How Air Pressure Signals Changes in the Weather
could be spotted by satellites from space, people would keep a wary
eye on their barometers during hurricane season. If the air pressure
dropped, that was usually a good time to board up windows and head
As hurricanes pass over coastal areas, air pressure can drop
significantly. At sea level air pressure is normally around 1013.25mb (29.92 inches of mercury).
Extremely strong hurricanes are accompanied by air pressure drops of
between 30 and 70mb. The greater the pressure difference between a low
pressure area and a high pressure area, the stronger the winds! Wind
is the natural result of having a low pressure area next to a higher
pressure area since the air molecules in the higher pressure zone will
migrate to the "more spacious" surroundings of the low pressure
Tornadoes, also known as Twisters, can be as destructive as
hurricanes on a smaller scale. A falling barometer can indicate bad
weather approaching and many people in the midwest and central plains
states will head into the cellar when the air pressure drops
Tornadoes account for millions of dollars of damage and significant
human suffering in the U.S. each year. Because of this, many
scientists are studying the way in which tornadoes form and how they
behave. For example, this graph shows wind
vectors across the U.S. and was created by the Global Tropospheric
Experiment (GTE) at NASA. GTE is one example of how scientists are
working to discover how our activities are affecting the weather.
How Does Weather Affect Air Pressure?
By Dr. Louis Walter of NASA's Earth Science Enterprise
The most fundamental thing you have to understand is that heavier gases
weigh more than lighter gases. Now that's pretty straightforward - but
what does it mean? Well, different chemical elements, as you know, have
different atomic weights. Those which form gases (like nitrogen, oxygen,
etc.) often combine two atoms at a time to form a gaseous molecule - like
N2 (two nitrogens) or O2 (two oxygens).
Now the ATOMIC weight of nitrogen (N) is 14 and of oxygen (O) it's 16.
The molecules N2 and O2 have MOLECULAR weights of 28 and 32, respectively.
Doesn't take a rocket scientist to see that a gallon of oxygen weighs more
than a gallon of nitrogen. Matter of fact, it turns out that - at room
temperature and normal (sea level) atmospheric pressure, 28
grams of nitrogen occupies a volume of 22.4 liters and 32 grams of oxygen
occupies the same volume!
In other words, under STP (standard temperature and pressure) the weight
of 22.4 liters of a gas in grams equals the molecular weight of the gas.
Now for some fun with numbers. Air is ABOUT 80% nitrogen and 20% oxygen.
How much does a liter of air weigh? Well.....
If 22.4 liters of nitrogen weighs 28 grams; 0.8 liters weighs
(0.8/22.4) X 28 grams = 1 gram (almost exactly) and 0.2 liters
of oxygen weighs (0.2/22.4) X 32 = 0.286 grams, so...
A liter of air weighs about 1.286 grams.
Now, let me change the subject just a bit. How much does gaseous water weigh?
Not LIQUID water - I mean steam or vapor. Let's figure it out.
The chemical formula for water is H2O. One oxygen atom (atomic weight 16)
and two hydrogen atoms (atomic weight 1). The total weight of the molecule is 18.
Now how much does 22.4 liters (of the gas) weigh? 18 grams. One liter weighs 18/22.4 grams or 0.8 grams.
Now we're getting close. You can see that air normally weighs 1.286 grams per
liter but, if we substitute water for some of the air, the mixture becomes lighter.
So, if there's water (otherwise known as humidity) in the air, the air mixure
becomes LIGHTER - and it doesn't push down so hard on the mercury and the barometer's lower.
Updated: July 1, 2005