Page:Handbook of Meteorology.djvu/119

 It has been pointed out that the cloudburst may be derived from the contents of a waterspout carried inland for a long way and dumped upon the nearest mountain crest which has a temperature low enough to chill it. This may be an explanation, but one is not certain that it is the real one. Any explanation must take into account the fact that an ordinary rain cloud cannot hold the moisture that is precipitated in a cloudburst.

Summer Precipitation.—The rainfall of summer months within the United States is rarely a result of cyclonic movements of the air. For the greater part, it is due to the updraughts that result from surface heating; and this also is the cause of most of the tropical rains. Summer showers are apt to be sporadic in character, and the area covered may be small. It is not uncommon to find a rainfall of 2 or 3 inches at one locality while scarcely more than a trace falls at another locality only a few miles distant. Occasionally the daily weather map shows half a dozen areas scattered over the eastern half of the United States in which rain is falling; less frequently, a belt 200 miles or more in width extending from the Gulf Coast to the Canadian border, sweeps eastward from the Mississippi Valley.

In summer the updraught, though strong, is more or less local, occurring over comparatively small areas. In winter the updraught, though weak, may involve an area more than 500 miles in diameter.