[AS. regent Water falling from the clouds in drops. This is the chief source of water-supply. By the heat of the sun water is evaporated from the surfaces of the seas and oceans and transported as water-vapor by winds. When it is condensed by cold, chiefly caused by the heated air rising into higher regions, it returns again to the liquid state, and falls down as drops of water in rain ; or, if the cold be very great, the water may pass at once into the solid state, and fall as snow or as hail. Rain-water is very soft, and in country places it is pure ; the air of large towns being full of impurities, the rain brings them down with it as it falls, and so purifies the air by washing it. In Britain the prevailing winds are westerly, and, being charged with moisture from the Atlantic, much rain falls on the western coasts, and pasture is abundant. In the eastern part of the United States, where there are no great mountains to catch the moisture, the rainfall is uniform ; but in the west and north the rainfall is determined by a centre of low atmospheric pressure in the Rocky Mountains. The heaviest rains occur in the tropics, and are confined to one part of the year called the rainy season. At a point 100 miles north of Calcutta the annual rainfall is from 500 to 600 inches. In Burmah the rainfall is 200 inches.