Page:Handbook of Meteorology.djvu/29

Rh 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. 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