CLIMATE. The Alpine region is at the meeting place of the high marine climate of Western Europe. the continental climate of Central Europe, and the low-latitude marine climate of the Mediterranean regions. While it does not lie directly in the main path of the cyclonic disturbances which sweep across North ern Europe from west to east, yet it does lie within the sphere of influence of these storm centres. Moreover, during the spring• numerous extended cyclones pass over the Alpine region; but they are less frequent in the winter and fall, and are almost totally lacking in the summer. This is the chief reason for the steady cold of the Alpine winter, with but few intensely cold waves, the serenity of its summer climate, and the harshness of its spring weather. The aver• age annual temperature on the northern Alpine boundary at altitudes of 1500 feet is about 43° F., while the seasonal averages range from about 30° F. in winter to 65° F. in summer. In winter temperatures usually descend as low as zero F., and in summer rise as high as 90° F. On the southern Alpine boundary, at altitudes of about S00 feet. the average temperature for the year is about 54° F., the variations ranging front 35° F. in winter to 72° F. in summer; but in winter the temperature usually does not de scend below 15° F., and in summer may reach even 95° F. With increase of altitude above these regions there is on the average for the year a decrease in temperature of about I° F. for each 330 feet of altitude; but the rate of decrease is much more rapid in summer than in winter. The average daily temperature is remarkably uniform in the Alps: hut the temperature changes from day to night are excessive, on account of the intense action of the sun by day and the rapid cooling by radiation by night, as in all elevated regions. The absolute humid ity decreases with the altitude, and is greater in summer than in winter. The relative humid ity, and, consequently, the degree of cloudiness, is least in winter in the Alps, while in the sur rounding region the relative and cloud iness are usually greatest in winter.
On the north side the annual rainfall is from 25 to 40 inches; but this increases irregularly to about 90 inches on the southern side, where the steep slopes deflect upward the mois ture-laden warm winds from the Mediterranean Sea. The average annual rainfall for the whole region cannot he far from fit) inches, while that of the surrounding lowlands is less than 35 inches. Where the high maintains have a
copious rainfall on the windward side, the val leys on the leeward side experience a deficiency; so that on one side of a mountain range the rain fall may be many times that on the other side. Of the total annual rainfall throughout the Alps about IS per cent. occurs in the spring and about 25 per cent. in winter. in summer the pro portion decreases from 37 per cent. in the north ern part to 25 per cent. in the south; but in the fall, on the contrary, the proportion increases from 20 per cent. in the north to 33 per cent. in the south. In the higher Alps much of the pre cipitation is of course in the form of snow, which is carried down to lower levels by glaciers and is there melted. The snow' line in the Alpine moun tains undergoes an annual variation. reaching its lowest altitude, about 2000 feet, toward the end of January, and its highest altitude, in the neighborhood of 9500 feet, about the middle of august. The limit differs for the northern and southern exposures, the snow line on the south ern slopes lying over 150 feet higher in mid winter. and about 1300 feet higher in the early fall. At low altitudes of 2000 to 3000 feet, the snowy days munch exceed the number of (lays on which the ground remains snow-covered, but at altitudes of 8000 feet, the first snow commonly remains throughout the season of snow. The lower limit of perpetual snow is at an altitude ranging from 8500 feet to 9500 feet.
The general winds of the Alps follow the •cyclonie and anti-cyclonic laws, which give a veering through the south when the cyclones pass to the north, as they usually do, and through the north when the cyclones pass to the south. Local winds are very prevalent ; among these the mountain and valley winds. blowing upward from the valleys by day and downward from the mountains by night, are the most charac teristic. In the Central and Northern Alps occur these hot, dry winds called the fan. These are the result of descending air on the leeward side of the mountains after much of the mois ture has been condensed by the cold high up on the windward side. These fan winds, while a source of discomfort to the inhabitants, are welcomed in the spring, for they clear the ground of snow much more rapidly than the sun can ac complish it. Such is the evaporating power of the fan winds that it may cause two feet of snow- to disappear in half a day.