HEAT: ITS NATURE, PROPERTIES AND DIFFUSION
The Nature of Heat.—The phenomena of heat and light are described, the one as “molecular motion,” the other as the “radiation of solar energy.” Roughly, either definition will apply to either phenomenon. Not much is known about the real essence of either, except that they are forms of energy which have been measured, and of which certain magnitudes have been established under the name of “wave lengths.”
Radiant Heat.—It is assumed that heat and light traverse space in “waves” or vibrations of the ether. Positive knowledge is confined to the fact that heat is radiated by the sun and stars in every direction. Practically all the heat received by the earth comes from the sun; and of the whole amount radiated by the sun, the earth intercepts less than one two-billionth part. Nevertheless, this small fraction of the sun's radiant heat produces all the results upon the earth which are manifested by life and its activities.
Some of the ether waves stimulate the nerves of the eye, producing the phenomena of light and vision. Others do not affect the nerves of sight; as they fall on the body they produce the sensation of warmth, thereby stimulating the growth of living matter. Meteorology is concerned chiefly with radiant energy of this character; they are conveniently called heat waves.
Perhaps our nearest approach to actual knowledge of heat is the recognition of the fact that when heat waves fall upon matter—say, a piece of metal—they set up a motion in the molecules composing it. If the intensity of the waves increases, molecular attraction little by little is overcome; the solid becomes a liquid and, finally, a vapor. This important change is explained as being due to increasing and to wider amplitude of the oscillations of the molecules. Perhaps the theory may not be satisfactorily established, but the facts cannot be denied; the heat has increased the motion of the molecules; finally it has overcome their cohesion.
Sources of Heat.—The warmth that is concerned with life and its activities is derived from the sun. The sun warms the rock envelope of the earth; the rock envelope radiates warmth to the atmosphere; the movements of the air diffuse the warmth, bringing cool air into warm regions and sending warm air into cold regions. Weather science is concerned chiefly with these movements of the air.
The earth itself is a source of heat. The interior of the rock envelope of the earth is intensely hot. Borings into the rock envelope show an increase of temperature with depth. The rate varies with the character of the rock, a rough average being 1° F for every 70 feet.[1] Some of the heat of the rock envelope, at such depths, is due to chemical action going on within the rocks themselves; some is due to vulcanism; but some is certainly due to the radiation of the heat of the interior of the rock envelope itself. In meteorology this source of heat practically is negligible.
The most common example of a terrestrial source of heat is the ordinary combustion of fuels. But even this is apparent rather than real terrestrial heat; in fact it is the heat of the sun stored and conserved by vital chemical processes. But all the original heat derived from within the earth and all the heat of vital chemical processes bears an infinitesimal ratio to that received from the sun. It is estimated that the heat received by the earth in one minute is sufficient to raise 42,000,000,000 tons of water from the freezing to the boiling point.
Heat as Motion.—If heat produces molecular motion, it must be assumed that the ether waves which traverse space warm nothing until they fall on matter—that is, on a substance composed of molecules which can be set in motion. But in many respects heat behaves to matter much as light does. It may pass through a substance just as light passes through glass; such a substance is said to be diathermous. Thus, glass itself is more or less diathermous, so also is clear water. Glass permits most of the heat of the sun to pass through it, but it intercepts some of it. Thereby molecular motion is set up in the glass itself and it becomes warm, radiating heat in just the same manner as a stove radiates it.
The heat “absorbed” by a substance—that is, converted into molecular motion—in time may be stored until it is again radiated and is given out as warmth. It is conveniently called “sensible heat.” It is thus distinguished from the ether waves
Convectional movements of the air; sectional view.
that have great physical power, but do not directly impart the sense of warmth. One cannot draw a line between the sensible and the ultra-sensible heat waves, however; and the use of the term, though convenient, is not exact. Weather science deals chiefly with the sensible heat of the air.
Diffusion of Heat.—As a body becomes warm, the heat may diffuse itself through the mass rapidly, as in the case of metals, or slowly, as in the case of non-metals. The former are good conductors; the latter are sometimes regarded as poor conductors, or insulators. Thus, steam pipes are wrapped with asbestos coverings. The metal pipe itself warms rapidly and, radiating heat rapidly, causes a loss of heat in the steam. The asbestos covering, being a non-conductor, or insulator, prevents the loss by radiation. In solid bodies, the diffusion of heat is accomplished by conduction. The motion imparted to molecules sets the molecules nearest to them in motion; finally the heat is diffused throughout the mass.
When liquids and gases are heated a movement of mixing occurs. This process is best observed when a handful of sawdust is placed in a beaker of water and the water is heated by a Bunsen burner. The rapid warming of the water carries the particles of sawdust upward, outward and downward through the water; they indicate the progressive movement of the water in different parts of the beaker. This mixing process is called convection.
Convection of the Air.—The convectional movements of the air are among the most important factors in weather science. Aside from the lateral movements of the air—the winds—convectional movements that are more or less vertical are going on all the time; that is, air is going up or coming down.[2] Warm air is ascending, cool air is descending.
The sun does not warm all parts of the earth evenly. In tropical latitudes the heat is far more intense than in extra-tropical latitudes. Because of the curvature of the earth’s surface, polar regions receive the sun’s rays very obliquely. The unequal heating results in a convectional movement of the air on a scale that affects the whole atmosphere. It produces an upward and poleward flow of air in tropical regions which is balanced by a downward and tropic-ward movement of the air in extra-tropical regions.
The principle of convection is one of the most important in meteorology, and is practically the foundation of that part of weather forecasts which concerns storms and cold waves.
Specific Heat.—Different substances vary greatly in their “capacity” for heat. That is a much greater amount of heat is required to produce a given intensity of molecular motion in one kind of matter than in another. For convenience, the amount is called the thermal capacity of the substance. For convenience also, the heat taken up by a given weight of water is taken as the unit of measurement. Thus, a pound of water has 9 times the thermal capacity of the same weight of iron and 30 times that of mercury.[3]
Latent Heat.—Reference has already been made to the fact that a very great amount of heat disappears when water at 212° F (100° C) is converted to steam at 212° F. The heat apparently lost reappears when the steam is condensed to water at 212° F. The heat thus employed in overcoming molecular attraction is called latent heat. The latent heat of evaporation is an important factor in the diffusion of heat. Thus, water vapor from tropical regions is borne to higher latitudes and there condensed, setting free an enormous amount of latent heat, which becomes “sensible” heat again. The latent heat set free when water freezes is also a factor in climate.
Adiabatic Heating and Cooling.—If a volume of gas, or of air, is compressed, a noticeable amount of heat is given off. The hand-operated tire pump is an example; after a dozen strokes of the plunger the barrel of the pump becomes hot. If the compressed air expands to its original volume, just as much heat is absorbed in the expansion as was given off during compression. The ordinary ammonia gas compressor furnishes an instructive illustration. The pipe near the compression valve may be at a low red heat; the pipe at the release valve is usually cased with a thick jacket of ice. Heat has not been added to the gas in the process of compression; it has not been taken away during expansion.
This phenomenon is an important principle of weather science. As has been pointed out, convection in the air is always going on. Ascending air expands in volume because of decreasing pressure; descending air is compressed in volume because of increasing pressure. Therefore it follows that ascending air cools by expansion and descending air becomes warmer by compression. This phenomenon is called adiabatic heating and cooling; to all intents and purposes it is merely a form of latent heat. Adiabatic heating and cooling of the air therefore is practically due to convectional movements.
Units of Measurement.—Various units are employed in the measurement of heat. Two aspects of heat measurement concern meteorology—quantity and intensity. Thus, the quantity of heat which a given weight of a substance may contain is less than that of a greater weight of the same substance, and, as has been noted, equal weights of different substances may differ greatly in thermal capacity. Several units of quantity, or thermal capacity, are employed. The calorie is the amount of heat required to raise one gram of pure water one degree centigrade in temperature. This unit is employed very generally in scientific research. In some instances, however, it is more convenient to employ the great calorie, or the amount of heat required to raise one kilogram of water one degree centigrade in temperature. The British thermal unit, the heat required to raise one pound of water one degree Fahrenheit, is also much used—chiefly, however, in expressing the heat value of fuels.
The unit of intensity is the degree, of which there are several, each differing in value from the others. All of them, however, have a common basis—namely, the difference in intensity of molecular motion between melting ice and boiling water, under certain standard conditions. The various scales of degrees are explained in another chapter.
The Solar Constant.—Weather science is concerned in the amount of heat received by the earth from the sun. For expressing this value the calorie is used. The measurements begun by Ångstrom and Langley, and continued by Abbott, Kimball and others, cover a period of about forty years. Simultaneous cooperative observations carried on in the United States and elsewhere show that the value is by no means constant, but that it varies from time to time. The mean of observations deduced by Abbott is 1.932 calories per minute, less the amount absorbed by the air.
The air, with its dust and its moisture content, intercepts a great deal of the heat radiated from the sun. When the sky is clear and the sun is overhead, it is found that a little more than two-thirds of the sun’s radiation reaches the earth, less than one-third being absorbed by the atmosphere. When the moisture content of the air increases, the value of the solar constant decreases. When the smoke pall that hovers over manufacturing centers thickens, the effect is the same. This also is true of any increase of atmospheric dust. The volcanic dust shot into the air by the eruption of Krakatoa lowered the value of the solar constant for a considerable length of time.
The moisture and dust content of the air acts as a blanket, intercepting and storing during the day a part of sun’s heat, and at night becoming a source of heat in itself.
The fixed constituents of the air, the oxygen and the nitrogen, vary so slightly in proportion and the amount of heat which they intercept, that their effects may be regarded as constant. The great changes in the effects of insolation are due chiefly to the varying proportions of the water vapor and the dust content of the air. The layer of water vapor is comparatively thin—practically not more than five or six miles. The dust blanket, on the other hand, may extend many miles into the upper air.
- ↑ At the Goff well, near Bridgeport, West Virginia, the temperature at a depth of 7310 feet is 159° F (106° C). The average increase of many measurements is somewhat less than 1° for each 70 feet.
- ↑ W. J. Humphreys points out the interesting paradox that “more air goes up than comes down.” Ascending air carries water vapor, an integral part of the air. But the updraught chills and condenses the water vapor which falls as rain or as snow. The descending air is less in quantity by the amount of water vapor that is lost by condensation.
- ↑ Thus, if the specific heat of water is 1, that of iron is 0.1138; of mercury 0.0333; of glass, 0.1977; of dry air at constant pressure, 0.2375; of steam at 212° F, 0.341; of ice, 0.50. Because of its great specific heat and good conductivity it is evident that water is well adapted to the heating of buildings. It holds its warmth steadily; it also holds a greater amount than any other available substance.