How a hurricane forms
Hurricanes are heat engines powered by warm ocean water, generally needing sea surface temperatures around 26.5 degrees Celsius or higher. Warm, moist air rises over the ocean, and as it ascends and cools, water vapor condenses, releasing latent heat that warms the surrounding air and drives further rising. This feedback lowers surface pressure and pulls in more air.
The Earth's rotation, through the Coriolis effect, deflects the inflowing winds and sets the whole system spinning, counterclockwise in the Northern Hemisphere. Because the Coriolis effect vanishes at the equator, hurricanes do not form right on it. Low vertical wind shear is also needed so the developing storm is not torn apart.
Anatomy: eye, eyewall, and rainbands
At the center sits the eye, a region of calm, sinking air and often clear skies, typically 30 to 65 kilometers across. Surrounding it is the eyewall, a ring of towering thunderstorms where the strongest winds and heaviest rain occur. Spiraling outward are rainbands, curved arms of showers and storms.
Storm intensity is graded on the Saffir-Simpson scale by sustained wind speed, from Category 1 to the most destructive Category 5. The same storm is called a hurricane in the Atlantic and Northeast Pacific, a typhoon in the Northwest Pacific, and a cyclone in the Indian Ocean and South Pacific.