Thunderstorms

Thunderstorms

Supercell Thunderstorm

A supercell, the largest of all thunderstorms, is likely to spawn tornadoes.
Credit: UCAR

Picture a thunderstorm - heavy raindrops beat the roof, lightning flashes through the windows, thunder booms, the dog whines from his hiding spot under your bed. Now picture two thousand thunderstorms.

Right now, at this very moment, there are about two thousand thunderstorms going on around the world. Even though thunderstorms are common, they are still dramatic events with intense rain, hail, wind, lightning, thunder, and even tornadoes.

Thunderstorms form when warm, moist air rises into cold air. The warm air becomes cooler, which causes its moisture, called water vapor, to form small water droplets - a process called condensation. The cooled air drops lower in the atmosphere, warms and rises again. This circuit of rising and falling air is called a convection cell. If this happens a small amount, a cloud will form. If this happens with large amounts of air and moisture, a thunderstorm can form.

Thunderstorms created by just one convection cell in the atmosphere are called single-cell storms. Most of these are small, lasting only about an hour, and are also called ordinary thunderstorms. These storms often form during summer and include towering cumulonimbus clouds that can grow 12 kilometers high in the atmosphere. Rain and lightning are common. Sometimes hail falls.

But a few single-cell thunderstorms, called supercells, are very large and last for hours releasing huge amounts of rain and sometimes even baseball-sized hail. They include fast moving convection – air zooming upward at as much as 175 miles (280 km) per hour. Rotation in supercells sometimes forms violent tornadoes, the largest and most damaging type, because the storms are so long-lived. Several tornadoes can be produced from one supercell thunderstorm. And clouds grow up to 18 km in the atmosphere. Supercells are one type of severe thunderstorm.

Other severe thunderstorms are made from many convection cells. These are called multicell thunderstorms. Often the cells form a line called a squall line up to 600 miles (1000 km) long at a cold or warm front where warm air is pushed above cold air, higher in the atmosphere. Strong wind gusts often blow just ahead of the storm.