_Chinook Winds._--On the Great plateaus adjacent to the Rocky Mountains, and in similar situations to the eastward of the Sierra Nevada and Cascade Mountains, warm, drying winds frequently occur, especially in winter, when they bring a balminess as of spring. The remarkable feature of these interesting winds is that they come from the snow-clad mountains, but are warm and dry in contrast with the preceding condition of the air on the plains. The capacity of the air brought by these winds for moisture is so great that evaporation is active, and the snow in the valleys and over the broad plains disappears without visible melting. The change in the previously winter aspect of a region within the influence of these _chinook_ winds, as they are termed, is truly surprising, and to their influence is due to a marked extent the value of the Great plateaus as stock-ranges, for the reason that the snow is removed from them so as to allow cattle to feed on the naturally dried grasses. The chinook winds are the counterpart of the _foehn_ winds of Switzerland, and are explained on the principle that descending air is made more dense by the increased pressure to which it is subjected, and its temperature correspondingly raised, its capacity for moisture being at the same time increased on account of its rise in temperature. The apparent anomaly of a warm, dry wind blowing from a snow-clad mountain range is no longer a mystery, if we consider that the air is drawn over the mountains towards a centre of low barometrical pressure owing to the wide-reaching influence of a cyclonic storm or other large atmospheric movement. The air as it rises in order to cross a mountain is cooled, largely on account of relief of pressure, and parts with a portion, possibly a large portion, of its moisture, which condenses on the mountain commonly as snow; on passing the mountain the air descends and is warmed by compression, and having less moisture than before, becomes a drying wind, which produces the sudden and surprising changes on the plains and valleys to the leeward. The chinook winds of the western portion of Canada and the United States occur principally to the eastward of high mountains, for the reason that the prevailing air-currents of that region are from the west.