Tropical cyclone

Tropical cyclones are warm-core low-pressure systems with an organized, closed circulation and no fronts. They form over tropical or subtropical waters when a set of atmospheric conditions comes together. Warm ocean water supplies heat and moisture, but does not by itself create a cyclone: a pre-existing disturbance, moist air in the middle troposphere and relatively weak vertical wind shear also help a system organize. Because the rotation that helps shape a cyclone is weak close to the Equator, most storms develop some distance away. As water vapour rises and condenses into clouds, it releases heat that warms the surrounding air and sustains further ascent and thunderstorms.

A mature cyclone commonly has spiral rainbands, an eyewall and, in many stronger storms, a central eye. The eyewall is a ring of intense thunderstorms around the eye and usually contains the strongest winds. Air sinking in the eye can suppress clouds and bring a temporary lull, but it does not mean the storm has ended: the eyewall and its dangerous winds may pass over again from another direction. Cyclones vary greatly in size and intensity, and not every storm develops a distinct eye.

The same broad kind of storm is commonly called a hurricane in the Atlantic and parts of the eastern Pacific, a typhoon in the western North Pacific, and a cyclone in the Indian Ocean and South Pacific. Terminology and operational warning practices are set by regional meteorological agencies, so a storm's regional name does not describe a fundamentally different kind of weather system. Individual storms are also assigned names under regional lists; the World Meteorological Organization says this makes warnings and information easier to communicate, especially when more than one storm is active.

A cyclone's danger is not measured by wind alone. Extreme winds can damage buildings and infrastructure, while heavy rain can cause flash floods and river flooding far inland. Along coasts, storm surge—an abnormal rise in sea level driven mainly by a storm's onshore winds—can inundate low-lying areas. Lightning and tornadoes can also occur. These hazards may overlap, making timely forecasts and clear warnings important for preparation. The WMO coordinates the sharing of observations and forecasting knowledge, while national meteorological services issue warnings for their regions.

Cyclones both draw energy from and alter the ocean surface. Their winds churn the upper ocean, bringing cooler water upward and mixing heat downward; NASA Earth Observatory describes these effects as a cooler wake left behind some storms. The amount of cooling varies with the storm and the ocean conditions beneath it. This interaction is one reason ocean structure matters to cyclone behavior, and why storms are studied as coupled atmosphere–ocean systems.