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What causes hurricanes?

Hurricanes are intense low pressure areas that form over warm ocean waters in the summer and early fall. Their source of energy is water vapor which is evaporated from the ocean surface.

Water vapor is the “fuel” for the hurricanes because it releases the “latent heat of condensation” when it condenses to form clouds and rain, warming the surrounding air. (This heat energy was absorbed by the water vapor when it was evaporated from the warm ocean surface, cooling the ocean in the process.)

Usually, the heat released in this way in tropical thunderstorms is carried away by wind shear, which blows the top off the thunderstorms. But when there is little wind shear, this heat can build up, causing low pressure to form. The low pressure causes wind to begin to spiral inward toward the center of the low.

These winds help to evaporate even more water vapor from the ocean, spiraling inward toward the center, feeding more showers and thunderstorms, and warming the upper atmosphere still more. The showers and thunderstorms where all of this energy is released are usually organized into bands (sometimes called “rainbands” or “feeder bands”), as well as into an “eyewall” encircling the center of the storm. The eyewall is where the strongest winds occur, which encircle the warmest air, in the eye of the hurricane. This warmth in the eye is produced by sinking air, which sinks in response to rising air in the thunderstorms. The winds diminish rapidly moving from the eyewall to the inside of the relatively cloud-free eye, where calm winds can exist.