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Occurrence
The greatest incidence of tornadoes is generally assumed to be in North America, and especially in the Mississippi Valley. On an equal-area basis, however, other countries, such as Italy, New Zealand, and the United Kingdom, exceed or at least challenge the incidence rate of the United States. In actual numbers observed, Australia, with about 20 annually, ranks second to the United States. The United States is notable for the incidence of severe tornadoes of scale 4 or 5. Tornadoes occurring in the tropics are usually extremely weak and often begin as waterspouts. The Stockholm and Saint Petersburg areas appear to be the northernmost regions that experience tornadoes.
Within the United States, Texas records the greatest number because of its size, usually about 15 to 20 percent of the nation's annual total of about 1,000. On an equal-area basis, however, Texas ranks ninth, far behind Oklahoma, Kansas, and Massachusetts. A rather steady increase in the annual total has been observed, probably as a result of the improving reporting system. The seasonal maximum occurs in spring and early summer, although tornadoes have been reported in all months. The height of activity in early spring is in the southern United States. Later it occurs in more northerly regions and, in July, in western Canada. Tornadoes occur most frequently during the middle and late afternoon. There is a large interannual variation, as well.
http://www2.scholastic.com/browse/article.jsp?id=5185
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