Tornadoes in the United States

Abstract

IN the Monthly Weather Review for last December there is a preliminary report on the frequency of tornadoes in the United States during 1931, on the number of deaths caused by them, and on the material damage. This report shows that in the past year tornadoes were notably scarce compared with other recent years, and did less damage than usual; also that, what is even more satisfactory, they caused less than half the number of deaths recorded in any of the previous fifteen years. Even so, the death roll, resulting from about ninety of these visitations, reached 34, of which no fewer than 14 occurred last December. The unit in which material damage is expressed is the sum of a thousand dollars, and in five months the total damage thus expressed ran into three figures; September was outstanding with 828,000 dollars' worth of damage caused by twelve or thirteen tornadoes. The figures for monthly frequency of occurrence of the storms show a characteristic early summer maximum, with a total of 32 storms in May and June, that is, of more than a third of the year's total. It may be noted that although the tornado and the commonest type of summer thunderstorm are probably both caused by local instability due to the juxtaposition of air masses of different temperature, of which the warmer generally carries a great quantity of water vapour, they must be regarded as in some way essentially different in their modes of origin.