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Rh 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