=Effect of Mountains on Climate.= The rarity of the atmosphere of mountains readily allows the rays of the sun to pass through it and thus the surface of mountains is quickly warmed, but the same conditions permit a rapid radiation at night, so that there are considerable extremes of temperature. Air cooled by contact with a mountain may flow down its sides at night and collect in depressions below, often causing frost on still nights where the temperature higher up is much above freezing. Mountains may be more cloudy and rainy than plains, for the currents of air that cross them must rise, and in rising they cool by expansion and often reach the dew point of the air, moisture being precipitated in the form of clouds, rain, or snow. Often a peak is constantly capped with a crown of clouds. Mountains may intercept vapor-bearing winds from oceans, force them to such an elevation that their vapor is largely precipitated on the windward side of the mountain, and receive them on the leeward side as dry, rainless winds. Vast desert areas are often the result. A good example is presented in the case of the Pacific coast mountains and the desert plateau to the east. Mountain peaks may be covered with snow, even though they be located in the tropics, if their elevation be sufficient. This is because the absorption of both incoming and outgoing radiation is so much greater in the lower reaches of the atmosphere, where the water vapor is densest. Wherever observations have been made they have shown that the temperature of the air on high mountain peaks and crests and for a distance of one to three hundred feet above them is cooler than adjacent free air of the same height, due to upward deflection of air currents and their cooling by expansion, and to radiation from the peak. The Himalayan Mountains exercise a profound effect on the climate of Asia. The monsoon (any wind that alternates annually in direction or force) of summer brings the moist air from the Bay of Bengal and precipitates torrential rains from it as it ascends to higher and higher elevations in passing over the great heights of the mountains. At a place four thousand feet above the sea and not distant from Calcutta, the annual rainfall is 466 inches, while the average for most of the region east of the Mississippi River is only forty inches. More than forty inches have been known to fall in one day in the Himalayan Mountains. As in the case of all very high mountains, the rainfall increases in these mountains up to a certain elevation and then decreases. North of the mountains the monsoon passes into the interior of Asia with withering dryness and vast deserts are the result. [Illustration: FIG. 28.—Average Monthly Temperature and Rainfall of Typical Places in North America. (Huntington and Cushing.)]