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Panama City condominium damaged by Hurricane Opal
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Storm Strength

A cyclone is an area of low pressure with winds blowing counter-clockwise around it in the Northern Hemisphere and clockwise around it in the Southern Hemisphere. A tropical cyclone is a cyclone which forms in the tropics--but that's not everything. Tropical cyclones form from one air mass while extratropical storms form when separate warm and cold air masses meet.

Tropical cyclones are given different names depending on how strong they are. The strength is gauged by the wind speed. Here are the different types of storms:

Tropical Disturbance: Winds weak and unorganized
Tropical Depression: Winds less than 39 mph
Tropical Storm: Winds 39 to 74 mph
Hurricane: Winds greater than 74 mph

Also, meteorologists use the wind speed as one factor to forecast how strong a hurricane is. A hurricane's strength is one factor emergency managers consider when they decide which areas to evacuate. They also use the storm's strength to guess the type of damage to expect.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA