According to the National Severe Storms Laboratory, thunderstorms occur most frequently in temperate climates where warm, moist air encounters pockets of cool air. The four categories of thunderstorms are single-cell storms, multi-cell storms, multi-cell storm lines and supercell thunderstorms. A closely related weather phenomenon is the dry thunderstorm, characterized by frequent lightning strikes that cause raging wildfires in late summer and early autumn.
The University of Illinois Department of Atmospheric Sciences explains that single-cell thunderstorms are common and rarely dangerous. They occur frequently over land in spring and summer, but in cold weather they are most common over the oceans. Multi-cell thunderstorms usually form near mountain ranges and contain many small storms. Although the component storms are usually small and pass quickly, the aggregate thunderstorms often last for hours and cover large distances.
The most powerful type of thunderstorm is the supercell. According to the University of Illinois Department of Atmospheric Sciences, these storms often include hail, driving rains, ground lightning strikes and dangerous winds. Most large, destructive tornadoes spawn from supercells. Supercell winds regularly exceed 80 miles per hour and the hailstones they drop measure up to four inches in diameter. These thunderstorms are severe weather events capable of causing massive property damage, injuries and deaths.
The term "severe thunderstorm" applies to any type of thunderstorm that causes significant damage to land, property, humans or livestock. It is a subjective label with standards that vary by country. According to The Weather Channel, within the United States this moniker applies to storms that meet at least one of three criteria: winds exceeding 58 miles per hour, hail at least 3/4 of an inch in diameter and tornado spawning.