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Hurricanes
Hurricanes are tropical cyclones with winds that exceed 64 knots
(74 mi/hr) and circulate counter-clockwise about their centers
in the Northern Hemisphere (clockwise in the Southern Hemisphere).
Hurricanes are formed from simple complexes of thunderstorms.
However, these thunderstorms can only grow to hurricane strength
with cooperation from both the ocean and the atmosphere.
First of all, the ocean water itself must be warmer than 26.5
degrees Celsius (81°F). The heat and moisture from this
warm water is ultimately the source of energy for hurricanes.
Hurricanes will weaken rapidly when they travel over land or
colder ocean waters -- locations with insufficient heat and/or
moisture.
Related to having warm ocean water, high relative humidities
in the lower and middle troposphere are also required for hurricane
development. These high humidities reduce the amount of evaporation
in clouds and maximizes the latent heat released because there
is more precipitation. The concentration of latent heat is critical
to driving the system.
The vertical wind shear in a tropical cyclone's environment
is also important. Wind shear is defined as the amount of change
in the wind's direction or speed with increasing altitude. The
video below shows how wind shear plays a role in hurricane formation.
When the wind shear is weak, the storms that are part of the
cyclone grow vertically, and the latent heat from condensation
is released into the air directly above the storm, aiding in
development. When there is stronger wind shear, this means that
the storms become more slanted and the latent heat release is
dispersed over a much larger area.
When cloud particles become too heavy to remain suspended in
the air, they fall to the earth as precipitation. Precipitation
occurs in a variety of forms; hail, rain, freezing rain, sleet
or snow. This portion of the Clouds and Precipitation module
focuses on precipitation and has been organized into the following
sections.
Atmospheric conditions that lead to the development of rain
and hail.
Freezing Rain
A detailed look at freezing rain, associated dangers and the
conditions that lead to its development.
Sleet
Atmospheric conditions that lead to the development of sleet.
Snow
Atmospheric conditions that lead to the development of snow.
Acknowledgments
Those who contributed to the Precipitation sections of the Clouds
and Precipitation module.
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