Page:Handbook of Meteorology.djvu/32

20 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.

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.