of violent thunderstorms. In North America, cold fronts form on
the eastern edges of high pressure systems. A warm front occurs when a
warm air mass moves into an area occupied by a colder air mass. The warm
air is lighter, so it flows up the slope of the cold air below it. Warm
fronts usually form on the eastern sides of low pressure systems, create
wide areas of clouds and rain, and move at an average speed of 15 mph.
When a cold front follows and then overtakes a warm front (warm fronts
move more slowly than cold fronts) lifting the warm air off the ground,
an occluded front forms.
Water condensation occurring on surfaces below freezing. Condensing
water turns to ice.
The hypothesis that the Earth's atmosphere, biosphere, and its
living organisms behave as a single system striving to maintain a
stability that is conducive to the existence of life.
The increase in signal power produced by an amplifier, usually
expressed in decibels as the ratio of the output to the input. A measure
of the effectiveness of a directional antenna as compared to a
non-directional antenna. See antenna.
A branch of applied mathematics concerned with measuring the shape
of the Earth and describing variations in the Earth's gravity
Geographic Information System (GIS)
A system for archiving, retrieving, and manipulating data that has
been stored and indexed according to the geographic coordinates of its
elements. The system generally can utilize a variety of data types, such
as imagery, maps, tables, etc.
A surface of constant gravitational potential around the Earth--an
averaged surface perpendicular to the force of gravity.
The physical elements of the Earth's surface, crust, and
Geostationary Meteorological Satellite (GMS)
Japan's geostationary weather satellite.
Geostationary Operational Environmental Satellite (GOES)
NASA-developed, NOAA-operated series of satellites that:
GOES observes the U.S. and adjacent ocean areas from vantage oints 35,
790 km (22,240 miles) above the equator at 75° west and 135°
satellites have an equatorial, Earth-synchronous orbit with a 24-hour
period, a resolution of 8 km, an IR resolution of 4 km, and a scan rate
of 1864 statute miles in about three minutes. See geostationary. The
transmission of processed weather data (both visible and infrared) by
GOES is called weather facsimile (WEFAX). GOES WEFAX transmits at 1691+
MHz and is accessible via a ground station with a satellite dish
antenna. GOES carries the following five major sensor systems:
- provide continuous day and night weather observations;
- monitor severe weather events such as hurricanes, thunderstorms,
and flash floods;
- relay environmental data from surface collection platforms to a
- perform facsimile transmissions of processed weather data to
low-cost receiving stations;
- monitor the Earth's magnetic field, the energetic particle flux
in the satellite's vicinity, and x-ray emissions from the sun;
- detect distress signals from downed aircraft and ships.
- The imager is a multispectral instrument capable of sweeping
one visible and four infrared channels in a north-to-south swath across
an east-to-west path, providing full disk imagery once every thirty
- The sounder has more spectral bands than the imager for
producing high-quality atmospheric profiles of temperature and moisture.
It is capable of stepping one visible and eighteen infrared channels in
a north-to-south swath across an east-to-west path.
- The Space Environment Monitor (SEM) measures the condition of
the Earth's magnetic
field, the solar activity and radiation around the spacecraft, and
transmits these data to a central processing facility.
- The Data Collection System (DCS) receives transmitted
meteorological data from
remotelylocated platforms and relays the data to the end users.
- The Search and Rescue Transponder can relay distress signals at all times,
but cannot locate them. While only the polar-orbiting satellite can
locate distress signals, the two types of satellites work together to
create a comprehensive search and rescue system.
Describes an orbit in which a satellite is always in the same
position (appears stationary) with respect to the rotating Earth. The
satellite travels around the Earth in the same direction, at an altitude
of approximately 35,790 km (22,240 statute miles) because that produces
an orbital period equal to the period of rotation of the Earth (actually
23 hours, 56 minutes, 04.09 seconds). A worldwide network of operational
geostationary meteorological satellites provides visible and infrared
images of Earth's surface and atmosphere. The satellite systems include
the U.S. GOES, METEOSAT (launched by the European Space Agency and
operated by the European Weather Satellite Organization–EUMETSAT), the
Japanese GMS, and most commercial, telecommunications satellites. See
Geosynchronous (aka GEO)
Synchronous with respect to the rotation of the Earth. See
A multi-year surplus accumulation of snowfall in excess of snowmelt
on land and resulting in a mass of ice at least 0.1 km2 in area that
shows some evidence of movement in response to gravity. A glacier may
terminate on land or in water. Glacier ice is the largest reservoir of
fresh water on Earth, and second only to the oceans as the largest
reservoir of total water. Glaciers are found on every continent except
Global Change Research Program (GCRP)
The USGCRP is a government-wide program whose goal is "to establish
a scientific basis for national and international policy-making relating
to natural and human-induced changes in the global Earth system."
Mission to Planet Earth is NASA's central contribution to the U.S.
Global Change Research Program. The Global Change Research Program
coordinates and guides the efforts of federal agencies. The program
examines such questions as, is the Earth experiencing global warming? Is
the depletion of the ozone layer expanding? How do we determine and
understand the causes of global climate changes? Are they reversible?
What are the implications for human needs and activities?
All of the activities required to specify a global variable, such as
ozone. These activities range from data acquisition to the generation of
a data-analysis product, and include estimates of the uncertainties in
that product. A global measurement often will consist of a combination
of observations from a spacecraft instrument (required for global
coverage) and measurements in situ (needed to provide reference points
for long-term accuracy).
Functions of space and time that describe the large scale state and
evolution of the Earth system. The Earth system's geosphere,
hydrosphere, atmosphere, and biosphere and their components are, or
potentially are, global variables.
See Geostationary Meteorological Satellite.
GOES I/GOES 8
NOAA geostationary satellite launched in April 1994 (alphabetical
designators are used while on the ground and before geostationary orbit,
after it achieves geostationary orbit it became GOES 8). GOES 8 is the
first in a series of five new geostationary satellites that will ensure
dual-satellite coverage of the U.S. into the next century, and will
provide better advanced warnings of thunderstorms, flash floods,
hurricanes, and other severe weather. GOES 8 will also contribute
important information to a new flood and water management system which
will assist decision-makers with the allocation of precious western
The next generation of NOAA geostationary satellites, scheduled for
launch beginning sometime after 2003. Currently in the planning phase,
these satellites will follow the series of five geostationary satellites
which are being launched beginning in 1994. See GOES I
See Geostationary Operational Environmental Satellite.
Environmental satellite scanners, rather than photographing a scene,
scan a scene line-byline measuring light or heat levels and transmitting
this information as a video image via an amplitude modulated (AM)
subcarrier contained in the satellite's FM signal. The video image--a
2400 Hz tone--is amplitude modulated to correspond to the light and dark
areas sensed, with the louder portion of the tone representing the
lighter areas of the image and the lower portion of the tone
representing the darker areas of the image. Intermediate volumes form
the shades of the grayscale (up to 256 shades) needed to complete the
image. This is an analog type of data transmission, and enables the
assessment of such features as heat, light, temperature, and cloud
Process by which significant changes in the chemistry of Earth's
atmosphere may enhance the natural process that warms our planet and
elevates temperatures. If the effect is intensified and Earth's average
temperatures change, a number of plant and animal species could be
threatened with extinction. Certain gaseous components of the
atmosphere, called greenhouse gases, transmit the visible portion of
solar radiation but absorb specific spectral bands of thermal radiation
emitted by the Earth. The theory is that terrain absorbs radiation,
heats up, and emits longer wavelength thermal radiation that is
prevented from escaping into space by the blanket of carbon dioxide and
other greenhouse gases in the atmosphere. As a result, the climate
warms. Because atmospheric and oceanic circulations play a central role
in the climate of the Earth, improving our knowledge about their
interaction becomes essential.
A gaseous component of the atmosphere contributing to the greenhouse
effect. Greenhouse gases are transparent to certain wavelengths of the
sun's radiant energy, allowing them to penetrate deep into the
atmosphere or all the way into the Earth's surface. Greenhouse gases and
clouds prevent some of infrared radiation from escaping, trapping the
heat near the Earth's surface where it warms the lower atmosphere.
Alteration of this natural barrier of atmospheric gases can raise or
lower the mean global temperature of the Earth. Greenhouse gases include
carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and water
vapor. Carbon dioxide, methane, and nitrous oxide have significant
natural and human sources while only industries produce
chlorofluorocarbons. Water vapor has the largest greenhouse effect, but
its concentration in the troposphere is determined within the climate
system. Water vapor will increase in response to global warming, which
in turn may further enhance global warming.
Greenwich Mean Time (GMT)
See Coordinated Universal Time.
Gross Feature Map
Map that displays geographic characteristics rather than political
Ground Control (Points)
Identifiable points on the ground whose locations on the surface of
the Earth are accurately known for use as geodetic references in
mapping, charting, and other related mensuration applications.
See Earth station.
The inclination of a satellite, together with its orbital altitude
and the period of its orbit, creates a track defined by an imaginary
line connecting the satellite and the Earth's center. The intersection
on the line with the Earth's surface is the subsatellite point. As the
Earth turns on its axis and the satellite orbits overhead, a line is
created by the satellite's apparent path over the ground (the series of
subsatellite points connected). A geostationary satellite has an
inclination of essentially zero, and, because its orbital period exactly
matches the Earth's rotation, its ground track is reduced to an apparent
stationary point on the equator.
NASA Goddard Space Flight Center, located in Greenbelt, Maryland.
See NASA Centers.
Electromagnetic or acoustic wave that is constrained within certain
boundaries, as in a wave guide (transmission line).
A warm, swift ocean current that flows along the coast of the
Eastern United States and makes Ireland, Great Britain, and the
Scandinavian countries warmer than they would be otherwise.
A large arm of an ocean or sea extending into a land mass.
The area or region where a particular type of plant or animal lives
Precipitation composed of balls or irregular lumps of ice. Hail is
produced when large frozen raindrops, or almost any particles, in
cumulonimbus clouds act as embryos that grow by accumulating supercooled
liquid droplets. Violent updrafts in the cloud carry the particles in
freezing air, allowing the frozen core to accumulate more ice. When the
piece of hail becomes too heavy to be carried by upsurging air currents
it falls to the ground.
The electrical and mechanical components of a system, as opposed to
Fine dry or wet particles of dust, salt, or other impurities that
can concentrate in a layer next to the Earth when air is stable.
The equilibrium existing between the radiation received and emitted
by a planetary system.
Heat Capacity Mapping Mission (HCMM)
A two-channel radiometer launched by NASA to measure the thermal
properties of the terrestrial surface. It had an application to identify
and locate rocks and minerals. One radiometer channel was in the visible
to near infrared (0.5–1.1 micrometers), and the other in the thermal
infrared (10.5–12.5 micrometers). The instantaneous field of view (IFOV)
was about 600 meters.
Half of the Earth, usually conceived as resulting from the division
of the globe into two equal parts, north and south or east and
The international unit of frequency equal to one cycle per second.
Radio frequencies are usually expressed in kilohertz/kHz (1,000 cycles
per second) or megahertz/MHz (1,000,000 cycles per second).
Radio waves or other electromagnetic radiation resulting from the
oscillations of electricity in a conductor.
High Resolution Doppler Imager (HRDI)
Carried on UARS, it measures stratospheric winds.
High-Resolution Infrared Radiation Sounder (HIRS)
Instrument carried by NOAA polar-orbiting satellites that detects
and measures energy emitted by the atmosphere to construct a vertical
temperature profile from the Earth's surface to an altitude of about 40
km. Measurements are made in 20 spectral regions in the infrared
High-Resolution Picture Transmission (HRPT)
Real-time, 1.1-kilometer resolution, digital images provided by
NOAA's polar-orbiting environmental satellites, containing all five
spectral channels and telemetry data transmitted as high-speed digital
transmissions. The Advanced Very High Resolution Radiometer (AVHRR)
provides the primary imaging system for APT and HRPT. See TIROS.
A digital logic state corresponding to a binary "1." See
The subtropical latitudes (30–35 degrees), where winds are light and
weather is hot and dry. According to legend, ships traveling to the New
World often stagnated in this region and had to throw dead horses
overboard or eat them to survive, hence the name horse latitudes. See
See High Resolution Doppler Imager.
See High Resolution Picture Transmission.
The amount of water vapor in the air. The higher the temperature,
the greater the number of water molecules the air can hold. For example:
at 60 °F (15 °C), a cube of air one yard on each side can hold up to
4.48 ounces of water. At 104 °F (40 °C), the same cube of air can hold
up to 17.9 ounces of water. Relative humidity describes the amount of
water in the air compared with how much the air can hold at the current
temperature. Example: 50
relative humidity means the air holds half the
water vapor that it is capable of holding; 100
relative humidity means
the air holds all the water vapor it can. At 100
humidity, no more
evaporation can occur until the temperature rises, or until the water
vapor leaves the air through condensation. Absolute humidity is the
ratio of the mass of water vapor present in a system of moist air to the
volume occupied by the mixture, that is, the density of water
Severe tropical storms whose winds exceed 74 mph. Hurricanes
originate over the tropical and subtropical North Atlantic and North
Pacific oceans, where there is high humidity and light wind. These
conditions prevail mostly in the summer and early fall. Since hurricanes
can take days or even weeks to form, time is usually available for
preventive or protective measures. From space, hurricanes look like
giant pinwheels, their winds circulating around an eye that is between 5
and 25 miles in diameter. The eye remains calm with light winds and
often a clear sky. Hurricanes may move as fast as 50 mph, and can become
incredibly destructive when they hit land. Although hurricanes lose
power rapidly as soon as they leave the ocean, they can cause high waves
and tides up to 25 feet above normal. Waves and heavy flooding cause the
most deaths during a hurricane. The strongest hurricanes can cause
One of a class of compounds used primarily as a CFC substitute. Work
on CFC alternatives began in the late 1970s after the first warnings of
CFC damage to stratospheric ozone. By adding hydrogen to the chemical
formulation, chemists made CFCs less stable in the lower atmosphere
enabling them to break down before reaching the ozone layer. However,
HCFCs do release chlorine and have contributed more to atmospheric
chlorine buildup than originally predicted. Development of non-chlorine
based chemical compounds as a substitute for CFCs and HCFCs
The pathways through which water is cycled in the terrestrial
The totality of water encompassing the Earth, comprising all the
bodies of water, ice, and water vapor in the atmosphere.
Instrument that measures water vapor content in the air and
communicates changes in humidity visibly and immediately through a graph
or a dial. There are three types of hygrometers:
- The hair hygrometer
uses a human hair as the sensing instrument. The hair lengthens when the
air is moist and contracts when the air is dry, but remains unaffected
by air temperature. However, the hair hygrometer cannot respond to rapid
fluctuations in humidity.
- An electric hygrometer uses a plate coated
with carbon. Electrical resistance of the carbon coating changes as the
moisture content of the air changes--changes that translate into relative
humidity. This type of hygrometer is used frequently in the
- An infrared hygrometer uses a beam of light containing two
separate wave lengths to gauge atmospheric humidity. One of the
wavelengths is absorbed by water vapor, the other is unaffected,
providing an extremely accurate index of water vapor for paths of a few
inches or thousands of feet. See psychrometer.
A thick mass of ice extending from a polar shore. The seaward edge
is afloat and sometimes extends hundreds of miles into the sea.
Instantaneous Field of View. See Multispectral Scanner for sample
See International Geophysical Year.
The area represented by each pixel of a satellite image. The smaller
the area represented by a pixel, the more accurate and detailed the
image. For example, if a U.S. map and a world map are printed on
identically sized sheets of paper, one square inch on the U.S. map will
represent far less area and provide for more detail than one square inch
on the world map. In this example the U.S. map has higher resolution.
APT has a resolution of 4 km, HRPT has a resolution of 1.1 km and WEFAX
resolution is 8 km.
Pictorial representation of data acquired by satellite systems, such
as direct readout images from environmental satellites. An image is not
a photograph. An image is composed of two-dimensional grids of
individual picture elements (pixels). Each pixel has a numeric value
that corresponds to the radiance or temperature of the specific ground
area it depicts. See grayscale.
A satellite instrument that measures and maps the Earth and its
atmosphere. Imager data are converted by computer into pictures.
Latin for "in original place." Refers to measurements made at the
actual location of the object or material measured. Compare with remote
Inclination (aka i)
One of the six Keplerian elements, it indicates the angle of the
orbit plane to the central body's equator. See Keplerian elements for
diagram. The elliptical path of a satellite orbit lies in a plane known
as the orbital plane. The orbital plane always goes through the center
of the Earth but may be tilted at any angle relative to the equator.
Inclination is the angle between the equatorial plane and the orbital
plane measured counter-clockwise at the ascending node. A satellite in
an orbit that exactly matches the equator has an inclination of 0°,
whereas one whose orbit crosses the Earth's poles has an inclination of
90°. Because the angle is measured in a counterclockwise direction, it
is quite possible for a satellite to have an inclination of more than
90°. An inclination of 180° would mean the satellite is orbiting the
equator, but in the opposite direction of the Earth's rotation. Some
sun-synchronous satellites that maintain the same ground track
throughout the year have inclinations of as much as 98°. U.S. scientific
satellites that study the sun are placed in orbits closer to the
equator, frequently at 28° inclination. Most weather satellites are
placed in high-inclination orbits so they can oversee weather conditions
worldwide. See orbital inclination.
All of the means and mechanisms for data receipt, processing,
storage, retrieval, and analysis. Information systems can be designed
for storage and dissemination of a variety of data products--including
primary data sets and both intermediate and final analyses-- and for an
interface providing connections to external computers, external data
banks, and system users. To be effective, the design and operation of an
information system must be carried out in close association with the
primary producers of the data sets, as well as other groups producing
integrated analyses or intermediate products.
Infrared Radiation (IR)
Infrared is electromagnetic radiation whose wavelength spans the
region from about 0.7 to 1000 micrometers (longer than visible
radiation, shorter than microwave radiation). Remote-sensing instruments
work by sensing radiation that is naturally emitted or reflected by the
Earth's surface or from the atmosphere, or by sensing signals
transmitted from a satellite and reflected back to it. In the visible
and near-infrared regions, surface chemical composition, vegetation
cover, and biological properties of surface matter can be measured. In
the mid-infrared region, geological formations can be detected due to
the absorption properties related to the structure of silicates. In the
far infrared, emissions from the Earth's atmosphere and surface offer
information about atmospheric and surface temperatures and water vapor
and other trace constituents in the atmosphere. Since IR data are based
on temperatures rather than visible radiation, the data may be obtained
day or night.
Indian National Satellite.
Solar radiation incident upon a unit horizontal surface on or above
the Earth's surface.
Instantaneous Field of View (IFOV)
The field of view of a scanning detector system at a given instant.
The range of angles scanned by the system is then called the field of
view, or swath width.
Integrated Circuit (IC)
A solid state electronic circuit that consists of several
micro-components constructed to perform a special function.
International Date Line
An imaginary line of longitude 180° east or west of the prime
An internationally agreed-upon naming convention for satellites. The
designator contains the last two digits of the launch year, the launch
number of the year, and the part of the launch, i.e., "A" indicates
payload, "B" the rocket booster, or second payload, etc.
International Geophysical Year (IGY)
(1957–58) The IGY was organized by the scientific community through
the International Council of Scientific Unions (ICSU) . It was
highlighted by international cooperation in the exploration of
world-wide geophysical phenomena and by the inauguration of the space
age through the launching of the first satellites (USSR's Sputnik I and
US Explorer 1) to study the upper atmosphere and Earth's nearby
International Space Year (ISY)
(1992) Designated the first international celebration of humanity's
future in the space age. Themes included the global perspective of the
space age, discovery, exploration, and scientific inquiry. An important
ISY scientific focus was Mission to Planet Earth. A wide range of
educational programs and public events emphasized ISY's global
perspective. 1992 also commemorated the 500th anniversary of Columbus’
voyage to the New World and the 35th anniversary of the International
International System of units (SI)
The International System of Units prescribes the symbols and
prefixes shown in the table to form decimal multiples and submultiples
of SI units. The following examples illustrate the use of these
prefixes. 0.000,001 meters = 10-6 meters =1 micrometer = 1µm; 1000
meters = 103 meters = 1 kilometer = 1 km; 1,000,000 cycles per second =
106 hertz = 1 megahertz = 1 MHz
Atom or molecule that has acquired an electric charge by the
loss/gain of one or more electrons.
Inches per second.
Lines drawn on a weather map joining places of equal barometric
Of or indicating equality of temperature.
Lines connecting points of equal temperature on a weather
Narrow strip of land located between two bodies of water, connecting
two larger land areas.
ITOS (Improved TIROS Operational Satellite)
Second generation, polar-orbiting, environmental satellites utilized
to augment NOAA's world-wide weather observation capabilities. ITOS were
launched from 1970–1976, but eventually replaced by the third generation
of polar-orbiting, environmental satellites TIROS-N (first launched in
1978). See TIROS.
Japanese National Space Development Agency (NASDA)
The agency reports to the Japanese Ministry of Science and
Rivers of high-speed air in the atmosphere. Jet streams form along
the boundaries of global air masses where there is a significant
difference in atmospheric temperature. The jet streams may be several
hundred miles across and 1–2 miles deep at an altitude of 8–12 miles.
They generally move west to east, and are strongest in the winter with
core wind speeds as high as 250 mph. Changes in the jet stream indicate
changes in the motion of the atmosphere and weather.
Joint Education Initiative (JEI)
The JEI project was developed by USGS, NOAA, NASA, industry, and
teachers to enable teachers and students to explore the massive
quantities of Earth science data published by the U.S. Government on
CD-ROM. JEI encourages a research and analysis approach to science
JPL (Jet Propulsion Laboratory)
See NASA Centers.
JSC (Johnson Space Center)
See NASA Centers.
Kepler's Three Laws of Motion
Any spacecraft launched into orbit obeys the same laws that govern
the motions of the planets around our sun, and the moon around the
Earth. Johannes Kepler formulated three laws that describe these
motions: 1) Each planet revolves around the sun in an orbit that is an
ellipse with the sun as its focus or primary body. Kepler postulated the
lack of circular 2) The radius vector--such as the line from the center
of the sun to the center of a planet, from the center of Earth to the
center of the moon, or from the center of Earth to the center of gravity
of a satellite--sweeps out equal areas in equal periods of time. 3) The
square of a planet's orbital period is equal to the cube of its mean
distance from the sun times a constant. As extended and generalized,
this means that a satellite's orbital period increases with its mean
distance from the planet. See Newton's law of universal gravitation and
laws of motion. orbits--only elliptical ones--determined by gravitational
perturbations and other factors. Gravitational pulls, according to
Newton, extend to infinity, although their forces weaken with istance
and eventually become impossible to detect. (See Newton's law of
universal gravitation.) Spacecraft orbiting the Earth are primarily
influenced by the Earth's gravity and anomalies in its composition, but
they also are influenced by the moon and sun and possibly other
Keplerian Elements (aka Satellite Orbital Elements)
The set of six independent constants which define an orbit--named for
Johannes Kepler [1571–1630]. The constants define the shape of an
ellipse or hyperbola, orient it around its central body, and define the
position of a satellite on the orbit. The classical orbital elements
are: a: semi-major axis, gives the size of the orbit, e: eccentricity,
gives the shape of the orbit, i: inclination angle, gives the angle of
the orbit plane to the central body's equator W: right ascension of the
ascending node, which gives the rotation of the orbit plane from
reference axis, w: argument of perigee is the angle from the ascending
nodes to perigee point, measured along the orbit in the direction of the
satellite's motion, q: true anomaly gives the location of the satellite
on the orbit.
One thousand hertz, i.e., one thousand cycles per second.
Metric unit of distance equal to 3,280.8 feet or .621 statute
Unit of speed of one nautical mile (6,076.1 feet) an hour.
KSC (Kennedy Space Center)
See NASA Centers.
A body of fresh or salt water entirely surrounded by land.
A nocturnal coastal breeze that blows from land to sea. In the
evening the water may be warmer than the land, causing pressure
differences. The land breeze is the flow of air from land to sea
equalizing these pressure differences. See sea breeze.
Land Remote-Sensing Satellite, operated by the U.S. Earth
Observation Satellite Company (EOSAT). Commercialized under the Land
Remote-Sensing Commercialization Act of 1984, Landsat is a series of
satellites (formerly called ERTS) designed to gather data on the Earth's
resources in a regular and systematic manner. Objectives of the mission
are: land use inventory, geological/mineralogical exploration, crop and
forestry assessment, and cartography. Landsat has a spatial resolution
of 28.5 meters. Restructured Federal agency responsibilities for the
Landsat program are effective for the acquisition and operation of
Landsat 7. New operating policy specifies that NOAA will be responsible
for satellites after they are placed in orbit, NASA will be responsible
for the development and launch of Landsat 7, and that the U.S.
government will provide unenhanced data to users at no cost beyond the
cost of fulfilling their data request.
Landsats (aka Earth Resources Satellites)
Any land remote-sensing satellites. Includes the U.S. Landsat system
and the French SPOT.
LaRC (Langley Research Center)
See NASA Centers.
Laser (Light Amplification by Stimulated Emission of
Active instrument that produces discretely coherent pulses of light
(light waves with no phase differences, or with predictable phases
differences, are said to be coherent).
The use of lasers to measure distances.
Latitude (aka geodetic latitude)
The angle between a perpendicular at a location, and the equatorial
plane of the Earth.
A listing that contains symbols and other information about a
LeRC (Lewis Research Center)
See NASA Centers.
1) Form of radiant energy that acts upon the retina of the eye,
optic nerve, etc., making sight possible. This energy is transmitted at
a velocity of about 186,000 miles per second by wavelike or vibrational
2) A form of radiant energy similar to this, but not acting on
the normal retina, such as ultraviolet and infrared radiation. Interplay
between light rays and the atmosphere cause us to see the sky as blue,
and can result in such phenomena as glows, halos, arcs, flashes, and
A discharge of atmospheric electricity accompanied by a vivid flash
of light. During thunderstorms, static electricity builds up within the
clouds. A positive charge builds in the upper part of the cloud, while a
large negative charge builds in the lower portion. When the difference
between the positive and negative charges becomes great, the electrical
charge jumps from one area to another, creating a lightning bolt. Most
lightning bolts strike from one cloud to another, but they also can
strike the ground. These bolts occur when positive charges build up on
the ground. A negative charge called the "faintly luminous streamer" or
"leader" flows from the cloud toward the ground. Then a positively
charged leader, called the return stroke, leaves the ground and runs
into the cloud. What is seen as a lightning bolt is actually a series of
downward-striking leaders and upward-striking return strokes, all taking
place in less than a second. Lightning bolts can heat the air to
temperatures hotter than the surface of the sun. This burst of heat
makes the air around the bolt expand explosively, producing the sound we
hear as thunder. Since light travels a million times faster than sound,
we see lightning bolts before we hear their thunderclaps. By counting
the seconds between a flash of lightning and the thunderclap and
dividing by five, we can determine the approximate number of miles to
the lightning stroke. See thunderstorm.
Limb Viewing (Occultation)
The process of viewing the atmosphere at a tangent to the Earth's
surface. The viewing signal, from a star or another satellite, is
occulted or obscured by the intervening atmosphere. The absorption of
light from the sun or star provides information on the properties of the
atmosphere at different heights. Limb viewing instruments can also sense
infrared or microwave-emitted radiation from the atmosphere.
Line-of-Apsides (aka Major-Axis of the Ellipse)
The straight line drawn from the perigee (point of orbit closest to
Earth) to the apogee (point of orbit farthest from Earth) is the
The line created by the intersection of the equatorial plane and the
Area within which visible contact can be made. For example, NOAA
polar-orbiting satellites continuously transmit the APT signal. Radio
reception of the APT signal is possible only when the satellite is above
the horizon of a particular location (not obstructed by the Earth's
surface), with a line-of-sight contact with the satellite.
Exponent of the power to which it is necessary to raise a fixed
number (the base) to produce the given number. For example, the
logarithm of 100 (base 10) is 2 because 102 = 100.
The angular distance from the Greenwich meridian (0°), along the
equator. This can be measured either east or west to the 180th meridian
(180°) or 0° to 360° W.
Loss of Signal (LoS)
The inability to receive a satellite signal because the satellite's
orbital path has taken it below the antenna's horizon. This term is
relevant to all satellites except geostationary.
Low or Low-Pressure System
A horizontal area where the atmospheric pressure is less than it is
in adjacent areas. Since air always moves from areas of high pressure to
areas of low pressure, air from these adjacent areas of higher pressure
will move toward the low pressure area to equalize the pressure. This
inflow of air toward the low will be affected by the Earth's rotation
(see Coriolis force) and will cause the air to spiral inward in a
counterclockwise direction in the northern hemisphere. The air
eventually rises near the center of the low, causing cloudiness and
precipitation. The air in a low rotates in a counterclockwise direction
in the Northern Hemisphere, and in a clockwise direction in the Southern
Hemisphere. Low-pressure cells are called cyclones.
A logic state corresponding to a binary "0". Satellite imagery is
displayed on a computer monitor by a combination of highs and lows. See
Region surrounding a celestial body where its magnetic field
controls the motions of charged particles. The Earth's magnetic field is
dipolar in nature. That is, it behaves as if produced by a giant bar
magnet located near the center of the planet with its north pole tilted
several degrees from Earth's geographic north pole. The Earth's magnetic
field presents an obstacle to the solar wind, as a rock in a running
stream of water. This obstacle is called a bow shock. The bow shock
slows down, heats, and compresses the solar wind, which then flows
around the rest of Earth's magnetic field. See Van Allen belts.
MAryland Pilot Earth Science and Technology Education NETwork.
NASAsponsored education project designed to complement NASA's Mission to
Planet Earth. MAPS-NET has been developed to enrich math and science
curricula and enhance teacher preparation in Earth system science.
Middle and high school teachers learn about Earth sciences and satellite
direct readout at graduate-level summer workshops; academia, federal
agencies, and the private sector form the support network.
Mean Anomaly (aka M0 or MA or phase)
Specifies the mean location (true anomaly specifies the exact
location) of a satellite on an orbit ellipse at a particular time,
assuming a constant mean motion throughout the orbit . Epoch specifies
the particular time at which the satellites position is defined, while
mean anomaly specifies the location of the satellite at epoch. Mean
anomaly is measured from 0° to 360° during one revolution.
It is defined
as 0° at perigee, and hence is 180° at apogee. See Keplerian
Mean Motion (aka N0)
Averaged speed of a satellite in a non-circular orbit (i.e.,
eccentricity>0). Diagram, page 19. Satellites in circular orbits travel
at a constant speed. Satellites in non-circular orbits move faster when
closer to the Earth, and slower when farther away. Common practice is to
compute the mean motion (average the speed), which is measured in
revolutions per day.
Measurement System Integrity
The tracking and documentation over the long term of all causes of
error or uncertainty in a final data-analysis product. These include
instrument calibration, adequacy of measurement validation, data
coverage and sampling density, availability and quality of ancillary
data, procedures for data analysis and reduction, the results of checks
against independent measurement, and quantitative error
The establishment of confidence in the numerical relationship
between the calibrated sensor output and the actual variable being
A method of making maps in which the Earth's surface is shown as a
rectangle with the meridians as parallel straight lines spaced at equal
intervals and the parallels of latitude as parallel straight lines
intersecting the meridians at right angles. Areas away from the equator
appear larger than they are, with the greatest distortion near the
The upper boundary of the mesosphere where the temperature of the
atmosphere reaches its lowest point.
The atmospheric layer above the stratosphere, extending from about
50 to 85 kilometers altitude. The temperature generally decreases with
Information describing the content or utility of a data set. For
example, the dates on which data were procured are metadata.
The former Soviet Union's series of polar-orbiting weather
satellites. The Meteor satellites transmit images in a system compatible
with the NOAA polar-orbiting satellites.
Study of the atmosphere and its phenomena.
METEOrological SATellite. Europe's geostationary weather satellite,
launched by the European Space Agency and now operated by an
organization called Eumetsat. METEOSAT transmits at 1691 and 1694.5
Generic term for meteorological (weather) satellites.
Micrometer (aka micron)
One millionth of a meter, used to measure wavelengths in the
Controlling unit of a microcomputer, laid out on a tiny silicon chip
and containing the logical elements for handling data, performing
calculations, carrying out stored instructions, etc.
Electromagnetic radiation with wavelengths between about 1000
micrometers and one meter.
Electromagnetic radiation between the near infrared and the thermal
infrared, about 2–5 micrometers.
One thousandth of a bar, a unit of atmospheric pressure. The average
atmospheric pressure at sea level is 1.01325 bars or 1013.25 mb. See
pascal (Pa), atmospheric pressure.
Mission to Planet Earth (MTPE)
International research program to understand our planet's
environment as a system. A major challenge of MTPE is to observe,
understand, model, assess, and eventually predict global change. Meeting
this challenge will help to evaluate the impact that human activity
(e.g., clearing forests and burning fossil fuels) has on our
environment, and to distinguish human-induced changes from the effects
of natural events (e.g. volcanic eruptions, erosion). NASA's MTPE uses
space-, aircraft-, and ground-based measurements to provide the
scientific basis for understanding global change. The program will
produce longterm global maps of clouds, land and ocean vegetation,
atmospheric ozone, sea-surface temperature, and other global processes
necessary to understand the state of the Earth and to detect any
patterns of change. This information will be available to scientists and
policy makers through the Earth Observing System Data and Information
System (EOSDIS). The centerpiece of NASA's MTPE will be the Earth
Observing System (EOS), a series of satellites planned for launch
beginning in 1998. Measurements from EOS will be complemented by the
Earth Probes, a series of discipline-specific satellites and instruments
designed to observe Earth processes where smaller platforms and/or
different orbits from EOS are required. Planned Earth Probes will
measure tropical rainfall, ocean productivity, ozone, and ocean surface
winds. In addition, MTPE includes current NASA Earth science missions
collecting important data on the global environment, such as the Upper
Atmosphere Research Satellite (UARS) and the Ocean Topography Experiment
(TOPEX/POSEIDON), Space Shuttle experiments such as ATLAS, and aircraft
A mathematical representation of a process, system, or object
developed to understand its behavior or to make predictions. The
representation always involves certain simplifications and
Device that allows two computers (which use binary data in the form
of bits) to communicate using a telephone line (which uses tones). When
the computer is transmitting data, the modem is needed to modulate
binary data into tones. When receiving data, the device is needed to
demodulate the tones to obtain the binary data required by the computer.
Since the computer must be both a transmitter and receiver of data, the
modem must be able to modulate and demodulate data.
Variation in the frequency of a radio wave in accordance with some
other impulse. Modulation is essential to communication systems in which
a number of different signals must all share the same medium. One way
this sharing can be accomplished is to place each signal in its own band
of frequencies in the medium. Amplitude modulation and frequency
modulation are two ways in which signals The combining of a number of
signals to share a communication medium by dividing it into different
frequency bands for each signal is called frequency-division
multiplexing. Amplitude modulation (AM) is technologically quite simple,
and the bandwidth of the amplitude-modulated carrier is at most twice
the bandwidth of the modulating signal. However, an amplitude-modulated
carrier is very prone to the effects of additive noise. Frequency
modulation (FM) is more complicated than amplitude modulation, and the
bandwidth of the frequency-modulated carrier can be many times that of
the modulating signal. However, the process of demodulating a
frequency-modulated carrier eliminates much of the deleterious effects
of additive noise. This trade-off between bandwidth and noise reduction
characterizes most communication situations.
Heavy winds characterized by a pronounced seasonal change in
direction. Winds usually blow from land to sea in the winter, while in
the summer, the flow reverses and precipitation is more common. Monsoons
are most typical in India and southern Asia.
An international agreement to drastically reduce CFC production, the
Protocol was adopted in Montreal in 1987. It was significantly
strengthened at a subsequent meeting in London in 1990 that called for a
complete elimination of CFCs by the year 2000. The agreement was again
amended by a Meeting of the Parties in Copenhagen in November 1992.
Consumption of controlled substances– such as CFCs and halons–was
greatly reduced or eliminated, and many accountability dates were moved
forward, often from January 2000 to 1 January 1996.
Molecules are the tiny particles which form various substances. For example, air is
composed of many billions of oxygen, nitrogen and Carbon Dioxide molecules.
A composite picture built up from a number of image segments. An
example of a mosaic is the WEFAX transmission, which includes both polar
and mercator mosaics derived from TIROS-N/NOAA polar orbit image
Mountain and Valley Breezes
System of winds that blow downhill during the night (mountain
breeze) and uphill during the day (valley breeze).
MSFC (Marshall Space Flight Center)
See NASA Centers.
See Mission to Planet Earth.
A device that combines several separate communications signals into
one and outputs them on a single line.
Multispectral Scanner (MSS)
A line-scanning instrument flown on Landsat satellites that
continually scans the Earth in a 185 km (100 nautical miles) swath. On
Landsats 1, 2, 4, and 5, the MSS had four spectral bands in the visible
and near-infrared with an IFOV of 80 meters. Landsat-3 had a fifth band
in the thermal infrared with an IFOV of 240 meters.
Point on Earth directly beneath a satellite, the opposite of zenith.
Compare with subsatellite point.
See International System of Units.
One billionth of a meter. Nanometers are used to measure wavelengths
in the electromagnetic spectrum.
The ten major NASA Centers are: (1) Ames Research Center (ARC)
Located at Moffett Field, California. ARC is active in aeronautical
research, life sciences, space science, and technology research. The
Center houses the world's largest wind tunnel and the world's most
powerful supercomputer system. (2) The Dryden Flight Research Center,
Edwards Air Force base, California, formerly part of ARC, became a
separate entity March 1994. Since the 1940s, this Mojave desert site has
been a testing ground for high-performance aircraft and is one of two
prime landing sites for the Space Shuttle. (3) Goddard Space Flight
Center (GSFC) Goddard was NASA's first major scientific laboratory
devoted entirely to the exploration of space. Located in Greenbelt,
Maryland, GSFC's responsibilities include design and construction of new
scientific and applications satellites, as well as tracking and
communication with existing satellites in orbit. GSFC is the lead center
for the Earth Observing System, a key element of Mission to Planet
Earth. GSFC also directs operations at the Wallops Flight Facility on
Wallops Island, Virginia, which each year launches some 50 scientific
missions to suborbital altitudes on small sounding rockets. (4) Jet
Propulsion Laboratory (JPL) Located in Pasadena, California, JPL is
operated under contract to NASA by the California Institute of
Technology. Its primary focus is the scientific study of the solar
system, including exploration of the planets with automated probes. Most
of the lunar and planetary spacecraft of the 1960s and 1970s were
developed at JPL. JPL also is the control center for the worldwide Deep
Space Network, which tracks all planetary spacecraft. (5) Lyndon B.
Johnson Space Center (JSC) Johnson Space Center, located between Houston
and Galveston, Texas, is the lead center for NASA's manned space flight
program. JSC has been Mission Control for all piloted space flights
since 1965, and now manages the Space Shuttle program. JSC's
responsibilities include selecting and training astronauts; designing
and testing vehicles and other systems for piloted space flight; and
planning and executing space flight missions. The center has a major
role in developing the Space Station. In addition, JSC directs
operations at the White Sands Test Facility in New Mexico, which
conducts Shuttle-related tests. The nearby White Sands Missile Range
also serves as a backup landing site for the Space Shuttle. (6) Kennedy
Space Center (KSC) Located near Cape Canaveral, Florida, KSC is NASA's
primary launch site. The Center handles the preparation, integration,
checkout, and launch of space vehicles and their payloads. All piloted
space missions since the Mercury program have been launched from here,
including Gemini, Apollo, Skylab, and Space Shuttle flights. KSC is the
Shuttle's home port, where orbiters are serviced and outfitted between
missions, and then assembled into a complete Shuttle "stack" before
launch. The Center also manages the testing and launch of unpiloted
space vehicles from an array of launch complexes, and conducts research
programs in areas of life sciences related to human spaceflight. (7)
Langley Research Center (LaRC) Oldest of NASA's field centers, LaRC is
located in Hampton, Virginia, and focuses primarily on aeronautical
research. Established in 1917 by the National Advisory Committee for
Aeronautics, the Center currently devotes two-thirds of its programs to
aeronautics, and the rest to space. LaRC researchers use more than 40
wind tunnels to study improved aircraft and spacecraft safety,
performance, and efficiency. (8) Lewis Research Center (LeRC) Lewis
Research Center, located outside Cleveland, Ohio, conducts a varied
program of research in aeronautics and space technology. Aeronautical
research includes work on advanced materials and structures for
aircraft. Space-related research focuses primarily on power and
propulsion. Another significant area of research is in energy and power
sources for spacecraft, including the Space Station, for which LeRC is
developing the largest space power system ever designed. (9) George C.
Marshall Space Flight Center (MSFC) The MSFC, located in Huntsville,
Alabama, is responsible for developing spacecraft hardware and systems,
and is perhaps best known for its role in building the Saturn rockets
that sent astronauts to the Moon during the Apollo program. It is NASA's
primary center for space propulsion systems and plays a key role in the
development of payloads to be flown on the shuttle (such as Spacelab).
MSFC also manages two other NASA sites: the Michoud Assembly Facility in
New Orleans where the Shuttle's external tanks are manufactured, and the
Slidell Computer Complex in Slidell, Louisiana, which provides computer
support to Michoud and to NASA's John C. Stennis Space Center. (10) John
C. Stennis Space Center (SSC) This Center, located on Mississippi's Gulf
Coast, is NASA's prime test facility for large liquid propellant rocket
engines and propulsion systems. The main mission of the Center is to
support testing, on a regular basis, of the Space Shuttle's main
propulsion system. SSC is responsible for a variety of research programs
in the environmental sciences and the remotesensing of Earth resources,
weather, and oceans, and is the lead NASA Center for the
commercialization of space remote sensing.
NASA Prediction Bulletins
Reports published by NASA's Goddard Space Flight Center providing
the latest orbit information on satellites. The report gives information
in three parts: 1) two line orbital elements, 2) longitude of the south
to north equatorial crossings, and 3) longitude and heights of the
satellite crossings for other latitudes.
See National Aeronautics and Space Administration.
See Japanese National Space Development Agency.
National Aeronautical and Space Administration (NASA)
U.S. Civilian Space Agency created by Congress. Founded in 1958,
NASA belongs to the executive branch of the Federal Government. NASA's
mission to plan, direct, and conduct aeronautical and space activities
is implemented by NASA Headquarters in Washington, D.C., and by ten
major centers spread throughout the United States. Dozens of smaller
facilities, from tracking antennas to Space Shuttle landing strips to
telescopes are located around the world. The agency administers and
maintains these facilities, builds and operates launch pads, trains
astronauts, designs aircraft and spacecraft, and sends satellites into
Earth orbit and beyond, and processes, analyzes, and distributes the
resulting data and information. See NASA Centers. NASA shares
responsibility for aviation and space activities with other federal
agencies, including the Departments of Commerce, Transportation, and
Defense. Much of the work on major projects such as the Space Shuttle
and the Space Station is done in the private sector by aerospace
companies under government contract. From its inception, NASA has been
directed to pursue the expansion of human knowledge of phenomena in the
atmosphere and space. NASA's programs of basic and applied research
extend from microscopic sub-atomic particles to galactic astronomy. In
addition to enhancing scientific knowledge, thousands of the
technologies developed for aerospace have resulted in commercial
applications. Science offices at NASA Headquarters carry out a wide
range of research activities to fulfill NASA's science goals. Science
offices within NASA are: (1) Office of Mission to Planet Earth (MTPE)
focuses on the "home planet" as a dynamic system of land, ocean,
atmosphere, and life that can be investigated on a global scale from
space using remote-sensing tools. See Mission to Planet Earth. (2)
Office of Life and Microgravity Sciences and Applications explores the
basic physics of how solids, liquids, and gases behave in space; seeks
an understanding of the basic mechanisms that underlie space adaptation
--developing more effective countermeasures to mitigate the physiological
affects of space flight; and studies the role of gravity on life. (3)
Office of Space Science includes the Space Physics and Astrophysics
Division which studies the entire universe of stars and galaxies,
including the sun. The Solar System Exploration division has launched
spacecraft to all the known planets except Pluto in its quest to study
the solar system. (4) National Center for Atmospheric Research (NCAR)
Non-profit organization dedicated to furthering understanding of the
Earth's atmosphere. Located in Boulder, Co., NCAR is operated by the
University Corporation for Atmospheric Research (UCAR) and sponsored by
the National Science Foundation (NSF).
National Oceanic and Atmospheric Administration (NOAA)
NOAA was established in 1970 within the U.S. Department of Commerce
to ensure the safety of the general public from atmospheric phenomena
and to provide the public with an understanding of the Earth's
environment and resources. NOAA includes: the National Ocean Service
which charts the oceans and waters of the U.S. and manages 265,000 acres
of estuarine reserves; the National Marine Fisheries Service which
maintains the world's largest and most complex marine fisheries
management system; the NOAA Corps which operates 18 NOAA research and
survey ships and flies 15 NOAA aircraft; and the Office of Oceanic and
Atmospheric Research which supports experiments, laboratories, and the
National Sea Grant College Program, among other efforts. NOAA has two
main components: the National Weather Service (NWS), and the National
Environmental Satellite, Data, and Information Service (NESDIS). The
National Weather Service provides weather watch and warning services to
the public through 57 Weather Service Forecast Offices (WSFO) and over
100 smaller local Weather Service Offices (WSOs) nationwide. Three
national forecasting centers provide general and specialized guidance to
WSFOs using computer forecast models, satellite data, and conventional
surface and upper air observations from around the world. The centers
NWS River Forecast
Centers (RFCs) provide river stage and flood forecasts. NESDIS provides
support to the Weather Service forecast mission by operating a series of
environmental satellites and disseminating satellite imagery and derived
products to the National Centers and WSFOs. NESDIS operates 44 three
national data and information centers: the National Geophysical Data
Center, the National Climatic Data Center (NCDC), and the National
Oceanographic Data Center (NODC). See SOCC NOAA organizations perform
numerous services in addition to monitoring weather conditions. They
assess crop growth and other agricultural conditions, sense shifting
ocean currents, and measure surface temperatures of oceans and land.
They relay data from surface instruments that sense tide conditions,
Earth tremors, river levels, and precipitation.
- National Meteorological Center, Camp Springs, Maryland;
- National Severe torms Forecast Center, Kansas City, Missouri;
- National Hurricane Center, Coral Gables, Florida.
National Space Science Data Center (NSSDC)
The NSSDC provides on-line and off-line access to a wide variety of
astrophysics, space plasma and solar physics, lunar and planetary, and
Earth science data from NASA space flight missions, in addition to
selected other data, models, and software. Located at Goddard Space
Flight Center (GSFC) in Greenbelt, Maryland, the NSSDC is sponsored by
the Information Systems Office of NASA's Office of Space Sciences. NSSDC
on-line data and services are currently free of charge, offline support
(e.g., replications and mailing of magnetic tapes) are available for the
cost of fulfilling the request. The NSSDC Master Catalog (NMC) provides
an on-line listing of available data sets and the forms that the data
are available in (such as CD-ROM), and provides information about the
spacecraft and experiments (including past, present, and future NASA and
non-NASA) from which these data were obtained. The on-line NASA Master
Directory (NMD) identifies and briefly describes data of potential
interest to the NASA research community, and where possible, provides
electronic links to publicly-accessible data at sites world-wide.
On-line information services are made available through the menu-based
NSSDC Online Data Information Service (NODIS). For more information
contact: CRUSO (Coordinated Request & User Support Office) National
Space Science Data Center c/o World Data Center-A-R&S (only if
corresponding from outside the USA) NASA Goddard Space Flight Center,
Code 633.4 Greenbelt, Maryland 20771 phone: (301) 286-6695, FAX: (301)
286-1771 Internet: REQUEST@NSSDCA.GSFC.NASA.GOV DECnet:
National Weather Service (NWS)
See National Oceanic and Atmospheric Administration.
A unit of distance (U.S.) equal to exactly 1.852 kilometers or about
6076.1 feet. A nautical mile is approximately equal to 1/60 of a degree
or 1 minute of arc of a great circle of the Earth (i.e., 1 minute of arc
of latitude or of longitude at the equator).
See National Center for Atmospheric Research.
National Climatic Data Center, located in Asheville, North Carolina.
See National Oceanic and Atmospheric Administration.
Electromagnetic radiation with wavelengths from just longer than the
visible (about 0.7 micrometers) to about two micrometers. See
A type of analysis using satellite cloud pictures to study the
relationship between cloud forms and storm systems. In classical
mythology, Nephele was a woman Zeus formed from a cloud.
Clouds that resemble recognizable shapes.
National Environmental Satellite Data and Information Service. See
National Oceanic and Atmospheric Administration.
Newton's Law of Universal Gravitation
All bodies attract each other with what is called gravitational
attraction. This applies to the largest stars as well as the smallest
particles of matter. The force of attraction between two small bodies
(or between two spherical bodies of any size) is proportional to the
product of their masses and inversely proportional to the square of the
distance between their centers. In other words, the closer two bodies
are to each other, the greater their mutual attraction. As a result, to
stay in orbit, a satellite needs more speed in a low than a high orbit.
Kepler's three laws of planetary motion, which had been derived
empirically by Johannes Kepler, were obtained with mathematical rigor as
a consequence of Newton's law of universal gravitation in conjunction
with his three laws of motion. See Kepler's three laws of
Newton's Laws of Motion
Newton's three laws of motion are: 1) Every body continues in a
state of uniform motion in a straight line unless acted upon by some
external force. 2) The time rate of change of momentum (mass x velocity)
is proportional to the impressed force. In the usual case where the mass
does not change, this law can be expressed in the familiar form: force =
mass x acceleration or F = ma. 3) To every force or action, there is
always an equal and opposite reaction. Kepler's three laws of planetary
motion, which had been derived empirically by Johannes Kepler, were
obtained with mathematical rigor as a consequence of Newton's law of
universal gravitation in conjunction with his three laws of motion. See
Kepler's three laws of motion.
National Geophysical Data Center, located in Boulder, Colorado. See
National Oceanic and Atmospheric Administration.
Four bits of data.
Nimbus Satellite Program
A NASA program to develop observation systems meeting the research
and development requirements of atmospheric and Earth scientists. The
Nimbus satellites, first launched in 1964, carried a number of
instruments: microwave radiometers, atmospheric sounders, ozone mappers,
the Coastal Zone Color Scanner (CZCS), infrared radiometers, etc.
Nimbus-7, the last in the series, provided significant global data on
sea-ice coverage, atmospheric temperature, atmospheric chemistry (i.e.
ozone distribution), the Earth's radiation budget, and sea-surface
temperature. See Total Ozone Mapping Spectrometer (TOMS).
See National Oceanic and Atmospheric Administration. Operational
designation for the U.S. polar-orbiting meteorological satellites.
Current NOAA spacecraft are variations of the TIROS-N bus.
National Oceanographic Data Center, located in Washington, D.C. See
National Oceanic and Atmospheric Administration.
NASA Research Announcement.
National Research and Education Network.
National Science Foundation.
National Science Foundation NETwork.
See National Space Science Data Center.
A spot in a desert made fertile by water, which normally originates
Occluded Front (Occlusion)
A composite of two fronts formed as a cold front overtakes a warm
front. A cold occlusion results when the coldest air is behind the cold
front. The cold front undercuts the warm front and, at the Earth's
surface, coldest air replaces less-cold air. A warm occlusion occurs
when the coldest air lies ahead of the warm front. Because the cold
front can not lift the colder air mass, it rides piggyback up on the
warm front over the coldest air.
The salt water surrounding the great land masses. The land masses
divide the ocean into several distinct portions, each of which also is
called an ocean. The oceans include the Pacific Ocean, the Atlantic
Ocean, the Indian Ocean, and the Arctic Ocean.
The unit of electrical resistance, equal to the resistance of a
circuit in which an electromotive force of one volt maintains a current
of one ampere. Named for German physicist Georg S. Ohm
The path described by a heavenly body in its periodic revolution.
Earth satellite orbits with inclinations near 0° are called equatorial
orbits because the satellite stays nearly over the equator. Orbits with
inclinations near 90° are called polar orbits because the satellite
crosses over (or nearly over) the north and south poles. See
See period decay.
An imaginary gigantic flat plate containing an Earth satellite's
orbit. The orbital plane passes through the center of the Earth.
A large area of intense stratospheric ozone depletion over the
Antarctic continent that typically occurs annually between late August
and early October, and generally ends in mid- November. This severe
ozone thinning has increased conspicuously since the late seventies and
early eighties. This phenomenon is the result of chemical mechanisms
initiated by man-made chlorofluorocarbons (see CFCs). Continued buildup
of CFCs is expected to lead to additional ozone loss worldwide. The
thinning is focused in the Antarctic because of particular
meteorological conditions there. During Austral spring (September and
October in the Southern Hemisphere) a belt of stratospheric winds
encircles Antarctica essentially isolating the cold stratospheric air
there from the warmer air of the middle latitudes. The frigid air
permits the formation of ice clouds that facilitate chemical
interactions among nitrogen, hydrogen, and chlorine (elevated from CFCs)
atoms, the end product of which is the destruction of ozone.
The layer of ozone that begins approximately 15 km above Earth and
thins to an almost negligible amount at about 50 km, shields the Earth
from harmful ultraviolet radiation from the sun. The highest natural
concentration of ozone (approximately 10 parts per million by volume)
occurs in the stratosphere at approximately 25 km above Earth. The
stratospheric ozone concentration changes throughout the year as
stratospheric circulation changes with the seasons. Natural events such
as volcanoes and solar flares can produce changes in ozone
concentration, but man-made changes are of the greatest concern.
Rapid, transient, polar-ozone depletion. These depletions, which
take place over a 50-kilometer squared area, are caused by weather
patterns in the upper troposphere. The decrease in ozone during a
mini-hole event is caused by transport, with no chemical depletion of
ozone. However, the cold stratospheric temperatures associated with
weather systems can cause clouds to form that can lead to the conversion
of chlorine compound from inert to reactive forms. These chlorine
compounds can then produce longer-term ozone reductions after the
mini-hole has passed.
Ozone-Measuring Satellite Instruments
Satellite-based ozone-measuring instruments can measure ozone by
looking at the amount of ultraviolet absorption reflected from the
Earth's surface and clouds. Some instruments provide data within the
different levels of the atmosphere. The Total Ozone Mapping Spectrometer
(TOMS) maps the total amount of ozone between ground and the top of the
atmosphere. The amount and distribution of ozone molecules in the
stratosphere varies greatly over the globe, changing in response to
natural cycles such as seasons, sun cycles, and winds. Utilizing
satellites has enabled scientists to assess ozone levels simultaneously
over the entire Earth, and has led them to conclude that global ozone
levels are being depleted.
An almost colorless, gaseous form of oxygen with an odor similar to
weak chlorine. A relatively unstable compound of three atoms of oxygen,
ozone constitutes--on the average-- less than one part per million (ppm)
of the gases in the atmosphere (peak ozone concentration in the
stratosphere can get as high as 10 ppm). Yet ozone in the stratosphere
absorbs nearly all of the biologically damaging solar ultraviolet
radiation before it reaches the Earth's surface where it can cause skin
cancer, cataracts, and immune deficiencies, and can harm crops and
aquatic ecosystems. See ozone layer. Ozone is produced naturally in the
middle and upper stratosphere through dissociation of molecular oxygen
by sunlight. In the absence of chemical species produced by human
activity, a number of competing chemical reactions among
naturally-occurring species--primarily atomic oxygen, molecular oxygen,
and oxides of hydrogen and nitrogen --maintains the proper ozone balance.
In the present-day stratosphere, this natural balance has been altered,
particularly by the introduction of man-made chlorofluorocarbons. If the
ozone decreases, the ultraviolet radiation at the Earth's surface will
increase. See greenhouse gas. Tropospheric ozone is a by-product of the
photochemical (light-induced) processes associated with air pollution.
See photochemical smog. Ozone in the troposphere can damage plants and
Climate as it existed in the distant past, particularly before
The study of ancient or prehistoric geography.
Sensitive to all or most of the visible spectrum.
The addition of one or more redundant bits to information to verify
Unit of atmospheric pressure named in honor of Blaise Pascal
(1632–1662), whose experiments greatly increased knowledge of the
atmosphere. A pascal is the force of one newton acting on a surface area
of one square meter. It is the unit of pressure designated by the
International System. 100,000 Pa = 1000 mb = 1 bar. See atmospheric
A system sensing only radiation emitted by the object being viewed
or reflected by the object from a source other than the system. See
The instruments that are accommodated on a spacecraft.
Perigee (aka periapsis or perifocus)
On an elliptical orbit path, the point where a satellite is closest
to the Earth. See Keplerian elements.
The point in the orbit of a planet or comet which is nearest the Sun
(as opposed to the aphelion, which is the point in the orbit farthest
from the Sun).
Period Decay (aka decay)
The tendency of a satellite to lose orbital velocity due to the
influence of atmospheric drag and gravitational forces. A decaying
object eventually impacts the surface of the Earth or burns up in the
atmosphere. This parameter directly affects the satellite's mean
Time required for a satellite to make one complete orbit.
Minor corrections to the Keplerian model of a satellite orbit as an
ellipse of constant shape and orientation. Since satellite orbits are
affected by Earth's gravity and drag caused by the Earth's atmosphere
(causing satellites to spiral downward), minor adjustments must be made
to the orbit.
A symbol for the degree of acidity or alkalinity of a solution.
Expressed as a negative logarithm of the hydrogen ion concentration in a
solution, pH = -log10[H+]. If the hydrogen ion concentration of a
solution increases, the pH will decrease, and vice versa. The value for
pure distilled water is regarded as neutral, pH values from 0 to 7
indicate acidity, and from 7 to 14 indicate alkalinity.
In direct readout, the time between the end of a satellite image
start tone and the start of the actual frame data. The phase interval
represents white level video, interrupted by a black level pulse marking
the start of each line and is used to set up phasing prior to image
Subdiscipline of agriculture, a science that treats relations
between climate and periodic biological phenomena that are related to or
caused by climatic conditions, such as the budding of trees and the
migration of birds.
A type of smog that forms in large cities when chemical reactions
take place in the presence of sunlight, its principal component is
ozone. Ozone and other oxidants are not emitted into the air directly
but form from reactions involving nitrogen oxides and hydrocarbons.
Because of its smog-making ability, ozone in the lower atmosphere
(troposphere) is often referred to as "bad" ozone .
A quantum (smallest unit in which waves may be emitted or absorbed)
Photosynthetically Active Radiation
Electromagnetic radiation in the part of the spectrum used by plants
Physical Climate System
The system of processes that regulate climate, including atmospheric
and ocean circulation, evaporation, and precipitation.
Smallest part (addressable element) of an electronically- coded
image, such as a computer display. Pixel is a contraction of "picture
The fraction of incident solar radiation that is reflected by a
planet and returned to space. The planetary albedo of the
Earth-atmosphere system is approximately 30 percent, most of which is
due to backscatter from clouds in the atmosphere.
A fourth state of matter (in addition to solid, liquid, and gas)
that exists in space. In this state, atoms are positively charged and
share space with free negatively-charged electrons. Plasma can conduct
electricity and interact strongly with electric and magnetic fields. The
solar wind is actually hot plasma blowing from the sun. See
Concept that the Earth's crust is composed of rigid plates that move
over a less rigid interior.
A satellite that can carry instruments. See bus. The same term is
applied to automatic weather data transmitters installed on buoys,
balloons, ships, and planes, and mounted in remote areas.
POES (Polar-orbiting Operational Environmental Satellite)
Operated by the National Oceanic and Atmospheric Administration,
they are designated "NOAA satellites." Included in this group are the
current series of TIROS-N satellites, the third-generation
polar-orbiting environmental spacecraft operated by NOAA.
An orbit with an orbital inclination of near 90° where the satellite
ground track will cross both polar regions once during each orbit. The
term is used to describe the near-polar orbits of spacecraft such as the
USA's NOAA/TIROS and Landsat satellites.
The comparatively slow torquing of the orbital planes of all
satellites with respect to the Earth's axis, due to the bulge of the
Earth at the equator which distorts the Earth's gravitational field.
Precession is manifest by the slow rotation of the line of nodes of the
orbit (westward for inclinations less than 90° and eastward for
inclinations greater than 90°).
Moisture that falls from clouds. Although clouds appear to float in
the sky, they are always falling, their water droplets slowly being
pulled down by gravity. Because their water droplets are so small and
light, it can take 21 days to fall 1,000 feet and wind currents can
easily interrupt their descent. Liquid water falls as rain or drizzle.
All raindrops form around particles of salt or dust. (Some of this dust
comes from tiny meteorites and even the tails of comets.) Water or ice
droplets stick to these particles, then the drops attract more water and
continue getting bigger until they are large enough to fall out of the
cloud. Drizzle drops are smaller than raindrops. In many clouds,
raindrops actually begin as tiny ice crystals that form when part or all
of a cloud is below freezing. As the ice crystals fall inside the cloud,
they may collide with water droplets that freeze onto them. The ice
crystals continue to grow larger, until large enough to fall from the
cloud. They pass through warm air, melt, and fall as raindrops. When ice
crystals move within a very cold cloud (10 °F and -40 °F) and enough
water droplets freeze onto the ice crystals, snow will fall from the
cloud. If the surface temperature is colder than 32 °F, the flakes will
land as snow. Precipitation Weights: one raindrop .000008 lbs; one
snowflake .0000003 lbs; one cumulus cloud 10,000,000 lbs; one
thunderstorm 10,000,000,000 lbs; one hurricane 10,000,000,000,000
Pressure is the force exerted on an object by something else. For example, when pressing
a tack into the wall, you are exerting pressure on the wall through the tack.
Winds in the middle latitudes (approximately 30° to 60°) that
generally blow from west to east. The subtropical high pressure regions
at the horse latitudes (30°) forces surface air poleward, and the
rotation of the Earth causes these winds to bear to the right (east) in
the Northern Hemisphere and to the left (east) in the Southern
Hemisphere (see Coriolis force). This is, to some extent, an idealized
picture of the atmospheric circulation. The actual circulation on
individual days includes modifications and variations due to the
migratory cyclones and anticyclones of middle latitudes, causing rapid
and often violent weather changes, as warm semi-tropical air from the
horse latitudes meets cold polar air from the high latitudes. See
An imaginary line running from north to south through Greenwich,
England, used as the reference point for longitude.
A fiber card on which integrated circuits and other electronic
components can be mounted. Connections between the components are etched
in the correct circuit patterns.
An organized, systematic investigation of a particular process
designed to identify all of the state variables involved and to
establish the relationships among them. Process studies yield numerical
algorithms that connect the state variables and determine their rates of
change; such algorithms are essential ingredients of Earth system
An association of phenomena governed by physical, chemical, or
biological laws. An example of a process is the vertical mixing of ocean
waters in the so-called surface-mixed layer; the state variables for
this process include temperature, salinity in the water on a vertical
scale of tens of meters, and heat flow and wind stress at the sea
surface. Other examples include the volcanic deposition of dust and
gases into the atmosphere, eddy formation in the atmosphere and oceans,
and soil development.
Orbits of the Earth in the same direction as the rotation of the
An instrument designed to measure dew point and relative humidity,
consisting of two thermometers (one dry bulb and one wet bulb). The dew
point and humidity levels are determined by drying the wet bulb (either
by fanning or whirling the instrument) and comparing the difference
between the wet and dry bulbs with preexisting calculations. See
Research and Development.
1. In optics, the point or object from which light proceeds. 2. In
geometry, a straight line proceeding from a given point, or fixed pole,
about which it is conceived to revolve. 3. In astronomy, the point in
the heavens from which a shower of meteors seems to proceed.
A measure of all the inputs and outputs of radiative energy relative
to a system, such as Earth. See Earth Radiation Budget
Energy transfer in the form of electromagnetic waves or particles
that release energy when absorbed by an object.
Cooling process of the Earth's surface and adjacent air, which
occurs when infrared (heat) energy radiates from the surface of the
Earth upward through the atmosphere into space. Air near the surface
transfers its thermal energy to the nearby ground through conduction, so
that radiative cooling lowers the temperature of both the surface and
the lowest part of the atmosphere.
Theory dealing with the propagation of electromagnetic radiation
through a medium.
Radio Frequency (RF)
A frequency that is useful for radio transmission, usually between
10 kHz and 300,000 MHz.
The complete range of frequencies or wave lengths of electromagnetic
waves, specifically those used in radio and television.
An electrical impulse sent through the atmosphere at radio
Giving off or capable of giving off radiant energy in the form of
particles or rays, as in alpha, beta, and gamma rays.
An instrument that quantitatively measures electromagnetic
radiation. Weather satellites carry radiometers to measure radiation
from snow, ice, clouds, bodies of water, the Earth's surface, and the
A balloon-borne instrument that measures meteorological parameters
from the Earth's surface up to 20 miles in the atmosphere. The
radiosonde measures temperature, pressure, and humidity, and transmits
or "radios" these data back to Earth. Upper air winds also are
determined through tracking of the balloon ascent. Radiosonde
observations generally are taken twice a day (0000 and 1200 UTC) around
the globe. NOAA's National Weather Service (NWS) operates a network of
about 90 radiosonde observing sites in the U.S. and its territories.
When the balloons burst, radiosondes return to Earth on a parachute.
Approximately 25 percent are recovered and returned to NWS for
reconditioning and reuse.
An evergreen woodland of the tropics distinguished by a continuous
leaf canopy and an average rainfall of about 100 inches per year. Rain
forests play an important role in the global environment. The Earth
sustains life because of critical balances and interactions among many
factors. Were there not processes at work that limit the effects of
other essential processes, Earth would become uninhabitable. Destruction
of tropical rain forests reduces the amount of leaf area in the tropics,
and consequently the amount of carbon dioxide absorbed, causing
increases in levels of carbon dioxide and other atmospheric gases. It is
estimated that cutting and burning of tropical forests contributes about
20 percent of the carbon dioxide added to the atmosphere each year. The
World Resources Institute and the International Institute for
Environment and Development have reported that the world's tropical
forests are being destroyed at the rate of fifty-four acres per minute,
or twenty-eight million acres lost annually. Rain forest destruction
also means the loss of a wide spectrum of biological life, erosion of
soil, and possible desertification.
Calibrated container that measures the amount of rainfall during a
specific period of time.
Random Access Memory. Computers use two types of memory, RAM and
ROM. RAM is the computer's working area, the primary location where the
microprocessor stores the information it needs. The designation "random
access" stems from the microprocessor's ability to access information in
memory randomly by knowing its location, or address, rather than hunting
through memory sequentially from beginning to end. Because information
in RAM is stored electronically, accessing data stored in RAM is much
faster than getting that data from a mechanical storage device such as a
disk drive. But because it is stored electronically, all information in
RAM is temporary (which is why you must store it on a more permanent
storage capability, such as a disk). Compare with ROM.
As it happens.
The ability of a receiver to detect weak signals through the noise
level of the receiving system, which includes the antenna and internal
thermal noise of the receiver. See signal-tonoise ratio.
The return of light or sound waves from a surface. If a reflecting
surface is plane, the angle of reflection of a light ray is the same as
the angle of incidence.
Flattening the Earth into a standard map projection. When the
spherical Earth is photographed by satellites, areas lying near the
outer edge of the picture are distorted. Remapping rectifies the
The technology of acquiring data and information about an object or
phenomena by a device that is not in physical contact with it. In other
words, remote sensing refers to gathering information about the Earth
and its environment from a distance, a critical capability of the Earth
Observing System. For example, spacecraft in low-Earth orbit pass
through the outer thermosphere, enabling direct sampling of chemical
species there. These samples have been used extensively to develop an
understanding of thermospheric properties. Explorer-17, launched in
1963, was the first satellite to return quantitative measurements of
gaseous stratification in the thermosphere. However, the mesosphere and
lower layers cannot be robed directly in this way--global observations
from space require remote sensing from a spacecraft at an altitude well
above the mesopause. The formidable technological challenges of
atmospheric remote sensing, many of which are now being overcome, have
delayed detailed study of the stratosphere and mesosphere by comparison
with thermospheric research advances. Some remote-sensing systems
encountered in everyday life include the human eye and brain, and
photographic and video cameras.
The smallest unit of area in an image of discrete elements. The area
represented by a pixel.
A measure of the ability to separate observable of an image. The
smaller the area represented by a pixel, the more accurate and detailed
the image. APT has a resolution of 4 km, i.e., each pixel represents a
square, 4 km on each side. HRPT has a resolution of 1.1 km at nadir (4
km at edge of scan), and WEFAX of 8 km. See resolution cell.
An east-to-west orbit of Earth (Earth spins west to east). See
Process of the Earth circling the sun in its orbit. Revolution
determines the seasons, and the length of the year. In addition,
differences in seasons occur because of Earth's inclination (tilt on its
axis) of about 23.5 degrees as it revolves around the sun. Compare with
See radio frequency.
Right Ascension of ascending node (aka ½, RAAN or RA of Node)
One of six Keplerian elements, it indicates the rotation of the
orbit plane from some reference point. Two numbers orient an orbital
plane in space; inclination is the first, this is the second. After
specifying inclination, an infinite number of orbital planes are
possible. The intersection of the equatorial plane and the orbital plane
(see diagram, line of nodes) must be specified by a location on the
equator that fully defines the orbital plane. The line of nodes occurs
in two places. However, by convention, only the ascending node (where
the satellite crosses the equator going from south to north) is
specified. The descending node (where the satellite crosses the equator
going from north to south) is not. Because the Earth spins, conventional
latitude and longitude points are not used to separate where the lines
of node occur. Instead, an astronomical coordinate system is used, known
as the right-ascension/declination coordinate system, which does not
spin with the Earth. Right ascension of ascending node is an angle,
measured at the center of the Earth, from the vernal equinox to the
ascending node. For example, draw a line from the center of the Earth to
the point where the satellite crossed the equator (going from south to
north). If this line points directly at the vernal equinox, then RAAN =
Read Only Memory. Refers to the computer memory chips that contain
information the computer uses (along with system files) throughout the
system, including the information it needs to get itself started.
Information in ROM is permanent; it doesn’t vanish when the power is
turned off. Compare with RAM.
Process of the Earth turning on its axis. Rotationm determines day
and night, and the length of the day. Compare with revolution.
One of the segments or bands into which the radio frequency spectrum
above 1000 MHz is divided, designated by letters. Signals from GOES and
other geostationary spacecraft transmitting on or near 1691 MHz are
transmitting on S-Band.
The process of obtaining a sequence of discrete digital values from
a continuous sequence of analog data.
See synthetic aperture radar.
Search and Rescue Tracking System carried on NOAA polar-orbiting
satellites that receives emergency signals from persons in distress. The
satellites transmit these signals to ground receiving stations in the
U.S. and overseas. Signals are forwarded to the nearest rescue
coordination center which computes the location from which the emergency
signals came and provides the coordinates of the emergency site to a
rescue team. See Search and Rescue.
Satellite Dish (aka Parabolic Reflector)
Bowl shaped antennas that collect and focus the signals that a
satellite beams down to Earth. The dish reflects the incoming radio
frequency energy to a focal point where it can be picked up by a
feedhorn antenna to transfer the RF energy to a transmission line. The
bigger the dish, the greater will be the intercepted RF energy and
hence, the gain. For example, a satellite dish is used to receive GOES
Satellite Operations Control Center (SOCC)
NOAA National Environmental Satellite Data and Information Service
(NESDIS) Satellite Operations Control Center located in Suitland,
Maryland. A principal operating feature of the NOAA system is the
centralized remote control of the satellite through command and data
acquisition (CDA) stations. The CDA stations transmit command programs
to the satellite, and acquire and record meteorological and engineering
data from the satellite. Data is transmitted from CDA to Suitland NESDIS
Data Processing Services Subsystem (DPSS). DPSS is responsible for data
processing and timely generation of meteorological products and
distribution of these products.
Satellite Orbital Elements
See Keplerian elements.
A procedure by which satellites are used to locate precise objects
or particular points on Earth.
The time from one perigee (the point of an elliptical orbit path
where a satellite is closest to Earth) to the next.
A free-flying object that orbits the Earth, another planet, or the
A system that optically scans its detector(s) across a scene and
records or stores the data in a two-dimensional format to form an
An imaging system consisting of lenses, moving mirrors, and
solid-state image sensors used to obtain observations of the Earth and
its atmosphere. Scanning radiometers, which are the sole imaging systems
on all current operational weather satellites, have far better long-term
performance than the vidicon TV camera tubes used with earlier
The process by which electromagnetic radiation interacts with and is
redirected by the molecules of the atmosphere, ocean, or land surface.
The term is frequently applied to the interaction of the atmosphere on
sunlight, which causes the sky to appear blue (since light near the blue
end of the spectrum is scattered much more than light near the red
Cloud pattern so named because some observers maintain they can see
the head of an eagle facing west in these cloud patterns. The pattern is
similar to a comma, only the pattern is disorganized and not solid.
Weather associated with screaming eagles consists of rain showers and
gusty surface winds up to about 25 knots. The eagles can intensify and
enlarge when moving into areas east of troughs; in that case, intense
thunderstorms can develop. Screaming eagles are common in the Pacific
Ocean between Hawaii and the equator, and are uncommon in the western
Local coastal wind that blows from the ocean to land. Sea breezes
usually occur during the day, because the heating differences of land
and sea cause pressure differences. Cooler, heavier air from the sea
moves in to replace rising warm air on the coastline. See land
The datum against which land elevation and sea depth are measured.
Mean sea level is the average of high and low tides.
Search and Rescue
International satellite-aided search and rescue project. COSPAS/
SARSAT satellites monitor the entire surface of the Earth, and transmit
distress signals to special ground receiving stations. The receiving
stations compute the location of the signal, and notify the nearest
rescue coordination center. Satellite search has cut recovery time from
days to hours, and has aided downed airplanes, capsized boats, and
persons in other emergencies.
See Space Environment Monitor, TIROS.
Semi-Major xis (aka a)
One of the six Keplerian elements, it indicates the size of an
orbit. The semi-major axis is onehalf of the longest diameter of an
orbital ellipse, e.g., one-half of the distance between the apogee and
perigee of an Earth orbit. (The semi-major axis is related to the
orbital period and mean motion by Kepler's third law. See Kepler's three
laws of motion.) See Keplerian elements for diagram.
The relationship between input and output for a given
Device that produces an output (usually electrical) in response to
stimulus such as incident radiation. Sensors aboard satellites obtain
information about features and objects on Earth by detecting radiation
reflected or emitted in different bands of the electromagnetic spectrum.
Analyzing the transmitted data provides valuable scientific information
about Earth. Weather satellites commonly carry radiometers, which
measure radiation from snow, ice, clouds, and bodies of water.
Spaceborne radars are used for Earth observations, bouncing radar waves
off land and ocean surfaces to study sea-surface conditions, ice
thickness, and land surface features. A wind scatterometer is a special
type of radar designed to measure ocean surface winds indirectly by
bouncing signals off the water and measuring them from various angles.
Infrared (IR) detectors measure heat generated by Earth features in the
IR band of the spectrum. Photographic reconnaissance sensors in their
simplest form are large telescope-camera systems used to view objects on
Earth's surface. The bigger the lens, the smaller the object that can be
detected. Camera-telescope systems now incorporate all sorts of
sophisticated electronics to produce better images, but even these
systems need cloudless skies, excellent lighting, and good color
contrast between objects and their surroundings to detect objects the
size of a basketball. Some of the satellites produce film images that
must be returned to Earth, but a more convenient method is to record the
image as a series of digital code numbers, then reconstruct the image
from the electronic code using a computer at a ground station.
Signal-to-Noise Ratio (SNR)
In decibels (dB), the difference between the amplitude of a desired
radio frequency (RF) signal and the internal or external RF noise level
in a system. A negative SNR indicates the signal is below the system
noise level and unusable. The greater the positive SNR, the less effect
noise will have on the final quality. SNR of at least +12dB is necessary
to produce imagery with minimal noise effects.
Electrical impulses, sound or picture elements, etc., received or
transmitted. Signals can exist in many different forms and media
(electrical/wires, acoustic/air, light/transparent fibers, etc.), but
all signals will vary with time. The signal shape plotted as a function
of time is called the waveshape or waveform. Some waveforms are
repetitive or periodic, that is, a small segment of the waveform repeats
itself regularly. Other waveforms, such as noise, are nonperiodic or
aperiodic. All waveforms can be distilled into the combination of pure
waves called sine waves. The frequency of a sine wave is the rate at
which the fundamental shape repeats itself. Most signals occupy a
limited range of frequencies between a lower limit and an upper limit.
This range or band of frequencies occupied by a signal is called the
bandwidth of the signal. Communication medium or channel can pass only a
specific range or band of frequencies, which is called the bandwidth of
the channel. The bandwidths of the channel and the signal determine the
number and types of signals that can be transmitted by a particular
communication channel. Signals often are too small and need to be made
larger through a process called amplification. The amount of
amplification is measured in decibels. However, amplification is an
imperfect process, and inadvertently introduces various distortions,
noise, and bandwidth limitations. Often, multiple signals must share the
same medium. One way the sharing can be accomplished is to place each
signal in its own band of frequencies within the total band of the
medium. The combining of a number of signals to share a medium by
dividing it into different frequency bands for each signal is called
frequency-division multiplexing. Frequency-division multiplexing
requires the ability to move signals around so that each multiplexed
signal occupies its own band. This is accomplished through a process
called modulation, in which a high-frequency sine wave carries the
signal into the specified band. Either the amplitude or the frequency of
the carrier wave can be varied, or modulated, in synchrony with the
information-bearing signal. These methods are called amplitude
modulation (AM) and frequency modulation (FM). FM is the more complex
process of the two, and the bandwidth of the FM carrier can be many
times that of the modulating signal. The process of demodulating a
frequency-modulated signal eliminates much of the deleterious effects of
additional noise. (The trade-off between bandwidth and noise immunity
characterizes most communication systems. Both are analog modulation
schemes for multiplexing signals in the frequency spectrum.) Digitizing
a signal requires a number of steps and results in a binary digital
signal that takes on one of two discrete values. This process results in
considerable immunity to additive noise, but requires a considerable
increase in bandwidth.
Simple Wind Chill Equation
Tw = TA - 1.5 x VA
Tw = wind chill
TA = air temperature
VA = wind speed
For example, if the temperature is 20° and the wind 20 mph:
Tw = 20 - 1.5 x 20
Tw = 20 - 30
Tw = -10° FSine Wave
A smoothly varying wave that repeats itself; its frequency is the
rate at which the fundamental shape repeats itself. Any waveform can be
distilled into a combination of pure sine waves of varying frequencies
The process of providing storage for a substance. For example,
plants--through photosynthesis --transform carbon dioxide in the air into
organic matter, which either stays in the plants or is stored in the
soils. The plants are a sink for carbon dioxide.
The first U.S. space station, launched unmanned in May 1973 and soon
after occupied in succession by three crews through November
See signal-to-noise ratio.
See Satellite Operations Control Center.
The programs, data, or routines used by a computer, distinguished
from the physical components (e.g., hardware).
Solar Backscatter Ultraviolet Radiometer (SBUV)
Instrument that measures the vertical distribution and total ozone
in the Earth's atmosphere. Data is used for the continuous monitoring of
ozone distribution to estimate long-term trends. SBUV instruments are
flown on NOAA polar-orbiting satellites.
Aka total solar irradiance. The constant expressing the amount of
solar radiation reaching the Earth from the sun, approximately 1370
watts per square meter. It is not, in fact, truly constant and
variations are detectable.
Eleven-year cycle of sunspots and solar flares that affects other
solar indexes such as the solar output of ultraviolet radiation and the
solar wind. The Earth's magnetic field, temperature, and ozone levels
are affected by this cycle.
Energy received from the sun is solar radiation. The energy comes in
many forms, such as visible light (that which we can see with our eyes).
Other forms of radiation include radio waves, heat (infrared),
ultraviolet waves, and x-rays. These forms are categorized within the
A continuous plasma stream expanding into interplanetary space from
the sun's corona. The solar wind is present continuously in
interplanetary space. After escaping from the gravitational field of the
sun, this gas flows outward at a typical speed of 400 km per second to
distances known to be beyond the orbit of Pluto. Besides affecting
Earth's weather, solar activity gives rise to a dramatic visual
phenomena in our atmosphere. The streams of charged particles from the
Sun interact the Earth's magnetic field like a generator to create
current systems with electric potentials of as much as 100,000 volts.
Charged electrons are energized by this process, sent along the magnetic
field lines towards Earth's upper atmosphere, excite the gases present
in the upper atmosphere and cause them to emit light which we call the
auroras. The auroras are the northern (aurora borealis) and southern
(aurora Australis) lights.
A special kind of radiometer that measures changes in atmospheric
temperature with height, as well as the content of various chemical
species in the atmosphere at various levels. The High Resolution
Infrared Radiation Sounder (HIRS), found on NOAA polar-orbiting
satellites, is a passive instrument. See passive system.
Space Environment Monitor (SEM)
Instrument that measures the condition of the Earth's magnetic field
and the solar activity and radiation around the spacecraft, and
transmits these data to a central processing facility. NOAA
polar-orbiting and geostationary satellites both carry SEMs. See
Scientific study of magnetic and electric phenomena that occur in
outer space, in the upper atmosphere of the planets, and on the
NASA's manned, recoverable spacecraft designed to be used as a
launch vehicle for Earth-orbiting experiments and as a short-term
A manned laboratory module built by the European Space Agency (ESA)
that accommodates dozens of experiments on each flight, mainly in the
categories of materials science and life science.
NASA electronic database for educators, with information stored on a
computer at the Marshall Space Flight Center. Via computer, educators
communicate with NASA education specialists and access the following
menus: current NASA news, aeronautics research, U.S. Space Program
historical information, aerospace research in the 1980s and beyond,
overviews of NASA and its Centers, NASA educational services, classroom
materials, and space program spin-offs. The computer access number is
205-895-0028, the data word format is 8 data bits, no parity, and 1 stop
bit-- 300, 1200, or 2400 baud modem required. Callers with Internet
access may reach NASA Spacelink at: spacelink.msfc.nasa.gov.
A finite segment of wavelengths in the electromagnetic
1. The series of colored bands diffracted and arranged in the order
of their respective wave lengths by the passage of white light through a
prism or other diffracting medium and shading continuously from red
(produced by the longest visible wave) to violet (produced by the
shortest visible wave). 2. Any of various arrangements of colored bands
or lines, together with invisible components at both ends of the
spectrum, similarly formed by light from incandescent gases or other
sources of radiant energy, which can be studied by a spectrograph. 3. In
radio, the range of wave lengths of radio waves, from 3 centimeters to
30,000 meters, or of frequencies of radio waves, from 10 to 10,000,000
kilocycles. Also radio spectrum. 4. The entire range of radiant
energies. See electromagnetic spectrum.
Company that markets data gathered by the SPOT satellite
Systeme Pour l’Observation de la Terre. French, polar-orbiting Earth
observation satellite(s) with ground resolution of 10 meters. SPOT
images are available commercially and are intended for such purposes as
environmental research and monitoring, ecology management, and for use
by the media, environmentalists, legislators, etc.
Five seconds of 300 Hz black to white square wave modulation of the
WEFAX subcarrier signaling the start of a frame transmission (the
beginning of a direct readout image).
Five seconds of 450 Hz black to white square wave modulation of the
WEFAX subcarrier, signaling the stop of a frame transmission (end of a
direct readout image).
Region of the atmosphere between the troposphere and mesosphere,
having a lower boundary of approximately 8 km at the poles to 15 km at
the equator and an upper boundary of approximately 50 km. Depending upon
latitude and season, the temperature in the lower stratosphere can
increase, be isothermal, or even decrease with altitude, but the
temperature in the upper stratosphere generally increases with height
due to absorption of solar radiation by ozone.
The 2400 Hz audio tone transmitted by APT and WEFAX spacecraft.
Amplitude modulation of this tone is used to convey video
Point where a straight line drawn from a satellite to the center of
the Earth intersects the Earth's surface.
See ground track.
- A subunit of either the physical climate system (e.g., ocean
dynamics) or the biogeochemical cycles (e.g., terrestrial ecosystems).
- A subunit of a spacecraft, e.g., the telemetry subsystem, the power
subsystem, the sensor subsystem, etc.
Describes the orbit of a satellite that provides consistent lighting
of the Earth-scan view. The satellite passes the equator and each
latitude at the same time each day. For example, a satellite's
sun-synchronous orbit might cross the equator twelve times a day, each
time at 3:00 p.m. local time. The orbital plane of a sun-synchronous
orbit must also precess (rotate) approximately one degree each day,
eastward, to keep pace with the Earth's revolution around the
The closest star to Earth (149,599,000 km away on average). The sun
dwarfs the other bodies in the solar system, representing approximately
99.86 percent of all the mass in the solar system. One hundred and nine
Earths would be required to fit across the Sun's disk, its interior
could hold over 1.3 million Earths. The source of the Sun's energy is
the nuclear reactions that occur in its core. There, at temperatures of
15 million degrees Celsius (27 million degrees Fahrenheit) hydrogen atom
nuclei, called protons, are fused and become helium atom nuclei. The
energy produced through fusion at the core moves outward, first in the
form of electromagnetic radiation called photons. Next, energy moves
upward in photon heated solar gas--this type of energy transport is
called convection. Convective motions within the solar interior generate
magnetic fields that emerge at the surface as sunspots and loops of hot
gas called prominences. Most solar energy finally escapes from a thin
layer of the Sun's atmosphere called the photosphere--the part of the Sun
observable to the naked eye. The sun appears to have been active for 4.6
billion years and has enough fuel for another 5 billion years or so. At
the end of its life, the Sun will start to fuse helium into heavier
elements and begin to swell up, ultimately growing so large that it will
swallow Earth. After a billion years as a "red giant," it will suddenly
collapse into a "white dwarf." It may take a trillion years to cool off
Refers to observational emphasis upon frequent global coverage,
usually with restricted spatial and spectral resolution, aimed at
developing a consistent, long-term data product for later
The area observed by a satellite as it orbits the Earth.
Chart showing mete-orological conditions over a region at a given
time; weather map.
The ability to see large areas at the same time.
Synthetic Aperture Radar (SAR)
A high-resolution ground-mapping technique that effectively
synthesizes a large receiving antenna by processing the phase of the
reflected radar return. The along-track resolution is obtained by timing
the radar return (time-gating) as for ordinary radar. The crosstrack
(azimuthal) resolution is obtained by processing the Doppler phase of
the radar return. The cross-track "dimension" of the antenna is a
function of the length of time over which the Doppler phase is
collected. See Doppler effect.
See Tracking and Data Relay Satellite System.
- Telecommunications transmission to a distance of measured
magnitude by radio or telephony with suitably coded modulation, e.g.,
amplitude, frequency, phase, pulse.
- Transmission of data collected at
a remote location over communications channels to a central station.
- Surveying measurement of linear distances by use of tellurometer--a
device that uses microwaves to measure distance.
Used to transmit sounds between widely removed points with or
without connecting wires.
Television and Infrared Observation Satellite (TIROS)
A series of NASA and NOAA satellites launched to monitor Earth's
weather from outer space. The era of the meteorological satellites began
with the launch of TIROS-1 on April 1, 1960. For the first time, it was
possible to monitor weather conditions over most of the world regularly
from space. A series of these satellites were launched throughout the
1960s, those funded by NASA for research and development were called
TIROS, and those funded by the Environmental Science Services
Administration (ESSA, the predecessor of NOAA) for the operational
system were called ESSA. A second generation of ITOS/NOAA* environmental
satellites was initiated by the launch of ITOS-1 in 1970, followed by a
number of NOAA satellites. The third generation of TIROSN/NOAA
environmental satellites was initiated by the launch of TIROS-N in 1978.
* Pairs of acronyms such as ITOS/NOAA arise because NASA funds and names
its prototype satellites and then the operating agency funds and names
the rest of the series.
Measure of the energy in a substance. The more heat energy in the
substance, the higher the temperature. The Earth receives only one
twobillionth of the energy the sun produces. Much of the energy that
hits the Earth is reflected back into space. Most of the energy that
isn’t reflected is absorbed by the Earth's surface. As the surface
warms, it also warms the air above it.
A trillion (1012) bits.
Thematic Mapper (TM)
A Landsat multispectral scanner designed to acquire data to
categorize the Earth's surface. Particular emphasis was placed on
agricultural applications and identification of land use. The scanner
continuously scans the surface of the Earth, simultaneously acquiring
data in seven spectral channels. Overlaying two or more bands produces a
false color image. The ground resolution of the six visible and
shortwave bands of the Thematic Mapper is 30 meters, and the resolution
of the thermal infrared band is 120 meters. Thematic mappers have been
flown on Landsats-4 and -5.
Electromagnetic radiation with wavelengths between about 3 and 25
The sound that results from lightning. Lightning bolts (static
electricity) produce intense heat. This burst of heat makes the air
around the bolt expand explosively, producing the sound we hear as
thunder. Since light travels faster than sound, we see the lightning
before we hear the thunder.
Local storm resulting from warm humid air rising in an unstable
environment. Air may start moving upward because of unequal surface
heating, the lifting of warm air along a frontal zone, or diverging
upper-level winds (these diverging winds draw air up beneath them). The
scattered thunderstorms that develop in the summer are called air-mass
thunderstorms because they form in warm, maritime tropical air masses
away from other weather fronts. More violent severe thunderstorms form
in areas with a strong vertical wind shear that forces the updraft into
the mature stage, the most intense stage of the thunderstorm. Severe
thunderstorms can produce large hail, forceful winds, flash floods, and
NOAA satellites that continuously orbit the Earth from North to
South Pole (hence, polar orbiting) at an altitude of approximately 470
nautical miles (870.44 km or 540.86 statute miles). These environmental
satellites collect visible and infrared imagery and provide
atmospheric-sounding data and meteorological data relay and collection.
A primary mission of TIROS-NOAA is to monitor the 70 percent of the
globe covered by water–where weather data is sparse and provide
continuous data to the National Weather Service for use in numerical
forecast modeling. Each TIROSN/NOAA carries six primary systems:
- The Advanced Very High Resolution Scanning Radiometer (AVHRR)
over both ocean and land, using the visible and infrared parts of the
spectrum. It stores measurements on tape, and later plays them back to
NOAA's command and data acquisition stations. The satellites also
broadcast in real time, and the broadcasts can be received around the
world by anyone equipped with a direct readout receiving station.
- The TIROS Operational Vertical Sounder (TOVS) is a 3-part TIROS system to
- Temperature profile of the Earth's atmosphere from the
surface to 10 millibars;
- Water content of the Earth's atmosphere;
- Total ozone content of the Earth's atmosphere
- The ARGOS Data
Collection and Platform Location System (DCS) collects data from sensors
placed on fixed and moving platforms, including ships, buoys, and
weather balloons, and transmits data to a ground station antenna.
Because ARGOS also determines the precise location of these moving
sensors, it can serve wildlife managers by monitoring and tracking the
transmitters placed on birds and animals.
- The Space Environment
Monitor (SEM) measures energetic particles emitted by the sun over
essentially the full range of energies and magnetic field variations in
the Earth's near-space environment. Readings made by these instruments
are invaluable in measuring the sun's radiation activity.
- Search and
Rescue Tracking (COSPAS/SARSAT) equipment receives emergency signals
from persons in distress. The satellites transmit the signals to ground
receiving stations. The signals then are forwarded to rescue
coordination centers. The rescue centers compute the location of the
signals and provide the coordinates of the emergency site (usually
within a few miles).
- Earth Radiation Budget Experiment (ERBE) is a
radiometer, flown on NOAA 9 and 10, designed to measure all radiation
striking and leaving the arth. This enables scientists to measure the
loss or gain of terrestrial energy to space. Shifts in this energy
"budget" affect the Earth's average temperatures. Even slight changes
can affect climatic patterns.
See Television and Infrared Observation Satellite.
See thematic mapper.
Thermal Noise Level.
See Tropical Ocean Global Atmosphere Program.
See Total Ozone Mapping Spectrometer.
Ocean Topography Experiment, United States (NASA)/France (CNES).
Launched in 1992, the mission carries a radar sensor--called an
altimeter--to measure the ocean's surface topography with unprecedented
precision. TOPEX/POSEIDON is a core element of the international World
Ocean Circulation Experiment (WOCE) and the Tropical Ocean Global
Atmosphere (TOGA) seagoing measurements program. Mission objectives are
- Study ocean circulation and its interaction with the atmosphere
to understand climate change better;
- Improve our knowledge of heat transport in the ocean;
- Model global ocean tides;
- Study the marine gravity field;
- Calculate sea-level variations on both global and local scales.
A twisting, spinning funnel of low pressure air. The most
unpredictable weather event, tornadoes are created during powerful
thunderstorms. As a column of warm air rises, air rushes in at ground
level and begins to spin. If the storm gathers energy, a twisting,
spinning funnel develops. Because of the funnel's cloud and rain
composition and the dust, soil, and debris it draws up, the funnel
appears blackish in color. The most energetic storms result in the
funnel touching the ground. In these tornadoes, the roaring winds in the
funnel can reach 300 mph, the strongest winds on Earth. Funnels usually
travel at 20 to 40 mph, moving toward the northeast. When tornadoes form
over lakes or oceans they suck water into the funnel cloud and are
Total Ozone Mapping Spectrometer (TOMS)
Flown on NASA's Nimbus-7 satellite, its primary goal is to continue
the high-resolution global mapping of total ozone on a daily basis. The
Nimbus-7 launch in 1978 enabled TOMS to begin delivering data in 1979
and continue providing information until 1993. TOMS has mapped the total
amount of ozone between the ground and the top of the atmosphere,
provided the first maps of the ozone hole, and continues to monitor this
phenomenon. Because of its longevity, TOMS also has obtained information
on the more subtle trends in ozone outside the ozone hole region. This
results from development of a powerful new calibration technique that
removes the instrument measurement drift that developed over the years.
With this technique applied to the TOMS 14.5-year data record, a global
ozone decrease of 2.69 percent per decade was detected. To ensure that
ozone data will be available through the next decade, NASA will continue
the TOMS program using U.S. and foreign launches. In 1991, the former
Soviet Union launched a Meteor-3 satellite carrying a TOMS instrument
provided by NASA. A third TOMS will be launched onboard a NASA Earth
probe satellite in 1994, and the Japanese Advanced Earth Observations
Satellite (ADEOS) will carry a fourth TOMS when it launches in
TIROS Operational Vertical Sounder. See Television Infrared
Operational Satellite (TIROS).
Tracking and Data Relay Satellite System (TDRSS)
An orbiting communications satellite, developed by NASA, used to
relay data from satellite sensors to ground stations and to track the
satellites in orbit.
Surface air from the horse latitudes that moves back toward the
equator and is deflected by the Coriolis Force, causing the winds to
blow from the Northeast in the Northern Hemisphere and from the
Southeast in the Southern Hemisphere. These steady winds are called
trade winds because they provided trade ships with an ocean route to the
New World. See wind.
See Tropical Rainfall Measuring Mission.
Tropical Ocean Global Atmosphere (TOGA)
TOGA is a program jointly sponsored by the United Nations World
Meteorological Organization (WMO); the International Council of
Scientific Unions (ICSU); the United Nations Educational, Scientific,
and Cultural Organization (UNESCO) Intergovernmental Oceanographic
Commission (IOC); and the ICSU Scientific Committee on Oceanic Research
(SCOR). TOGA has four major objectives:
- To collect and catalog observations of the tropical atmosphere and ocean;
- To assess the evolution of the tropical atmosphere/ocean system
in real time;
- To promote the development of short-term climate-prediction
computer models for the tropics;
- To study the influence of the tropical atmosphere/ocean system
on the climate at higher latitudes.
Tropical Rainfall Measuring Mission (TRMM)
A joint NASA/NASDA mission planned for launch in 1997. The goal of
TRMM is to obtain a minimum of 3 years of climatologically significant
observations of rainfall in the tropics. Because rainfall is such a
variable phenomenon, adequate sampling is a difficult problem. By
averaging the instantaneous rainfall rates for 30 days over a 5° by 5°
grid, TRMM will obtain observations that meet climatological
requirements. TRMM measurements, used together with cloud models, also
will provide accurate estimates of vertical distributions of latent
heating in the atmosphere. The present uncertainty about the quantity
and distribution of precipitation, especially in the tropics, prohibits
definition of the mass and energy exchange between the tropical ocean
and atmosphere. Since the tropical atmosphere and oceans are closely
coupled, cloud radiation and rainfall are likely to have significant
effects on ocean circulation and marine biomass. TRMM data will play a
significant role in global change studies, especially in developing an
interdisciplinary understanding of atmospheric circulation,
ocean-atmospheric coupling, and tropical biology. TRMM data on tropical
clouds, evaporation, and heat transfer will be used to understand the
larger scale coupling of the atmosphere to oceans. See Earth
Tropical Storm Formation
Tropical storms generally form in the eastern portion of tropical
oceans and track westward. Hurricanes, typhoons, and willy-willies all
start out as weak low pressure areas that form over warm tropical waters
(e.g., surface water temperature of at least 80 °F). Initially, winds
and cloud formations over the warm tropical waters are minimal. Both
intensify with time. Formation of tropical storms also requires a
significant Coriolis effect to induce proper spin in the wind formation.
As the storm begins to organize itself into a coherent pattern, it will
experience increased activity and intensity. When a storm develops a
clearly recognizable pattern, it is referred to as a tropical
depression. When wind speeds reach 35 knots (40.3 mph), it is called a
tropical storm and is given a name. When wind speed equals or exceeds 74
mph, the storm is called a hurricane. In the western Pacific, a
hurricane is referred to as a typhoon. In waters around Australia it is
called a cyclone or willy-willy. Hurricanes intensify when moving over
areas of increased water temperatures, and weakend over colder water
surfaces. Upper atmosphere wind shear (different wind direction and
speeds at different elevations) will frequently prevent or slow
intensification of tropical storms by "spreading out" the storm
horizontally and preventing the formation of strong updrafts of warm,
humid air. Movement over a land-mass will weaken hurricane winds but
will result in large-scale rain that can result in large-scale flooding.
When encountering a strong frontal system (such as a polar front) the
hurricane will curve and track along the leading edge of the front or
become implanted in it. Satellite infrared imagery can identify surface
water temperatures that will foster tropical storm development.
The area between 23.5 degrees north and south of the equator. This
region has small daily and seasonal changes in temperature, but great
seasonal changes in precipitation.
The lower atmosphere, to a height of 8-15 km above Earth, where
temperature generally decreases with altitude, clouds form,
precipitation occurs, and convection currents are active. See
Tropospheric Emission Spectrometer
A high-resolution infrared spectrometer for monitoring the minor
components of the lower atmosphere.
Elongated area of low atmospheric pressure, either at the surface or
in the upper atmosphere.
True Anomaly (aka J)
One of six Keplerian elements, it locates a satellite on an orbit.
True anomaly is the true angular distance of a satellite (planet) from
its perigee (perihelion) as seen from the center of the Earth (sun). See
Hurricanes in the Western Pacific Ocean.
See Upper Atmosphere Research Satellite
The energy range just beyond the violet end of the visible spectrum.
Although ultraviolet radiation constitutes only about 5 percent of the
total energy emitted from the sun, it is the major energy source for the
stratosphere and mesosphere, playing a dominant role in both energy
balance and chemical composition. Most ultraviolet radiation is blocked
by Earth's atmosphere, but some solar ultraviolet penetrates and aids in
plant photosynthesis and helps produce vitamin D in humans. Too much
ultraviolet radiation can burn the skin, cause skin cancer and
cataracts, and damage vegetation.
United States Geological Survey (USGS)
A bureau of the Department of the Interior. USGS was established in
1879 following several Federally-sponsored independent natural resource
surveys of the West and Midwest. The Department of the Interior has
responsibility for most of our nationally owned public lands and natural
resources. The USGS monitors resources such as energy, minerals, water,
land, agriculture, and irrigation. The resulting scientific information
contributes to environmental-policy decision making and public safety.
For example, USGS identifies flood- and landslide-prone areas and
maintains maps of the United States.
United States Global Change Research Program (USGCRP)
The USGCRP addresses significant uncertainties concerning the
natural and human-induced changes to Earth's environment. The USGCRP has
a comprehensive and multidisciplinary scientific research agenda. See
Global Change Research Program.
Upper Atmosphere Research Satellite (UARS)
UARS is part of a long-term, international program of space research
into global atmospheric change. Beginning in 1991, NASA's UARS program
began to carry out the first systematic, detailed satellite study of the
Earth's stratosphere, mesosphere, and lower thermosphere; establish the
comprehensive data base needed for an understanding of stratospheric
ozone depletion; and bring together scientists and governments around
the world to assess the role of human activities in atmospheric change.
Launched on September 12, 1991, UARS became the first official space
component of Mission to Planet Earth.
See United States Global Change Research Program.
See Coordinated Universal Time.
Ultraviolet. See ultraviolet radiation.
Van Allen belts or Van Allen Radiation belts
Doughnut-shaped regions encircling Earth and containing high energy
electrons and ions trapped in the Earth's magnetic field (the magnetic
field has definite boundaries, and is distorted into a tear-drop shape
by the solar wind). Explorer I, launched by NASA in 1958, discovered
this intense radiation zone. These regions are called the inner and
outer Van Allen Radiation belts, named after the scientist who first
observed them. See magnetosphere.
The beginning of spring in the Northern Hemisphere. The time/day
that the sun crosses the equatorial plane going from south to
Very High Frequency (VHF)
Referring to the 50–400 MHz portion of the radio frequency spectrum.
Polar-orbiting satellite transmissions (APT) are made in the 136–138 MHz
range using FM modulation.
A signal containing information on the brightness levels of
different portions of an image along with information on line and frame
synchronization. In the case of satellite signals, the video information
is transmitted in the form of an AM modulated subcarrier.
Visible/Infrared Spin Scan Radiometer (VISSR)
High-resolution, multi-spectral imaging system flown on the
pre-GOES-8 geostationary GOES spacecraft. Similar systems are flown on
the METEOSAT and GMS spacecraft.
That part of the electromagnetic spectrum to which the human eye is
sensitive, between about 0.4 and 0.7 micrometers. See spectrum.
A naturally occurring vent or fissure at the Earth's surface through
which erupt molten, solid, and gaseous materials. Volcanic eruptions
inject large quantities of dust, gas, and aerosols into the atmosphere.
A major component of volcanic clouds is sulfur dioxide, a strong
absorber of ultraviolet radiation. Chemical interactions between sulfur
dioxide and water cause sulfuric acid aerosols which can scatter some of
the incident solar radiation back to space, thus causing a global
cooling effect. For example, Mt. Pinatubo in the Philippines erupted in
June 1991, and in the following year the global surface temperature was
observed to decrease by about 0.3° C.
The unit of electromotive force, or difference of potential, which
will cause a current of one ampere to flow through a resistance of one
ohm. Named for Italian physicist Alessandro Volta (1745–1827). Greenwich
The space that contains an object. In the case of a gas (or air) the volume of the container will tell you the volume of the gas.
Volume is typically measured in liters or milliliters.
For example, an open 1-liter bottle at sea-level contains 1-liter of air.
Degrees west longitude, referenced to the Greenwich (prime)
Water Vapor (aka moisture)
Water in a gaseous form.
1. In electricity, a periodic variation of an electric current or
voltage. 2. In physics, any of the series of advancing impulses set up
by a vibration, pulsation, or disturbance in air or some other medium,
as in the transmission of heat, light, sound, etc.
Physical distance of one period (wave repeat).
Weather Facsimile (WEFAX)
A system for transmitting visual reproductions of weather forecast
maps, temperature summaries, cloud analyses, etc. via radio waves. WEFAX
transmissions are relayed by NOAA's geostationary GOES
Some commonly used symbols are illustrated in the chart on the
- Clear: Sky cloud-free to 30 percent covered.
- Sunny: Sunshine 70-100 percent of the day.
- Partly sunny and partly cloudy: Both terms
refer to 40 to 70 percent cloud cover. Partly sunny is used in the day;
partly cloudy is used at night.
- Fog: A cloud on the ground. Fog is
composed of billions of tiny water droplets floating in the air.
- Snow: Precipitation of ice crystals.
- Snow flurries: Intermittent
snowfall that may result in little accumulation.
- Sleet: Pellets of
ice that form when rain or melting snowflakes freeze while falling.
(Occurs in cold weather; hail usually occurs in summer.)
- Freezing Rain: Rain that turns to ice on impact with the surface.
- Rain: Extended period of precipitation. Associated with
large storm systems
rather than single clouds or thunder storms.
- Showers: Brief interval
of rain that does not affect a large area.
- Squall: Fast-moving
thunderstorm or line of thunderstorms that often can produce damaging
winds, hail, and tornadoes.
- Hail: Pieces of ice that fall from
thunderstorms. Hail often is composed of concentric rings of ice that
form as the particle moves through "wet" and "dry" areas of the
Statement that dangerous weather is likely or is occurring. Take
Statement about a particularly dangerous weather system that may
occur at some specified time in the future.
Atmospheric condition at any given time or place. Compare with
See weather facsimile.
Australian term for tropical cyclone, hurricane.
The wind can reduce significantly the amount of heat your body
retains. The following wind chill chart does not take into account such
variables as type of clothing worn, amount of exposed flesh, and
physical condition, all of which would alter body heat.
An instrument used to indicate wind direction.
Arrow representing wind velocity. The arrow points in the direction
of the wind. The length of the arrow is proportional to wind
Vector term that includes both wind speed and wind
A natural motion of the air, especially a noticeable current of air
moving in the atmosphere parallel to the Earth's surface. Winds are
caused by unequal heating and cooling of the Earth and atmosphere due to
absorbed, incoming solar radiation and infrared radiation lost to
space--as modified by such effects as the Coriolis force, the
condensation of water vapor, the formation of clouds, the interaction of
air masses and frontal systems, friction over land and water, etc. The
chart above is an abbreviated version of the Beaufort Wind Scale, named
for the British admiral who invented it in 1805. Wind is the movement of air from a higher pressure zone to a lower pressure zone. This
phenomenon can be observed while opening a bottle of soda, the compressed gas in the soda
bottle escapes as the bottle is opened which causes a small wind around the opening for a
Term used to denote a region of the electromagnetic spectrum where
the atmosphere does not absorb radiation strongly.
See World Ocean Circulation Experiment.
A "smart" computer terminal that serves as a primary scientific
research tool, offering direct access to experimental apparatus,
information files, internal computers, and output devices, usually
connected to an external communications network.
World Ocean Circulation Experiment (WOCE)
A study of the general global circulation of the oceans. It
emphasizes the measurements and understanding needed to describe and
understand the circulation, to simulate it, and to predict its changes
in response to climatic changes.
World Weather Watch
A type of receiving antenna that has several rod elements mounted
ono a beam. Its directional pattern of sensitivity and ease of
construction make it ideal for APT direct readout stations. See
A Mediterranean term for any soft, gentle breeze.
Updated: February 10, 2003