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 with the map of rain frequency published by the Weather Bureau. From this table a coefficient of the probability of rainfall for the particular station may be deduced by dividing the number of rainy days by 365, or 366, as the case may be. In a similar manner, the probability of rain for each month may be established. It is pertinent to add, however, that forecasts made from such coefficients are by no means certain; often they are disappointing.

The average duration of rainfall may be deduced by dividing the total number of hours during which rain has fallen for the month by the number of rainy days. If the duration is to be based on the average length of storms, the number of storms may be taken as the divisor. In the northeastern part of the United States the average duration of rainstorms is five hours; in the southeastern part, four hours; in the western highland region, including the plains, about three hours; and in the basin region probably not more than one hour. The intensity, or rate of rainfall per hour, is a matter of great importance. It is tabulated at regular intervals at Weather Bureau stations.

It is well to bear in mind that the artificial production of rain is a delusion. No appreciable fall of rain will occur unless a continued updraught of air is produced, and neither cannonading nor explosions at a considerable height has accomplished this. Possibly the conjunction of planets may affect the movement and the formation of storms; if so, however, the connection has not been established.

Secondary Storms; Tornadoes.—When a whirlpool forms in a stream, smaller whirlpools almost always occur near its edge. These secondary whirls result from the formation of the larger whirl. Similarly, secondary whirls of the air are very apt to accompany the cyclonic storms which pass over the Great Lakes and down the St. Lawrence Valley. In the winter the secondary storms thus formed appear along the Virginia coast, or perhaps to the north of it, and move north or northeast with heavy snow squalls and high, gusty winds. In the summer they are attended by hailstorms, thunder-storms and tornadoes. These occur usually on the south or the southeast side of the low.