Page:Handbook of Meteorology.djvu/109

 pressure, and the belt of descending air. The region covered by it is one of calms over the sea and of light, variable winds over the land. Along the Pacific Coast from San Diego almost to Manzanillo, Mexico, the yearly rainfall is very light and uncertain. Along the Atlantic Coast of Africa, the region of calms is a desert. Eastern Mexico receives a more generous rainfall; and southern Mexico and the Central American states are within the zone of rain-bearing winds.

Southern coasts in general have an abundance of rain. Along the Indian Ocean and the Guinea coast the rainfall is seasonal. On the Gulf Coast of the United States rain falls pretty evenly throughout the year.

The Precipitation of Cyclonic Storms.—Cyclonic storms, or lows, are rather more frequent in the United States and Canada than in Europe. They are much more frequent in occurrence east of the Western Highlands, and they also are much more energetic.

Rather more than half of the storm-whirls of this sort are noted first between the Columbia River and the Strait of Juan de Fuca — not because they do not occur elsewhere, but for the reason that there are fewer weather stations north of Vancouver. When a low is crossing the mountains it is in a region of dry air; therefore it does not possess much energy. When it has crossed the Rocky Mountains it is in a region where both the absolute and the relative humidity are greater. Therefore it is apt to develop a much greater energy; for the latent heat set free by the condensation of water vapor is the fuel of a cyclone. The northerly cyclone usually traverses the Great Lakes, where the increased humidity imparts greater precipitation, finally moving out into the Atlantic. As a rule, the rainfall of such storms is not very heavy. It may drop a little more than 1 inch of rain over the track of its passage, but usually the precipitation is not much greater.

The more southerly cyclones frequently bend towards the Gulf of Mexico, and begin to curve towards the northeast after passing the Mississippi River. They are much more energetic than the northerly storms and drop perhaps as much as 2 inches of rain along their tracks. In various instances a storm first discovered in the plains of Texas finally travels a course between the St. Lawrence River and the coast.