An Atlantic hurricane is a large low pressure storm which forms off the coast of Africa and begins to spins counterclockwise. The hurricane starts as a band of thunderstorms. These storms can become a hurricane when warm air over the warm western Atlantic Ocean combine with wind patterns. The counterclockwise motion is perpetuated by strong easterly trade winds and temperate westerlies, and a hurricane is born.

A hurricane forms in three stages:
1. Tropical Depression - Organized thunderstorms with maximum sustained winds of 38 mph or less.
2. Tropical Storm - Same structure as a tropical depression, but with maximum sustained winds of 39 to 73 mph.
3. Hurricane - The storm is officially designated a hurricane when the winds reach a maximum sustained wind speed of 74 mph.

Hurricanes are also recognized by an eye. The eye is an area of calm contained near the rotational axis of the hurricane. The eye is surrounded by thick clouds forming the eye wall and it is kept open by a strong upward air motion The most violent area of the hurricane is the eye wall.

The thunderstorms and wind associated with hurricanes are not the only things which pose a threat to life and property. Preceding the storm is a storm surge. This is a large dome of water displaced by the low pressure surrounding the hurricane. This water is forced on shore ahead of the hurricane. If the storm surge occurs during high tide, it is called a storm tide and it creates a greater loss of life and property due to the extra amount of water.

Hurricanes weaken when they hit land and are cut off from the warm moist ocean. But this does not stop the on land threat. Hurricanes can still create sustained winds over 100 mph on land as well as spawning tornadoes. Since a hurricane is composed of thunderstorms, the rainfall often saturates the ground and causes massive flash flooding.

Not all hurricanes kill, the severity of a hurricane is rated by the Saffir-Simpson Hurricane Scale. This scale is based on the maximum sustained wind speed of a hurricane.

Saffir-Simpson Hurricane Scale

Category	Sustained Winds (MPH)	Damage/Effects	Examples
1	74 to 95	No damage to permanent structures. Some damage to unanchored homes and plants, some flooding and pier damage.	Charley (1988) in NC
2	96 to 110	Structure damage includes roofing, doors, and windows. Considerable damage to vegetation, unanchored structures, and piers. Low lying areas flood before arrival of eye and small crafts break moorings.	Bob (1991)
3	111 to 130	Mobile homes destroyed. Flooding along coast can destroy smaller structures, also flooding continue inland if terrain is continuously 5 ft. below sea level. Some structural damage to small residence.	Alicia (1983) in TX
4	131 to 155	Extensive structural damage including colaspe of roof structures and damage to lower floors of buildings. Major beach erosion and terrain continuously 10 ft. below sea level is flooded inland. Massive evacuation upto 6 miles inland.	Andrew (1992) in FL
5	greater than 155	Complete roof failure on residences and some complete building failure. Some structures are blown away. Major flood damage and flooding to areas continually 15 ft. below sea level. Massive evacuation upto 10 miles inland	Camille (1969) in MS

Return to main page