A hurricane is a severe kind of tropical cyclone, which is a low-pressure system with defined wind circulation that occurs over the tropics. During a hurricane, sustained winds reach speeds of 74 mph or higher, while air pressure in the center of the cyclone drops, and the Coriolis force causes these winds to spiral counterclockwise.
As winds reach greater speeds, a tropical cyclone passes through four distinct phases. Tropical disturbance refers to a system of clouds, showers and thunderstorms that lasts more than 24 hours. A tropical depression occurs when closed-circulation winds develop and sustained one-minute winds reach 38 mph. A tropical depression becomes a tropical storm when sustained one-minute winds reach 39 to 73 mph. In the final phase, sustained one-minute winds reach 74 mph, and a tropical depression becomes a hurricane.
As a tropical cyclone develops, the air pressure drops, attracting warm air from the ocean's surface. The winds spiral in a counterclockwise direction because of the Coriolis effect, which is from the deflecting force of the earth's rotation. The low-level winds at the center of the cyclone often form an "eye" when the storm system evolves into a hurricane. The winds are strongest along the walls of this eye, and the storm system refuels itself by continuing to circulate heat energy from the ocean water to the storm and back.