thunderstorm, violent, local atmospheric disturbance accompanied by lightning, thunder, and heavy rain, often by strong gusts of wind, and sometimes by hail. The typical thunderstorm caused by convection occurs when the sun's warmth has heated a large body of moist air near the ground. This air rises and is cooled by expansion. The cooling condenses the water vapor present in the air, forming a cumulus cloud. If the process continues, the summit often attains a height of 4 mi (6.5 km) above the base, and the top spreads out in the shape of an anvil. The turbulent air currents within the cloud cause a continual breaking up and reuniting of the raindrops, which may form hail, and builds up strong electrical charges that result in lightning. As the storm approaches an area, the gentle flow of warm air feeding the cloud gives way to a strong, chilly gust of wind from the opposite direction, blowing from the base of the cloud. Intense rain begins, then gradually diminishes as the storm passes. Night thunderstorms are caused by the cooling of the upper layers of air by radiation; others are caused by approaching cold air masses that advance as a wedge near the ground, forcing the warmer air in its path to rise. Even a forest fire or a volcanic eruption may create a thunderstorm. Thunderstorms occur most frequently in the equatorial zone (some localities have as many as 200 a year) and seldom in the polar regions. In the United States they are most frequent along the E Gulf Coast (averaging more than 70 a year) and least frequent on the Pacific coast (less than 4 a year).