Spring storm season is upon us. You know the routine. Dark clouds move in, heavy rain drops hit the roof, lightning flashes, thunder roars and winds start moving trees side to side.
Right now, at this very moment, there are about 2,000 thunderstorms going on around the world. And even though thunderstorms are common, they are still dramatic events.
Thunderstorms form when warm, moist air rises into cold air. The warm air becomes cooler, which causes 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 continuous circuit of rising and falling air is called a convection.
Thunderstorms can consist of just one convection cell, multiple convection cells, or even one extremely large and powerful convection cell. Below is a description of three types of thunderstorms, classified by their structure: single-cell, multi-cell and supercell.
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 4,000 feet high in the atmosphere. Rain and lightning are common. Sometimes hail falls.
Some thunderstorms are made from many convection cells moving as a single unit. These are called multi-cell thunderstorms. Often the convection cells are arranged as a cluster, with each cell at a different stage of the thunderstorm cycle. Multi-cell storms along a cold or warm front, where warm air is pushed high into the atmosphere above cold air, often form a line, called a squall line. The squall line can be up to 600 miles long. Strong wind gusts often blow just ahead of the storm.
Thunderstorms with deep, rotating updraft winds, 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 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 6,000 in the atmosphere. Supercells are the least common type of thunderstorm.