An Introduction to Supercells and Tornadoes

 

The American Meteorological Society (AMS) is an international community dedicated to advancing weather, water, and climate science and numerous related services in those areas. At www.ametsoc.org, the organization provides a comprehensive glossary of weather events and terms, such as supercells and supercell tornadoes.

A supercell is a potentially dangerous convective storm, commonly referred to as a thunderstorm. In the case of a supercell, the storm features a strong, quasi-steady rotating updraft that continues well beyond the time for an air parcel to travel the length of the updraft. An air parcel, similar to a fluid parcel, is an imaginary volume of air that can carry any or all properties of atmospheric air.

This type of updraft, known as a mesocyclone, allows powerful supercells to carry on for several hours under the right conditions. By comparison, the average thunderstorm lasts about 30 minutes. A supercell’s mesocyclone is often prone to cyclonic vorticity, meaning it is not uncommon for tornadoes to form in the mesocyclone. Doppler radar technology tracks the duration and vertical growth of mesocyclones to monitor tornado threats.

While the internal workings of a supercell sound chaotic, a supercell’s structure is simple compared to other storms, despite its size and power. It is the supercell’s architecture that allows the storm to last much longer than other thunderstorms.

As with many potentially dangerous meteorological events, supercells are difficult to predict. They most commonly form in environments that produce strong vertical wind shears, meaning wind speed and direction change with altitude in a concentrated area of the atmosphere. That said, it is not uncommon for cells to travel in different directions and produce different wind speeds than the surrounding wind shears. In these cases, the supercell has the potential to split into a right-moving cyclonic supercell and a left-moving anticyclonic supercell.

Supercells bring high winds, hail, and other forms of precipitation. Tornadoes that form in a supercell’s mesocyclone can grow into powerful, long-lived tornadoes in their own right.

In the winter of 1973, a supercell produced the San Justo tornado in Santa Fe, Argentina. It developed into the most powerful tornado in the history of the entire Southern Hemisphere. The F5-rated tornado brought winds that approached 250 miles per hour. Damage included 63 deaths, 350 injuries, numerous buildings, and more than 500 homes. Despite spawning from a supercell, the San Justo tornado lasted just seven minutes, reaching F5 intensity within two minutes of formation.

Supercells are not graded in the same way as hurricanes and tropical storms, as the various characteristics of individual storms can overlap with one another and change over time. Meteorologists sometimes use the terms low precipitation supercell, high precipitation supercell, and mini-supercell, also known as a low-topped supercell.

Any environment capable of producing thunderclouds involves rising currents of warm air alongside falling currents of cool air. These streams combine to form spinning air currents. Currents begin spinning in a horizontal direction but have the potential to take on a vertical orientation and fall from the cloud, taking on the properties of a tornado.