Tropical Twisters - Hurricanes: How they work and what they do.

How are Hurricanes Created?

The birth of a hurricane requires at least three conditions. First, the ocean waters must be warm enough at the surface to put enough heat and moisture into the overlying atmosphere to provide the potential fuel for the thermodynamic engine that a hurricane becomes. Second, atmospheric moisture from sea water evaporation must combine with that heat and energy to form the powerful engine needed to propel a hurricane. Third, a wind pattern must be near the ocean surface to spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and grow stronger: the beginnings of a hurricane!

Often, the feature that triggers the development of a hurricane is some pre-existing weather disturbance in the tropical circulation. For example, some of the largest and most destructive hurricanes originate from weather disturbances that form as squall lines over Western Africa and subsequently move westward off the coast and over warm water, where they gradually intensify into hurricanes.

Hurricane winds in the northern hemisphere circulate in a counterclockwise motion around the hurricane's center or "eye," while hurricane winds in the southern hemisphere circulate clockwise.

The eye of a hurricane is relatively calm. It is generally 20 to 30 miles wide (the hurricane istself may extend outward 400 miles). The most violent activity takes place in the area immediately around the eye, called the "eyewall". At the top of the eyewall (up to 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.

Tropical Rainfall Measuring Mission

TRMM view of Hurricane Susan Hurricanes are huge heat engines, converting the warmth of the tropical oceans and atmosphere into wind and waves. The heat dissipates as the system moves toward the poles, sometimes causing a great deal of hardship for people living along the vulnerable coastlines.

NASA scientists are using the TRMM satellite to understand which parts of a hurricane produce rainfall and why. In addition, TRMM may answer the question of how much latent heat or "fuel" hurricanes release into the atmosphere and whether they affect global weather patterns.

Most importantly to people endangered by hurricanes, TRMM will add to the knowledge needed to improve computer-based weather modeling. With this data, meteorologists may be more able to precisely predict the path and intensity of these storms.

For more information about TRMM visit the ESE Kids Only TRMM site and the official NASA TRMM web site.

Updated: January 22, 2003
Take a Tour
Take a VR Tour of a hurricane, and see how the winds within a hurricane interact.

The Most Killed
The 1900 Galveston, Texas, hurricane took 6,000 lives -- more than any other natural disaster in United States history. Click here for more information about hurricane damage.

Population Growth
There are now some 45 million permanent residents along the hurricane-prone U.S. coastline -- and the population is still growing. Florida, where hurricanes are most frequent, also leads the United States in new residents. In addition, holiday, weekend, and vacation populations swell in some coastal areas.

Destroy the Hurricane?
NOAA and its predecessor tried to weaken hurricanes by dropping silver iodide into the rainbands of the storms. The idea was that the silver iodide would enhance the thunderstorms of the rainband helping it to grow at the expense of the eyewall. Two hurricanes were massively seeded with silver iodide: one in 1969 and one in 1971. The first storm weakened temporarily after the seeding; there was no effect on the second. The hurricane seeding program was discontinued in 1972 because of budget cuts. Later analysis of the 1969 storm suggested that it would have weakened even if the seeding had not occurred.

National Hurricane Center
The National Hurricane Center (NHC) maintains a continuous watch on tropical cyclones over the Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific.

Find out more about Hurricanes.

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