A hurricane is a type of tropical cyclone, which is the general term for all circulating weather systems over tropical waters. In the Northern Hemisphere these storms circulate in a counterclockwise rotation. Tropical cyclones are classified as follows:
- Tropical Depression: An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 38 Miles Per Hour (MPH) (33 knots) or less.
- Tropical Storm: An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39 to 73 MPH (34-63 knots).
- Hurricane: An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 MPH (64 knots) or higher (See the Saffir-Simpson Scale).
What They Do
Hurricanes are products of the tropical ocean and atmosphere. Powered by heat from the sea, they are steered by the easterly trade winds and the temperate westerlies as well as by their own ferocious energy. Around their core, winds grow with great velocity, generating violent seas. Moving ashore, they sweep the ocean inward while spawning tornadoes and producing torrential rains.
The Atlantic Hurricane Season begins June 1 and ends November 30 each year. The peak hurricane threat for the U.S. exists from mid-August to late October. Developing hurricanes gather heat and energy through contact with warm ocean waters. The addition of moisture by evaporation from the sea surface powers them like giant heat engines.
The process by which a disturbance forms and subsequently strengthens into a hurricane depends on at least three conditions. Warm waters and moisture that were mentioned and the third condition is a wind pattern near the ocean surface that spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and allow for additional strengthening.
The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud free area.