Quantity of Solar Rays Absorbed by the Atmosphere.— On the average the atmosphere of the earth absorbs about 76 per cent of the total incident solar energy. About one-half is absorbed by a cloudless atmosphere, and nearly all absorbed or reflected away by a cloudy air. On the average 52 per cent of the earth's sur face is obscured by clouds all the time which, according to Abbot, reduces the total amount of insolation that reaches the earth to but 24 per cent.
Terrestrial Radiation. When solar radia tion is absorbed either by the gases of the air or by dust in suspension in it, or by the earth itself, the energy of the ether is transmuted into molecular vibrations of the matter that absorbed it. The temperature of the matter is raised and it itself sends out radiations in the form of heat-waves, which readily escape through the atmosphere unless intercepted in their passage by cloud or water vapor. There is no permanent increase in the temperature of the earth or atmosphere, therefore there must be a continual loss of heat in some way, and this is brought about by the outward radiation of earth and air toward interplanetary space. During the spring and summer the gain of heat by the atmosphere is greater than the loss; during the autumn and winter these con ditions are reversed.
Why Mountain Peaks are Cold.—The absorption of solar and of terrestrial radia tion by the air is greater in its lower levels, where dust, water vapor and clouds are densest, while the transmission of both incom ing and outgoing radiation is more rapid through the pure air aloft. Thus we account for the coolness of all mountain peaks.
Nocturnal Cooling of Earth and Air.— Terrestrial radiation goes on day and night, winter and summer. During daylight the pin of heat is greater than the loss, while at night the reverse is true. After the sun has passed below the horizon both the earth and the air continue to cool by radiation, unretarded by the counteracting effect of insolation. The loses heat, even under a clear sky, more freely than the air, with the result that the surface of the ground and of vegetation and the air in close proximity thereto may sink to a tem perature 10° or 15° lower, than that of the air at considerable elevations. This condition is called temperature reversion. It occurs both winter and summer. The greater difference will occur when there is but little wind to mix the air. On a clear night in summer there may
occur an increase in temperature up to a height of 200 to 600 feet, and then a steady fall, reaching the surface temperature at about 2,01SO feet elevation, unless radiation be dimin ished by saturation of the soil or the location be adjacent to a considerable body of Heating of Land and Water.— The thermal effect of a given quantity of heat falling upon various forms of matter differs with the substance; this is notably apparent when the matter is land, water or air. The same amount of heat will raise the tempera ture of a water surface only about one-fourth as much as it will raise the temperature of a land surface. Water rejects by reflection a considerable amount of the solar radiation that reaches it, while land reflects but a small part, and of that which falls upon the top layer of i water much is expended in the process of evaporation and does not impart warmth to the water. Solar radiation also penetrates water to a considerable depth and is quite uniformly absorbed by the whole stratum penetrated. These conditions give to large water surfaces and the air immediately over them a much lower temperature during the day and a higher temperature during the night, and also lower temperatures during summer and higher temperatures during winter, than occur over a land surface of the same latitude and approxi mate elevation.
Ocean Temperatures.— Because of the dis placement of oceanic isothermals by ocean cur rents it is impossible to name a definite tem perature as prevailing over oceans at all places on a given parallel of latitude. But in a gen eral way it may be stated that at the equator there is a surface temperature of 82° to 84° F., which changes less than a degree between and night, and not over 5° between winter and summer; at a depth of 400 fathoms the tem perature is 44° and unchangeable, and below 1,000 fathoms it is but little above the freezing point of fresh water—namely, 34° to 36°. At latitude 70° north the temperature has but little diurnal variation, and a yearly range of from 35° in winter to 45° for summer; at a depth of 400 fathoms it remains steady at 32°. From this level there is a gradual decrease to a depth of 1,000 fathoms, where a constant temperature of 28° exists, and below this to the bottom there is no change.