Development of a Hurricane

A hurricane is a tropical storm with winds that have reached a constant speed of 74 miles per hour or more. Hurricane winds blow in a large spiral around a relative calm center known as the "eye." The "eye" is generally 20 to 30 miles wide, and the storm may extend outward 400 miles. The most violent activity takes place in the area immediately around the eye, called the "eyewall". At the top of the eyewall (up to 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. As a hurricane approaches, the skies will begin to darken and winds will grow in strength. As a hurricane approaches land, it can bring torrential rains, high winds, and storm surges. A single hurricane can last for more than 2 weeks over open waters and can run a path across the entire length of the eastern seaboard. August and September are peak months during the hurricane season that lasts from June 1 through November 30.

The development  of a hurricane requires at least three conditions: (1) the ocean waters must be warm enough at the surface to put enough heat and moisture into the overlying atmosphere to provide the potential fuel for the thermodynamic engine that a hurricane becomes. (2) atmospheric moisture from sea water evaporation must combine with that heat and energy to form the powerful engine needed to propel a hurricane. (3) a wind pattern must be near the ocean surface to 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 grow stronger: the beginnings of a hurricane!

 Often, the feature that triggers the birth of a hurricane is some pre-existing weather disturbance in the tropical circulation. For example, some of the largest and most destructive hurricanes originate from weather disturbances that form as squall lines over Western Africa and subsequently move westward off the coast and over warm water, where they gradually intensify into hurricanes.
Hurricane winds in the northern hemisphere circulate in a counterclockwise motion around the hurricane's center or "eye," while hurricane winds in the southern hemisphere circulate clockwise.

 The eye of a hurricane is relatively calm. It is generally 20 to 30 miles wide (the hurricane itself may extend outward 400 miles).