A powerful tornado on Sunday tore through the southwestern Missouri town of Joplin, killing over 20 people and leaving several injured.

The damage, which appeared to be the worst in this southwest Missouri city, caused substantial damage to St. John's Hospital and several apartment buildings as well as to other areas of the city.

The storm system that led to the Joplin tornado also generated twisters from Oklahoma to Wisconsin, leaving at least one person dead in Minneapolis.

Joplin is a city in southern Jasper County and northern Newton County in the southwestern corner of the U.S. state of Missouri and is the center of what is regionally known as the Four State Area: Oklahoma, Arkansas, Missouri, and Kansas.

On the flip side, Joplin is situated in Tornado Alley, a broad region where cold air from the Rocky Mountains and Canada collides with warm air from the Gulf of Mexico, leading to the formation of powerful storms.

A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone.

Tornadoes often develop from a class of thunderstorms known as supercells. Supercells contain mesocyclones, an area of organized rotation a few miles up in the atmosphere, usually 1-6 miles (2-10 km) across. The severity of tornadoes is measured by Enhanced Fujita Scale and most intense tornadoes would come between EF3 and EF5.

In May 1971, Joplin was hit by a EF3 tornado. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms.