One of the most notable features in the cli mate of the United States is the great contrast between the Atlantic and Pacific coasts. This is due to the fact that the cold, dry air following each area of low pressure is confined in its movement southward between the Rocky Moun tains and the Appalachian ranges. Occasionally dry, cold air descends from the north or northeast upon 'The Great Valley' of California with dis astrous consequences, but in general the Pacific coast is tinder the influence of south and west winds, which bring abundant winter rains to northern California, Oregon, and Washington without severe cold weather. On the Atlantic coast the winter rains and snows are followed by very cold weather. The contrast between the mean temperatures on the immediate coast of the United States in January and July are shown in the following table. The temperatures here given are as read off on charts of isotherms, and refer to locations on the land at sea level near the shore. These temperatures are controlled largely by the prevailing winds and the adjacent oceans.
Of course a similar great contrast prevails between the temperatures on the east side of Asia and west side of Europe. but nothing like this occurs in the Southern Hemisphere, owing to the presence of the great southern ocean and the small area and low altitude of the lands.
Another striking peculiarity in the climate of the United States is the great contrasts of dry ness and rainfall. The heaviest precipitation and
also a fairly uniform humidity prevail on the coast of Oregon and Washington ; next to this are the humidity and rainfall of the coasts of Florida and the adjacent States. Throughout the Mis sissippi Valley and along the Atlantic coast from Cape Hatteras to Nova Scotia there are great fluctuations in moisture and evaporation. On the average the driest portion of the continent extends from southern California and Arizona eastward over the eastern slopes of the Rocky Mountain region. Not only is the rainfall here a minimum, but the humidity of the atmosphere is also very small and the proportion of the clear sky very large. Yuma, in Arizona, Santa Fe, in New Mexico, and Pueblo, in Colorado. have mean annual relative humidities of 43. 45. and 46 per cent, respectively. The amount of evapora tion from the soil is correspondingly large in these regions, and artificial irrigation is quite essential for successful agriculture. Even the summits of the mountains of this portion of the continent show hut little permanent snow, and that in sheltered spots. What would remain on the ground in an ordinary, moist climate is rapidly evaporated in this dry air. See map showing average annual relative humidity in the United States, accompanying IlumunTY. See ETEONOLOGY CLIMATE; ;STOIRNI.