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44 of the earth as a whole. The fact that the pressure over the southern hemisphere is lowest at the time when it is highest over the northern hemisphere, and vice versa, indicates the shifting of an enormous volume and weight of air from one hemisphere to the other, twice a year. Davis estimates that the weight of air thus moved is equivalent to a pressure of 0.12 inch (4.1 mb), or between 30 and 35 million tons in weight.

Density.—It is evident that a close relation exists between the temperature, pressure and density of the air. With lowering temperature, the volume of air contracts, and air flows in to equalize the loss. The cold air contains a greater number of molecules per given volume and therefore its density is increased; because it contains more matter per given volume its weight, and therefore its pressure is increased. Density varies directly with pressure and inversely with temperature. It also varies inversely with the moisture content of the air.

For the greater part, human activities are carried on at the plane of contact, where earth and air meet. Weather science, however, includes a study of density and pressure at all observable altitudes from sea level upward. Density of the air at different heights also affects air flight and the flight of projectiles. Hence, a knowledge of the density of the air at different altitudes is necessary.

The changes in the density of the air are most marked at the earth’s surface. The daily range in density may be as much as 10 per cent, and the extreme range in a year has exceeded 20 per cent. The range is greatest in temperate latitudes at or near sea level. The changes in density due to temperature variations explain the high midwinter pressure over inland regions and the high midsummer pressure in oceanic regions; the rock envelope of the earth radiates its heat more rapidly than does the water. Very low temperatures in winter increase the density of the air. High temperatures in summer decrease the density, with the result that oceanic regions are cooler in summer and warmer in winter than far-inland continental regions.

Diurnal and Semi-Diurnal Changes in Pressure.—At stations of considerable elevation a maximum daily pressure at the warmest part of the day is observable. It is attributed to the heating of the air, thereby causing an accumulation which