GEOGRAPHICAL DISTRIBUTION OF RAIN. As all animal and vegetable life on the earth's surface depends more or less directly upon the quantity of rainfall and its distribution throughout the months of the year. therefore the geographical and seasonal distribution of rain claims ono• first attention. From the preceding section it is seen readily that the distribution of rain over the earth's surface must depend upon the influences that force air to ascend rapidly. Thus on warm clear days, when the surface of the ground or water is highly heated, the lower stratum of air acquires a decided upward motion by reason of its buoyancy. :Masses of hot air are rising while the cooler air near by is descending. Thunder storms are usually formed in this way,. and nearly every station in the torrid and temperate zones has a preponderance of local rains in the afternoon. Whenever an ocean breeze or a monsoon wind rises high enough on a mountain side it gives rise to cloud and rain, so that the ocean winds bring more rain than the land breezes. The finest illustrations of this principle are seen in the rains of the southwest monsoon in India, in the rains that fall with .southwest winds on the coast of Europe, or in the southerly winds with rain on the Gulf coasts of the United States. Again, when a moist warm wind meets a cold dry wind, the latter generally flows under and lifts up the former, because of the greater density of the cold air compared with the warm. Therefore above the cold air is
formed a layer of cloud and oftentimes of rain due to the rapid elevation of the warm air. Illustrations of this are to be found on the south east and southwest sides of the areas of low pressure that pass eastward over the United States throughout the year, and especially in the winter season. From the preceding it fol lows ( ) that every rising slope, whether of mountain ranges or interim• plains, should, other things being equal, show a greater rain fall as we proceed up the slope, and this distribu tion of rainfall with altitude has been found to agree with observations in Great Britain, Germany, India, the East Indies, and the United States; (2) considering the world at large, the heavier rainfalls should occur in regions where warm moist winds steadily impinge upon the mountain slopes. The above principles are exem plified in the rainfall charts contained in Bar tholomew's Physical _Atlas (London, I599), and especially in the twenty-six maps showing the monthly and annual distribution of the rainfall on the land surface throughout the globe com piled by A. J. Derbertson and published by the Royal Geographical Society of London in 1900. Iferbertson's charts of annual rainfall on the land surfaces are reproduced herewith.