=Different Temperatures with the Same Quantity of Solar Heat.= On the same day of each year at the same place practically the same amount of heat falls upon and into the earth’s atmosphere from the sun, but rarely does the same temperature and weather occur, and often there is wide variation in the weather of the same day of two different years. The first of July may be cold enough to wear an overcoat at midday, or the first of January may be so temperate as to permit the donning of summer habiliments, while, according to the amount of heat received from the sun, there would have occurred the usual seasonal conditions on the days named had there been no other influence than the direct action of the sun’s heat. The cause of these seeming inconsistencies is due to the motions of the atmosphere in a stratum only five to seven miles in depth, air cooling by expansion as it ascends in cyclonic whirls and heating as it descends in anti-cyclonic movements. Condensation, in the form of cloud or rain or snow, also introduces complications, usually producing a cooling effect in summer and a warming in winter. In other words: interference in the uniform and gradual change in temperature, of the lower stratum of air in which we live, from the heat of summer to the cold of winter, and then the reverse process, is due entirely to the heating and the cooling of the lower air by its upward and downward motions. [Illustration: FIG. 22.—Summer Solstice, June 21. North Pole leans towards Sun’s rays.] [Illustration: FIG. 23.—Winter Solstice, December 21. North Pole is dark now instead of light, as at Summer Solstice. Pole leans in same direction but Earth being on opposite side of its orbit rays come from opposite direction. Refer to Figure 24.] If the earth’s axis were vertical to the plane of its orbit all places on its surface always would have days of twelve hours each and the nights would be of the same length; sunshine would just touch both poles (Figure 21) throughout the entire course of the earth around the sun and there would be no seasons. One would need to change one’s location on the earth in order to get a change of weather, which would be monotonous and quite different from the active conditions of the atmosphere that we now enjoy. The whole conditions of life would be altered for the worse. You have seen a top tilt over to one side as it spun on the floor. In the same way the earth spins on its axis as it pursues its course around the sun without changing the direction towards which its axis points, as shown by Figure 24. [Illustration: FIG. 24.—Note that direction of axis does not change as Earth moves around Sun. This causes variation in area of surface illuminated. If axis were perpendicular to plane of orbit there would be no seasons.] [Illustration: FIG. 25.—As angle of incidence decreases from 90° to 10° the heat received on upper end of blocks is spread over greater area at bottom, and its temperature diminished. (Abbe.)]