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 explained as being due to a slowly moving point at which intense discharge is taking place; but this explanation is merely a possibility, not an established fact.

Occurrence of Thunder-storms.—Roughly speaking, the lower the latitude of moist regions, the greater the frequency of thunder-storms. In general, of two regions of the necessary warmth, one having moist air and the other dry air, the former is more likely to be visited by thunder-storms. They are more prevalent in the United States than in Europe; they are more prevalent in the southern part of the United States than in the northern part, so far as the region east of the Rocky Mountains is concerned.

According to A. J. Henry of the United States Weather Bureau, the regions of greatest frequency are in Florida, where thunder-storms occur 45 days in the year; in the central Mississippi Valley, where they occur 35 days in the year; and in the upper Missouri Valley, where the average is 30 days in the year. Thunder-storms rarely occur in the Pacific Coast states, but they are common in the Plateau Region.

Practically all the violent thunder-storms of the United States occur in the warm months. By far the greater number occur in June, July and August, during the hottest part of the day. There is also a period of minor frequency between 9 o’clock at night and midnight. Over the sea, however, the period of frequency is apt to be in the early morning, before daylight. Occasionally the updraught of the ordinary cyclone may produce a thunder-storm; the thunder-storms of winter are of this sort and they are rarely severe.

Pressure Waves.—The accompanying barogram, recorded at the Mount Vernon Meteorological Laboratory, illustrates pretty clearly the progress of a thunder-storm. The barometer had fallen steadily for more than twelve hours preceding the storm; and this continued until well along in the afternoon. The slight rise of the barometer in the morning is the diurnal pressure wave. The jump in pressure in the afternoon is the characteristic “thunder-storm nose” which usually is found on barograph records of thunder-storms. An expert observer does not need to refer to his daily reports to find the records of thunder-storms; the barograms show them in most instances. The rise in pressure occurs when the descending wind lifts