The intensity of the sun’s rays at sunrise and at sunset is less than at midday because the quantity of heat received at the outer limits of the atmosphere on a given area, as for instance at the area of the upper ends of the blocks in Figure 25, passes through a deeper stratum of air the lower the angle of incidence, and because it is distributed over a larger area when it reaches the surface of the earth. As the heat of day increases from morning until midday and then decreases, so does the heat of the year increase from midwinter to midsummer and then decrease, and for the same reason: change in obliquity of the sun’s rays, to which must be added change in distance from the central luminary. Figure 26 shows that the sun reaches its greatest midday altitude on June 21st and its least on December 21st. [Illustration: FIG. 26.—Observer at center of picture at latitude 45°. Showing altitude attained by the Sun at midday and length of its track above the horizon at the Summer and Winter Solstices and at the two Equinoxes.] =Solar Rays Absorbed by the Atmosphere.= The atmosphere of the earth absorbs about seventy-six per cent. of the solar rays that pass through it. About one half is absorbed by a cloudless atmosphere, and nearly all is absorbed or reflected away by a cloudy air. On the average about fifty-two per cent. of the earth’s surface is obscured by clouds all the time, which reduces the total amount of heat that reaches the earth to but twenty-four per cent. But in regions like the high plateau of the Rocky Mountains, where there is little cloudiness or moisture in the air, fully fifty per cent. reach the earth. At the equator, when the sun is in the zenith at noon, the rays strike the earth perpendicularly and reach the earth through the shortest air distance possible; but for latitudes far north or south of the equator, the rays are more oblique and must pass through an ever-increasing thickness of air as the latitude increases. Consequently the heat that reaches the earth at high latitudes decreases, not only on account of the greater obliquity of the sun’s rays, but also because of the longer path of atmosphere traversed, which causes a further loss by absorption.