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Hurricane
formation is usually initiated in the tropics, typically at latitudes
around 10 degrees north of the equator. In the late summer and early
fall, the water temperatures exceed 80 degrees. This provides the heat
and moisture needed for thunderstorm development. |
Converging surface winds along a tropical wave can cause
grouping of thunderstorms.
Photo
courtesy of NASA. |
Thunderstorms (which are commonly found in the tropics anyway) can be
bunched together by weak waves of low pressure in the easterly surface
winds. When such a cluster of thunderstorms forms in the presence of
jet stream winds or other forms of shear, they are not able to organize
any further. However, given light winds both near the surface and aloft
the group can remain and intensify. |
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With such intensification under way, latent heat is released as the
thunderstorms condense water from vapor to liquid form (think of it this
way, boiling water on a stove changes water from liquid to vapor and
requires the input of heat, whereas water changing from vapor to
liquid releases large amounts of energy). The combination of the
heat release and the upward motion of the thunderstorms induce a
mid-level low pressure within the complex of storms, now called a
tropical disturbance. The presence of the mid level low draws up more
warm and moist air and a feedback begins to develop. In the Northern
Hemisphere the Coriolis force causes low pressure and thus the group of
thunderstorms to begin to rotate counter-clockwise. Usually by this
time the system is called a tropical depression. Further strengthening
will cause the more familiar swirling appearance of a tropical storm and
eventually a hurricane. |
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Once formed, even a strong hurricane must remain in areas of light winds
aloft. Otherwise its orderly spiral bands will begin to get sheared.
This will weaken the system as a whole. Landfall also quickly weakens
the storm both by causing friction and by cutting off the supply of warm
and moist air at the surface. |
Hurricane Lili Oct. 2002 Photo Courtesy NHC |
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Want to learn more? Check out the
Hurricane FAQ Page at the
National Hurricane Center |
www.nws.noaa.gov 
