A tornado is a violently rotating column of air that extends from a thunderstorm cloud down to the ground. Picture a massive, spinning funnel made of wind, sometimes appearing gray or black from all the dust and debris it picks up as it tears across the landscape.

Tornadoes form when different layers of air moving at different speeds create a horizontal spinning effect, which a powerful thunderstorm then tilts vertically into that distinctive funnel shape. The spinning winds can reach over 300 miles per hour in the strongest tornadoes, powerful enough to lift cars, demolish houses, and strip bark from trees. Most tornadoes are much weaker, lasting only a few minutes and causing limited damage, but the strongest ones can stay on the ground for over an hour, carving paths of destruction dozens of miles long.

The United States experiences more tornadoes than any other country, with “Tornado Alley” (a region stretching from Texas up through Oklahoma, Kansas, and Nebraska) seeing the most activity. People who travel to observe and study tornadoes are often called storm chasers, and they follow severe thunderstorms to gather data that helps meteorologists predict when tornadoes might form.

When weather forecasters detect that a tornado has been sighted or indicated by radar, they issue a tornado warning, which means people should immediately seek shelter in a basement or an interior room away from windows.