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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 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 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 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
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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.
successive years, 1916, 1917, and 1918.
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.
from which they are bred.