How do hurricanes form?

A hurricane is a giant, rotating engine that converts ocean heat into wind. It starts over tropical seas where surface water is at least about 26.5 degrees Celsius (80 degrees Fahrenheit). Warm, moist air rises from the ocean, and as it climbs and cools, water vapor condenses into clouds, releasing latent heat. That heat warms the surrounding air, making it rise faster and lowering the pressure at the surface. More moist air rushes in to replace it, evaporates more water, and the cycle intensifies. Earth's rotation deflects the inflowing air, setting the whole system spinning, counterclockwise in the Northern Hemisphere. When winds organize around a calm central eye and reach 74 miles per hour (119 kilometers per hour), the storm is classified as a hurricane.

At its core, a hurricane is a heat engine, much like the ones physicists describe with thermodynamics. It draws energy from the temperature difference between the warm sea surface and the cold upper atmosphere, roughly 15 kilometers up. Evaporation loads the air with latent heat; condensation releases that heat high in the storm, and the surplus energy becomes the kinetic energy of wind. That is why sea-surface temperature matters so much, since warmer water charges the engine with more fuel. Two other conditions are essential. The storm must form far enough from the equator, generally beyond about 5 degrees of latitude, for Earth's rotation to impart spin. And the surrounding winds must be fairly uniform with height, because strong wind shear tears a developing storm apart before it can organize.

The same kind of storm goes by different names around the world: hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and cyclones in the Indian Ocean and near Australia. Most Atlantic hurricanes form between June and November, when seas are warmest, often from clusters of thunderstorms that drift westward off West Africa. Forecasters rank them one to five on the Saffir-Simpson scale by wind speed. Once a hurricane moves over land or cooler water, it loses its warm-water fuel and weakens, though it can still unleash devastating rain, wind, and storm surge, the wall of seawater pushed ashore that causes much of the deadliest flooding. Because warmer oceans hold more energy and warmer air holds more moisture, scientists expect a warming climate to make the strongest hurricanes more intense and rainier.

Myth: the eye is the most dangerous part of the storm. Reality: the eye is eerily calm, and the ferocious winds ring it in the surrounding eyewall. Myth: a storm's category number tells you how deadly it will be. Reality: the Saffir-Simpson scale measures only wind speed, yet water, from storm surge and inland rain flooding, causes most hurricane deaths, and even a Category 1 can be lethal. Myth: opening your windows equalizes pressure and protects your house. Reality: this is false and dangerous, inviting wind and debris inside. Myth: hurricanes can spin up anywhere the water is warm. Reality: they also need enough distance from the equator for Earth's rotation to start them turning, so they almost never form right at the equator.