Curiosities of Heat

by Lyman Beecher Tefft

“The bottom, I suppose, for the higher a man goes up upon the lofty mountains or in a balloon, the colder he finds the air.”

“That is right; and we need to ascend only about three miles, even in the tropics, to reach the region of perpetual snow, while in the polar regions the line of perpetual freezing comes down to the sea level. What would be the effect, Ansel, if the atmosphere were as warm, or warmer, at the top than at the surface of the earth? How would that affect the rate of radiation from the earth?”

“It must, of course, increase the radiation very much. With the temperature twenty or fifty or seventy degrees below zero, the radiation must be very little.”

“By some means, then, the atmosphere is kept warm at the surface of the earth and cold in the higher regions, and in this manner the radiation of heat into open space is prevented. This is accomplished notwithstanding that the top of the atmosphere is nearer the chief source of heat, the sun. This would be no very easy problem if its solution were left to human ingenuity. The explanation is very simple, however, when once suggested. The atmosphere is diathermic, that is, it permits the luminous heat from the sun to pass directly through it without heating the air, but the solid earth stops the heat by absorption, and is warmed by it. The warm surface of the earth imparts, in turn, its heat to the atmosphere resting upon it. This warm air, being expanded by the heat received, becomes lighter than the cold air around, and rises, or rather is forced, upward by the greater weight of the colder air. But as it rises and the pressure of the air is diminished it expands still further. By this expansion its sensible heat becomes latent, that is, the heat is transmuted into force, and, as force, is incapable of being radiated. In this manner radiation from the upper surface of the atmosphere is greatly hindered and waste of heat is in a good degree prevented.

“In respect to this heating of the atmosphere from the surface of the earth, a layer of clouds sometimes forms a kind of second surface which receives the sun’s rays and warms the air above. A few years ago I saw a balloon ascension in Providence, R. I. The day was bleak and chilly, and the sky entirely covered with clouds. The aeronauts were expecting a chilly voyage. The balloon shot like an arrow toward the zenith, and in five minutes was completely hidden by the clouds. But to the surprise of the voyagers of the sky, on passing through the clouds their thermometer rose ten degrees. This, doubtless, must very often be the case. The air above the clouds must often be warmer than that below.

“I think you all must have noticed illustrations of this principle on a small scale. Have you not seen that snow and ice often melt around straws and sticks, the snow or ice remaining still frozen at a little distance, as if the sticks and straws were warm and had melted them? Have you not seen a dark-colored board covered with ice, and the ice remain firm till the sun shone upon it, and then the ice melt upon the under surface, leaving the upper surface unaffected?”

“I have seen such things a great many times,” said Peter, “and wondered what the reason was.”

“The reason is that ice is diathermic. Heat passes through the ice without warming; but when the rays of heat fall upon the stick or stone or board, the heat is absorbed, the dark body is heated and in turn warms and melts the ice. In the same manner the atmosphere is warmed. The heat-rays of the sun pass through the atmosphere and fall upon the surface of the earth; the earth is warmed, and in turn warms the air resting upon it.

“The gases and watery vapor contained in the air also hinder the radiation of heat from the earth. Pure atmospheric air is perfectly diathermic to both luminous and dark heat, and vapors and gases are also diathermic to luminous heat. But to dark heat some of the gases are almost impenetrable. Ammonia stops dark heat almost completely. In a smaller degree watery vapor does the same. Gases and vapors thus serve as blankets to keep the earth warm. The heat of the sun, being luminous heat, penetrates the atmosphere with its vapors and foreign gases, and falls upon the earth almost without loss, but, being absorbed by the earth, it becomes dark heat, and cannot be radiated back through the same gases and vapors. Vapor serves thus as a valve: it admits the heat of the sun to the surface of the earth, but prevents its escape. Prof. Youmans calls watery vapor the barb of heat; it catches the heat of the sun and holds it fast.

“Who can sufficiently admire the simplicity of these arrangements for preventing the radiation of heat into the stellar regions?—and their efficiency is no less admirable than their simplicity. Arrangements like these show that the Creator had a definite object in view, and that object is benevolent. For the advantage and enjoyment of the inhabitants of this world these arrangements were made.