Page:Handbook of Meteorology.djvu/34

22 by Abbott is 1.932 calories per minute, less the amount absorbed by the air.

The air, with its dust and its moisture content, intercepts a great deal of the heat radiated from the sun. When the sky is clear and the sun is overhead, it is found that a little more than two-thirds of the sun’s radiation reaches the earth, less than one-third being absorbed by the atmosphere. When the moisture content of the air increases, the value of the solar constant decreases. When the smoke pall that hovers over manufacturing centers thickens, the effect is the same. This also is true of any increase of atmospheric dust. The volcanic dust shot into the air by the eruption of Krakatoa lowered the value of the solar constant for a considerable length of time.

The moisture and dust content of the air acts as a blanket, intercepting and storing during the day a part of sun’s heat, and at night becoming a source of heat in itself.

The fixed constituents of the air, the oxygen and the nitrogen, vary so slightly in proportion and the amount of heat which they intercept, that their effects may be regarded as constant. The great changes in the effects of insolation are due chiefly to the varying proportions of the water vapor and the dust content of the air. The layer of water vapor is comparatively thin—practically not more than five or six miles. The dust blanket, on the other hand, may extend many miles into the upper air.