THUNDERSTORMS ARE produced by cumulonimbus clouds and include thunder and lightning. They form when the troposphere is unstable or conditionally unstable for a large portion of its depth and may have tops of 40,000 to 50,000 foot (12,000 to 15,000 meter) or higher. In addition, thunderstorms require a source of atmospheric moisture and a “trigger”—a feature that initiates upward vertical motion in the atmosphere. Thunderstorms are built of units called cells. A mature cell contains an updraft and a downdraft. Single cell thunderstorms may be only 2 to 3 mile (a few kilometer) across and multiple cell thunderstorms may be a few hundred miles across. Cells have 1 of 2 types of structures: ordinary cells and supercells. Multiple cell thunderstorms may contain cells of both types. A thunderstorm with a single ordinary cell is known as an air mass thunderstorm. These storms are not known for producing severe weather but do so on occasion. An ordinary cell goes through a life cycle that consists of 3 stages. The first stage is called the cumulus stage and has only an updraft. Bubbles of warm air rise through the lower troposphere and liquid water condenses when their temperatures cool to their dew point. A cumulus cloud is present that has not grown large enough to produce precipitation that can fall through the updraft. As the cumulus cloud grows taller, the hydrometeors grow larger and can begin to fall through the updraft. This precipitation drags surrounding air down with it. In addition, on the edge of the cloud, the liquid water mixes with unsaturated air. Some of the liquid water evaporates, cooling the air. This cooler air is negatively buoyant and encourages downward motion. Some precipitation in downward moving air leaving the cloud through its bottom will evaporate, further cooling the air in the downdraft. When the precipitation reaches the Earth’s surface, the storm enters its mature stage. As the cool downdraft contacts the Earth’s surface, it spreads out away from the storm. Its leading edge forms a gust front or an outflow boundary. As the downdraft air moves away from the storm, it cuts off the rising bubbles of air that originally started the storm. When the downdraft has spread through the entire storm and no updraft is left, the storm enters the dissipating stage. The storm weakens and begins evaporating from the bottom up. This entire life cycle lasts 30 to 60 minutes. The updraft of a supercell thunderstorm rotates. This characteristic distinguishes it from the ordinary cell thunderstorm. The rotation is not influenced by the Coriolis force and may go in either direction but cyclonic circulation is preferred in . The supercell forms in an environment with considerable vertical wind shear. The updraft and downdraft will tilt and twist as they move through the storm. The updraft and the downdraft remain separated so that the downdraft does not cut off the updraft. Thus, a single supercell storm may last for several hours. The updraft is strong and precipitation does not have time to form within it.