The basic definition of a thunderstorm is a storm containing lightning and thunder. Thunderstorms come in a variety of shapes and sizes, but there are four categories in which thunderstorms generally fall. They are: single cells, multi-cell clusters, multi-cell lines, and supercells. Each category is unique and can be either visually identified in the field or by looking at radar images. Below are basic definitions and examples of these kinds of thunderstorms.
|Figure A: Single Cell Thunderstorm|
Single cell thunderstorms are sometimes referred to as "airmass", âpopcornâ, or âpulse" thunderstorms, and usually do not produce severe weather. They often form in regions of warm, moist air with no strong fronts in the area. Each thunderstorm follows the life cycle of a typical thunderstorm with development, maturity, and decay occurring over a period of 30 minutes to an hour. They may contain heavy rain and can also produce occasional downbursts, small hail, and (rarely) weak tornadoes, but these are fairly rare in single cell storms.
|Figure B: Multi-cell Clusters|
|Image from NC State Climate Office|
Multi-cell clusters are basically what the title says; a cluster of thunderstorms that form and move together. But how do multi-cell clusters form?
When a single cell thunderstorm forms, it produces a downdraft that can create a gust front of cool air near the ground. The gust front contains denser air than its surroundings, and as it spreads out, it âliftsâ the surrounding warm air around it. That âliftâ may be just enough to create another updraft which would lead to the formation of one or more additional thunderstorms. This cycle can repeat itself for hours before the cluster of thunderstorms finally dies down. Much like single cell thunderstorms, multi-cell thunderstorms are usually just heavy rain makers, but can produce hail, strong winds, and occasional tornadoes.
|Figure C: Squall Line Thunderstorm|
Multi-cell lines are like multi-cell clusters except that they form in a line rather than in a group together. These lines of storms are often called squall lines and they can stretch up to several hundred miles long. The gust front in squall lines is focused mostly ahead of the storms rather than spreading out in all directions. The most dangerous feature with squall lines are often the winds, as winds with these lines can sometimes exceed over 70 mph! Other threats include isolated tornadoes, hail, and flash flooding. Squall lines most often form in the warm, moist air ahead of an advancing cold front and can move through several hours before the main front passes.
|Figure D: Supercell Thunderstorm
Supercells are the most dangerous kind of thunderstorms as they can produce large hail, dangerous winds, and violent tornadoes. In 2009, a supercell in Aurora, Nebraska dropped hail so large that it left holes in a roof of a house that were big enough to crawl through! Other supercells that day produced tornadoes in Kansas and Nebraska, but luckily no one got hurt. Thankfully, supercells are one of the rarest kind of thunderstorms that exist, and only form under certain conditions. What makes a supercell unique from all other thunderstorms is the presence of a mesocyclone. A mesocyclone is basically an area of strong updrafts which spin as the air moves upwards. The main way mesocyclones form are from the wind shear in the atmosphere. On radar, supercells can often be seen in a typical "hook echo" pattern, which shows a rain-free area right where the strongest updraft is occurring. The large hail and/or tornadoes are most often found in the wrap-around hook near the back of the storm as it is seen on radar.