A tornado warning means that a tornado has been spotted on radar and cover should be taken immediately. A tornado watch is less severe than a warning, meaning that conditions are favorable for the development of a tornado, but one has yet to develop.
The National Oceanic and Atmospheric Administration issues tornado watches and warnings. Tornadoes are violently rotating columns of air. They are very dangerous storms that cause an average of 70 deaths in the United States per year. Tornadic winds have been registered as high as 250 miles per hour, and swaths of destruction a mile wide have been created by tornadoes that have stayed on the ground for over 50 miles.