=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. =Why Should Not Mountain Peaks Be Warm? They Are Nearer the Sun.= The absorption by the atmosphere of both solar and terrestrial radiation is greater in the lower levels of the air, where water vapor, cloud, and dust are the densest, while the transmission of both incoming and outgoing radiation is more rapid through the pure air aloft. Thus we account for the coolness of all mountain peaks, and the perpetual freezing temperatures of some, even though they be located in the tropics, and though their tops occupy positions several miles nearer the sun than the bases from which they rise. =How the Earth Cools at Night.= Radiation from the earth goes on day and night, winter and summer. During daylight the gain of heat is greater than the loss, while at night the reverse is true. After sunset both the earth and the air continue to cool by radiation unchecked by the incoming heat of the daytime. The earth loses heat, even under a clear sky, more freely than the air, with the result that the surface of the ground and of vegetation may fall to a temperature ten to fifteen degrees lower than that of the air at a few hundred feet elevation. This condition is called “temperature inversion.” The greater difference will occur when there is little wind to mix the air. On a clear night the radiation outward will be rapid; then, if the wind be light, there may occur an increase in temperature up to a height of two hundred to four hundred feet, and then a fall, reaching the surface temperature at about two thousand feet elevation, unless the ground be wet, or the location be adjacent to a considerable body of water.