The process by which a disturbance forms and subsequently strengthens into a
hurricane depends on at least three conditions. Warm waters and moisture are
mentioned above. 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.