Page:Handbook of Meteorology.djvu/48

36 temperatures are never high; towards the foot-hills they occasionally exceed 100°.

East of the Rocky Mountains moist, southerly winds are common during the summer months, and occasionally these extend to the northern border. In the southern half of the United States the prevailing winds are persistent, moist, and hot. In the northern part they are not so moist, but very warm. Rhode Island and Delaware possibly excepted, summer temperatures of 100° and over occur in every state, when hot westerly winds prevail for a few days.

It is obvious that sea winds are more equable in temperature than land winds. Thus, summer days in San Francisco do not often reach 90°, and freezing weather occurs perhaps two or three times in a decade. When such temperatures occur they come almost always with land winds. The normal wind at this station is from the Pacific Ocean. In New York City, on the other hand, prevailing winds are land winds; and within a period of eight months a range of 115 degrees, &minus;13° and 102°, has occurred.

Temperature and Radiation.—Very dry, clear air permits the sun’s rays to pass readily to the earth with but little perceptible loss—that is, dry, clean air is diathermous to the heat rays that impart the feeling of warmth to living bodies. The heat is in turn absorbed by the earth. Earth temperature at the surface, or to a depth of an inch or two, may be many degrees higher than that of the air. Thus, in desert regions, pieces of metal lying on the ground in the sun become so hot they cannot be held in the hand. All this is due to the absorption of rays to which the air is diathermous.

But if dry, clean air permits excessive absorption, it also