As applied to moisture-laden and near-surface winds, the effective agents of adiabatic cooling are the upland areas upon the continents, and especially the ranges of mountains. These barriers force the moving clouds to rise, cool, and deposit their moisture. It is, therefore, the highland barriers which face the oncoming, moisture-laden winds that receive the heaviest precipitation. Within temperate regions, because of the prevalence of westerly winds, those barriers which face the western shores receive the heaviest fall. Within the tropics, on the other hand, it is the barriers facing the eastern shores which, because of the easterly “trades”, are most favorable to precipitation. Thus it is in the Sierra Nevadas of California, and not in the Rockies or the Appalachians, that the glaciers of the United States are found. The highland of the Swiss Alps lying likewise athwart the “westerlies” of the temperate zone acquires the moisture for nourishment of its glaciers from the western ocean—here the Atlantic (Fig. 291). Within the tropics the conditions are reversed, and it is in general the ranges which lie nearer the eastern coasts that are the more favored. If no barrier is found upon this coast, the clouds may travel over vast stretches of country before being arrested by mountains and robbed of their moisture. Thus in tropical Brazil the glaciers are found in the Andes upon the Pacific coast though nourished by clouds from the Atlantic. [Illustration: FIG. 291.—Map showing the distribution of existing glaciers, and the two important wind poles of the earth.] =Sensitiveness of glaciers to temperature changes.=—How sensitive is the adjustment between snow precipitation and temperature may be strikingly illustrated by the statement on excellent authority that if the average annual temperature of the air within the Scottish Highlands should be lowered by only three degrees Fahrenheit, small glaciers would be the result; and a moderate temperature fall within the region surrounding the Laurentian lakes of North America would bring on glaciation, otherwise expressed as a depression of the snow line of the region. =The cycle of glaciation.=—Though to-day buried beneath its ice mantle, it is known that Greenland had more than once in earlier geological ages a notably mild climate, and in some future age it may revert to this condition. In other regions, also, we have evidence that such a rotation of climatic changes has been successively accomplished, the climate having steadily increased in severity towards a culminating point, and been followed by a reverse series of changes. Such a complete period may be called a _cycle of glaciation_. While the climate is steadily becoming more rigorous, we have to do with an _advancing hemicycle_ of glaciation, but after the culminating point has been reached, the period of amelioration of climate is the _receding hemicycle_.