by Ann Marie Imbornoni
Average number of tornadoes per year (1950–2010): 1,253
Top 5 states with highest incidence of tornadoes: Texas, Oklahoma, Kansas, Florida, Nebraska
States with lowest incidence of tornadoes): Alaska, Rhode Island, Hawaii, Vermont, Oregon
Most tornadoes in one day: 209 (April 2011)
Most tornadoes in one month: 817 (April 2011)
Most tornado deaths in one year: 550 (2011)
Fewest tornado deaths in one year: 15 (1986)
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. Two of the all-time worst tornado disasters occurred in April. On April 3-4, 1974, 148 twisters struck 13 states, causing more than 300 deaths, and on April 27, 2011, 137 reported tornadoes swept through the south, killing nearly 300 people in six states. Most of the fatalities occurred in Alabama. May holds the record for experiencing the most tornadoes. In May 2003, there were 543 recorded tornadoes. On average, 60 people die each year as a result of tornadoes, mostly from flying and falling debris.
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.
Did you know?
April is the deadliest month for tornadoes [more]
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.