Fair weather over central portion of Pacific slope. Storm coming in upon coast of Washington Stormy weather over the western half of the United States] In winter the isotherms curve far to the north over the North Pacific and North Atlantic oceans; but over the intervening land they curve as much to the south. In summer the isotherms are almost reversed in position, at least as far as the land is concerned, for they bend to the north in the heart of the continent. There are important reasons for the slight variation of the isothermal lines upon the western borders of North America and Europe, and their great change of position in the interior from winter to summer, but these reasons are not at all difficult to understand. The temperature of large bodies of water changes but little throughout the year, for water warms and cools slowly. The surface of the land, on the contrary, heats rapidly, and then as quickly loses its heat with the changing season. The air over the ocean is cooler in summer and warmer in winter because of the influence of the water, but over the land, in districts far from a large body of water, the changes in temperature between day and night, summer and winter, are very great. It was formerly thought that the warm Japan current, which flows against the western shore of North America, was responsible for the exceptionally mild climate there, and that the Gulf Stream produced a similar climate upon the coast of western Europe. More careful study, however, has shown that not the warm ocean currents, but rather the winds blowing from the water, are the cause of the mild climate in those lands across which they blow. In temperate latitudes there is a slow movement of the air in an easterly direction, and in consequence the climate of the western coast of North America is not marked by such extremes in winter and summer as are the interior and the eastern sections. It is also surprising to find how nearly alike the average winter and summer temperature is at San Francisco. It is also surprising to note that the average temperature of Seattle differs so little from that of San Diego, although these two places are separated by sixteen degrees of latitude. In some places the climatic conditions which we should naturally expect seem to be reversed. Oranges are grown in the Great Valley of California as far north as Red Bluff, and actually ripen a month sooner than they do near Los Angeles, five hundred miles farther south. The early ripening of fruits in the Great Valley may be explained by the presence of the inclosing mountain ranges: the Sierra Nevada mountains upon the northeast shut off the cold winds of winter, while the Coast ranges upon the west break the cool summer winds which come from off the Pacific.