A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and the northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or the Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls. The location of where the situation could be occurring is crucial as it determines the strength of the storm.
"Tropical" refers to the geographical origin of these systems, which form over tropical seas. "Cyclone" refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which recondenses into clouds and rain when moist air rises and cools to saturation. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are almost unknown in the South Atlantic due to consistently strong wind shear and a weak Intertropical Convergence Zone.
Coastal regions are particularly vulnerable to the impact of a tropical cyclone. The primary energy source for these storms is warm ocean waters. These storms are typically strongest when over or near water, and weaken quite rapidly over land. Coastal damage may be caused by strong winds and rain, high waves, storm surges, and the potential of spawning tornadoes. Tropical cyclones also draw in air from a large area, which can be a vast area for the most severe cyclones, and concentrate that air's water content into precipitation over a much smaller area. This continual replacement of moisture-bearing air by new moisture-bearing air after its moisture has fallen as rain may cause multi-hour or multi-day extremely heavy rain up to 40 kilometers (25 mi) from the coastline. This in turn can lead to river flooding, overland flooding, and a general overwhelming of local man-made water control structures across a large area. The effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate.