The new air world

by Willis L. Moore

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.

The Lag of Earth Temperatures. The solar rays reach their greatest intensity on June 21st, in the Northern Hemisphere, when the sun attains the farthest point north, and the obliquity of its rays is the least, but the highest temperature of the air for the year does not occur on the average for a month or six weeks later, due to the capacity of the earth and air to absorb heat; and the maximum for the earth does not occur until still later. The sun is the farthest south on December 21st, but the minimum air temperature of the year, on the average, does not occur until a month later, and at a later period in the earth. At Munich, Bavaria, at a depth of four feet, the minimum annual temperature occurs on the 2d of March, and the maximum on the 24th of August. For each increase of four feet in depth the time of occurrence of either maximum or minimum temperature is retarded twenty-one days, the minimum not occurring until the 23d of May at a depth of 20.2°, and the maximum being retarded until the 17th of November.

Annual Range in Air Temperature. The difference in temperature between winter and summer increases from the equator northward and from all oceans toward the interior of continents, and is greater in the middle latitudes on the eastern side of large bodies of land than on their western side. Yakutsk, Siberia, has experienced 80° below zero in January and 102° above in July, making a range of 182°. Dawson, Canada, has a record of 68° below for winter and 94° above for summer, making a range of 162°. In marked contrast with these large differences, shown in the northern interior of continents, is the annual range at Samoa, from a maximum of 92° to a minimum of 62°, a range for the year of only 30° for this island of the Pacific, located near the equator.

Reversal of the Seasons in the Two Hemispheres. The summer is shorter in the Southern Hemisphere than in the Northern and the winter is longer, but the Southern Hemisphere is nearer to the sun in the summer and farther away in winter, conditions that tend to add to the extremes of both seasons. Because of the slowness of the earth in passing through one half of its orbit, the northern summer lasts ninety-three days, while that of the Southern Hemisphere lasts but eighty-nine days. The result is that during like seasons and during the whole year the two hemispheres receive exactly the same quantity of heat.

Only Water Vapor Protects the Earth from Death by Freezing. In [Chapter IV] you are told that the earth is surrounded by four atmospheres that conduct themselves each quite independently of the others, and that water vapor (aqueous vapor) is one of them. Water vapor plays the most important part in absorbing incoming rays and in absorbing and reflecting back outgoing heat rays from the earth. Without the vaporous atmosphere the sun’s rays would be but slightly absorbed as they entered and radiation from the earth would readily escape through the atmosphere to outer space. No matter how fiercely the sun might shine, life on the earth would be entirely destroyed by cold.

When water vapor, clouds, or dust motes intercept certain portions of the sun’s rays, they change them from vibrations in ether to the motions of molecules, and the motions of these molecules are expressed in a rise in temperature in the vapor, cloud, or dust. Earth radiations of heat, having longer and slower wave lengths than those that come from the sun, are more readily absorbed by the atmosphere.

One of the principal functions of the atmosphere is to protect the earth from the intense cold of outer space, which must be near or at absolute zero—459° below the zero mark.