The distribution of rainfall over the earth (using the word “rainfall” in the broad sense to include snow reduced to its water equivalent) is most conveniently described in terms of mean annual values. This element is very unevenly divided between different parts of the globe and between the different regions of every large country. The raininess or dryness of a climate is determined especially by the prevailing movement of moisture-bearing winds and the relief of the land, while a second important control is the location of a region with respect to storm tracks. The rainiest regions are found on the windward slopes of mountain ranges not far from the ocean, where the moist winds, forced by the mountains to ascend rapidly, cool dynamically and shed their moisture. Thus the southern slopes of the eastern Himalaya receive an enormous rainfall from the southwest monsoon, blowing from the Indian Ocean, and an abundant rainfall also prevails on the south slopes of the high mountains of eastern Tibet, while northern Tibet, in the lee of the mountains, is a desert. For more than half a century the little hill station of Cherrapunji, in Assam, at an altitude of 4,100 feet, has been credited with having the heaviest rainfall in the world. According to the latest official record, its average annual precipitation is 426 inches. Recently it has come to light that Cherrapunji has a serious rival in the Hawaiian Islands. A fragmentary record totaling 90 months, between 1911 and 1921, kept on Mount Waialeale (altitude 5,075 feet), in the island of Kauai, showed an average of 455 inches per annum, and there is reason to believe that a longer record will give an even higher average for this place. The spot in question, which has been described as “Uncle Sam’s dampest corner,” is so difficult of access that it can be reached only after a three-day trip on foot over mountain trails. Hence the United States Geological Survey has installed here a huge rain gauge--said to be the largest in the world--capable of holding an entire year’s rainfall, so that measurements need be made only once a year. The heaviest average annual rainfall in the United States (not including Alaska) is about 130 inches in Tillamook County, Ore. Over most of our Atlantic seaboard States the rainfall ranges from forty to fifty inches. Extensive tracts in southern California and western Nevada have a rainfall of five inches or less. Any region with an annual rainfall of less than ten inches is normally a desert, though irrigation or “dry farming” methods may enable its inhabitants to practice agriculture.