Source: Storm Prediction Center at the National Weather Service

Although tornadoes can happen at any time of year, they are especially common during the spring and early summer. May and June are the peak months in terms of numbers of tornadoes, but April appears to be the deadliest month—an average of 27 tornado deaths occurred during this month between 1950 and 1999; and one of the all-time worst tornado disasters occurred on April 3-4, 1974, when 148 twisters struck 13 states causing more than 300 deaths.

A tornado is a dark funnel-shaped cloud made up of violently rotating winds that can reach speeds of up to 300 m.p.h. The diameter of a tornado can vary between a few feet and a mile, and its track can extend from less than a mile to several hundred miles. Tornadoes generally travel in a northeast direction (depending on the prevailing winds) at speeds ranging from 20-60 m.p.h.

Tornadoes are most often spawned by giant thunderstorms known as "supercells." These powerful, highly organized storms form when warm, moist air along the ground rushes upward, meeting cooler, drier air. As the rising warm air cools, the moisture it carries condenses, forming a massive thundercloud, sometimes growing to as much as 50,000 ft. in height. Variable winds at different levels of the atmosphere feed the updraft and cause the formation of the tornado's characteristic funnel shape.

The conditions that lead to the formation of tornadoes are most often met in the central and southern U.S., where warm, humid air from the Gulf of Mexico collides with cool, dry air from the Rockies and Canada. This area, dubbed "tornado alley," extends roughly from the Rocky Mountains to the Appalachians, and from Iowa and Nebraska to the Gulf of Mexico. Tornadoes can also occur elsewhere, though, including all U.S. states, Europe, Asia, and Australia.

The Fujita scale classifies tornadoes according to the damage they cause. Almost half of all tornadoes fall into the F1 or "moderate damage" category. These tornadoes reach speeds of 73-112 m.p.h. and can overturn automobiles and mobile homes, rip off the roofs of houses, and uproot trees. Only about 1 percent of tornadoes are classified as F5, causing "incredible damage." With wind speeds in excess of 261 m.p.h., these tornadoes are capable of lifting houses off their foundations and hurling them considerable distances.