Tornadoes strike in many areas of the world, but nowhere are they as frequent or as fierce as in the United States. More than 1,100, for example, were reported there during 1973 alone. Direct comparisons of relative tornado frequencies in various countries are biased because observational data are often lacking in sparsely settled regions. It appears, however, that Australia, where several hundred per year have been reported, has the dubious honour of second place. Other countries reporting tornadoes include, but are not limited to, Great Britain, Canada, China, France, Germany, The Netherlands, Hungary, India, Italy, Japan, and even Bermuda and the Fiji Islands.
A vast "tornado belt" embracing the Great Plains of the United States and the southeastern portion of the country is threatened by tornadoes every year. Every state in the nation, including Alaska and Hawaii, has experienced twisters. The average frequency varies from more than 100 in Texas to fewer than three in the far western and most of the northeastern states.
A more important criterion for gauging tornado severity is the average number of tornadoes in a unit area, such as a square kilometre or a two-degree square on a map. Computations of this tornado density for a 45-year period show that the greatest concentration of tornadoes per unit area is found in the states of Oklahoma and Kansas. The area with the greatest potential for casualties is that which combines a high tornado incidence rate with a thick population concentration. Southwestern Oklahoma, for example, has the highest tornado incidence per unit area, but because it is thinly populated as compared with the Chicago area, which has less than one-half its tornado incidence, casualty potential for Chicago is much greater. Although the probability that a specific locality will be struck by a tornado in any one year is very small, this low probability does not mean that such an event will not happen. An extreme example of an apparent defiance of probability statistics took place in Codell, western Kansas, a small town hit by tornadoes on the same date, May 20, in three successive years, 1916, 1917, and 1918.
Tornadoes may occur during any month of the year. Normally, for the United States as a whole, the month with the most tornadoes is May; and more than half the year's total occurs during the three months of April, May, and June. The lowest frequency is in December and January. No season of the year is free from tornadoes, but in spring and summer they are five times as numerous as in winter and fall.
Tornadoes are generated from severe thunderstorms, which form readily between warm, moist air from the south or southwest and contrasting cool, dry air from the west or northwest. A squall line of severe local storms often develops along this boundary, and sometimes a family of tornadoes is spawned. In February, warm, moist air from the Gulf of Mexico begins to penetrate the Gulf states. As it continues its northern and eastern penetration, tornado frequencies reach their peaks during April over the southern Atlantic states, during May over the southern Plains states, and during June over the area extending from the northern plains to western New York state. In the summer and fall the decreasing contrast between the air masses results in a reduction of tornado incidence in most sections, although they may develop from the severe thunderstorms associated with unstable air masses, especially in Florida, where tornadoes are often associated with hurricanes. During December and January, cold air dominates the country, and the moisture-temperature relationships required for tornado genesis are not usually present.
Although tornadoes may strike at any hour of the day or night, they generally form during the middle or late afternoon, between 3:00 to 7:00 PM, the period most favourable for the development of severe thunderstorms from which they are bred.