The effect of relief upon movements of the air and thus upon tem perature is so common that most people have noticed it. At night, for instance, one feels chilly in a hollow, and then is surprised that after going one or two hundred feet up hill the temperature becomes so warm that one feels quite comfortable.
How Relief Influences Rainfall.—Aaide_fmm cyclonic storms and the great equatorial belt of low pressure, the relief of the lands is the chief cause of rainfall. When a wind reaches a mountainous region the slopes force it to rise. As we have seen in the equatorial belt of low press e and elsewhere, rising air expands, cools, and loses part of its capacity to hold moisture. Hence clouds form, and rain or snow falls. The process is illustrated in Fig. 69. A good example is seen in the western United States. The shaded part of Fig. 80 shows the altitude of the land from the Pacific Ocean eastward to central Nevada. Where the westerly winds laden with water from the Pacific Ocean strike the low hills at San Francisco the rainfall in creases from 18.5 inches to about 23 because the air rises and hence grows cool. Beyond the hills the rainfall decreases a little, but at the foot of the Sierras, where the air once more ascends, it increases rapidly to more than 50 inches. Beyond the mountains part of the air descends down the eastern slope. The descent compresses and warms it, so that its capacity for and it sucks up moisture instead of giving it out. Hence at the eastern base of the Sierras there would be no rainfall were it not for occasional cyclonic storms which raise the air to high levels. Thus Reno gets 6 inches or rain and Wadsworth a little over 4.
Regions like Nevada, lying to the leeward of the mountains and thus sheltered from rain-bearing winds, are said to be in the "rain shadow." Places in a rain-shadow get little rain; ju,st as places in.an
ordinary shadow get little sunlight. The rain-shnow often causes deserts where scraggly little bushes at wide intervals replace the splendid forests which lie at the same altitude on the windward side.
The Wonderful Effect of the Himalayas on Rainfall.—The Him alayas furnish the most remarkable example of the effect of moun tains on rain. The southerly monsoon winds from the Bay of Bengal bring an abundant supply of water whip they deposit as they rise over the lower slopes of the mountains. At a place called Cherra punji, 4000 feet above the sea and not far north of Calcutta, the aver age rainfall each year is 466 inches. Compare this with the part of the United States east of the Mississippi where the average is only a little over 40 inches. In 1861 the enormous amount of 918 inches, or 761 feet, actually fell at Cherrapunji. More than a third of this, or 372 inches, fell in July alone, and 421 inches in one day. Think of it. As much rain in one day as most Americans see in a year. The heavy rains wash all the soil from the slopes and leave only naked rock, practically bare of vegetation. Yet in the flat places there is a perfect tangle of trees and vines, and plants grow as much in a month as they do with us in a year, for even during the rainy period there is some sunshine almost every day.
At higher altitudes on the same side of the Himalayas the rainfall greatly diminishes. The air has lost so much moisture that it cannot give up much. Hence here, as on the windward slope of every moun tain, the rainfall increases only up to a certain level after which it decreases. Beyond the Himalayas the air has been so robbed of moisture that vast regions in central Asia are deserts. They lie in the world's greatest rain-shadow.
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