Residents living in the Washington, D.C., suburbs are learning that tornadoes are not just a threat to the Great Plains. More than 200 people were injured and several homes and businesses were destroyed Monday when three twisters tore through Virginia. Gov. Tim Kaine declared a state of emergency as hazardous weather continued to plague the central part of the state, just one day after tornadoes slammed into St. Charles and Hyattsville in suburban Maryland. Hyattsville is just 10 miles (16 kilometers) from the nation's capital.

Tornadoes are rare but not unheard of in densely populated areas, as Atlanta residents discovered last month when a twister flattened 20 homes in the city's Cabbagetown neighborhood. In 1999 a tornado blew through Oklahoma City, killing 36 people. Most twisters, though, show a preference for wide-open spaces. Kansas, Oklahoma and Texas are particularly prone to such tempests, thanks to their location between the Gulf of Mexico to the south and the Rocky Mountains to the west, which provide ideal meteorologic conditions for them.

One common misconception is that, if the sky turns green during a thunderstorm, a twister (most likely accompanied by hail) is on the way. Although green skies have been observed during thunderstorms, there is little scientific evidence to support claims that hail and tornadoes are sure to follow.