Page:Handbook of Meteorology.djvu/107

 in an overhead position rain is falling in heavy showers. The passage of the cloud-belt north and south provides an unusual and an interesting distribution of rainfall. Roughly speaking, the cloud-belt halts and turns backward in the latitude of each tropic; in these latitudes, therefore, there is theoretically one rainy season each year. Between the tropical circles the cloud-belt passes twice, resulting in two rainy and two dry seasons—in some localities strongly marked, in others not so distinguishable.

South of the equator the zone of constant rains is not so well marked as it is north of the equator; moreover, the belt of easterly winds at times covers the whole of the Gulf of Mexico. In general, the lands of the Torrid Zone receive an average of about ioo inches of rain per year—rather more in the northern than in the southern part.

In the temperate zones, on the coasts facing westerly sea winds, the rainfall for the greater part is seasonal. Along the Pacific Coast of the United States, it increases with the latitude. Thus at San Diego, California, the annual fall is 10 inches; at Los Angeles, 16 inches; at San Francisco, 23 inches; at Portland, Oregon, 45 inches; at the coast stations of Alaska from 80 inches to more than 100 inches. At San Diego practically all the rain falls between October 15 and April 15; in the seventy-two years from 1850 to 1912, a shower amounting to more than a trace of rain fell only twenty times in July. In San Francisco there is an average of about one rainy day in July; in Portland, Oregon, the July rainfall averages 0.54 inch; in Seattle it is 0.69 inch.

On the west coast of Europe, owing chiefly to higher latitude, the seasonal character of the annual rainfall is not so marked as in California. In Portugal and southern Spain, most of the rain falls in the winter months. At Bordeaux, a little higher in latitude than Seattle, the July rainfall is in excess of 2 inches every month in the year. But while the average of the winter months in Seattle is above 5 inches, in Bordeaux it is about 3 inches.

The thirtieth parallel crosses the northern part of Mexico and also the northern part of Africa. A zone several hundred miles in width along this parallel—in sailors’ vernacular, the belt of “horse latitudes”—is the region of high barometric