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4 . 1 . 2 Hurricanes/Tropical Storms 4. 1 . 2. 1 Hazard Identification Hurricanes are tropical cyclones with winds that exceed 74 mph and blow counter-clockwise about their centers in the Northern Hemisphere . They are essentially heat pumping mechanisms that transfer the sun ' s heat energy from the tropical to the temperate and polar regions . This helps to maintain the global heat budget arid sustain life . Hurricanes are formed from thunderstorms that form over tropical oceans with surface temperatures warmer than 81 OF (26 . 50C ) . The ambient heat in the sea ' s surface and moisture in the rising air column set up a low pressure center and convective conditions that allow formation of self sustaining circular wind patterns . Under the right conditions , these winds may continue to intensify until they reach hurricane strength . This heat nd moisture from the warm ocean water is the energy source of a hurricane . Hurricanes weaken rapidly when deprived of their energy source by traveling over land or entering cooler waters . When a hurricane threatens the coast, advisories are issued by thE National Hurricane Center ( NHC ) . The storm ' s current location and intensity are descr bed along with its projected path . Advisories are issued at 6 - hour intervals : 5 : 00 A . M . , 11 : 00 . M . , 5 : 00 P . M . , and 11 : 00 P . M . , Eastern Time . In addition to advisories , the NHC may issue a hurricane watch or arning . A hurricane watch indicates that hurricane conditions are possible and may threaten the area within 36 hours . A hurricane warning is issued when winds of at least 74 mph are to be expected in the area within 24 hours . Advisories and hurricane watches and warnings will frequently refer to the category of a storm . Hurricanes are classified using the Saffir-Simpson scale as follows : • Category 1 — Winds 74 to 95 mph ; • Category 2 — Winds 96 to 110 mph ; • Category 3 — Winds 111 to 130 mph ; • Category 4 — Winds 131 to 155 mph ; and • Category 5 — Winds > 155 mph . Hurricane damage occurs through two means — high winds and storm surge . Generally it is the wind that produces most of the property damage associatec with hurricanes , while the greatest threat to life is from flooding and storm surge . Although hurricane winds can exert tremendous pressure against a structure , a large percentage of hurricane damage is caused not from the wind itself, but from flying debris . Tee limbs , signs and sign posts , roof tiles , metal siding , and other loose objects can become airborne missiles that penetrate the outer shells of buildings , destroying their structural integrity and allowing hurricane winds to act against interior walls not designed to withstand such forces . Once a structure ' s integrity is breached , the driving rains associated with hurricanes can enter the structure and completely destroy its contents . 4-20