The Global Radiation and Heat Balances. - Man's physical environment lies at an interface experiencing intense fluxes of energy and matter from the atmosphere to land and water surfaces and from land and water surface to the atmosphere. These exchanges of energy and matter are not only physical, but organic as well Plants and animals gain energy and matter from the atmosphere. This they place in storage. Organisms also release stored energy and matter to the atmosphere.
Concept of the radiation and heat balances. - All life processes as well as practically all exchanges of matter and energy at the interface between the earth's atmosphere and the surfaces of the oceans and lands are supported with radiant energy supplied by the sun. The planetary circulation systems of atmosphere and oceans are driven by solar energy. Exchanges of water vapour and liquid water from place to place over the globe depend upon this single energy source. It is true that some heat flows upward through the lithosphere to the earth's surface from internal radioactive and volcanic sources, but the amount is trivial in comparison with the energy that the earth intercepts from the sun's rays.
The flow of energy from sun to earth and then out into space is a complex system, since it involves not only electromagnetic radiation, but also energy storage and transport as heat in the gaseous, liquid, and solid matter of the atmosphere, hydrosphere and lithosphere.
Organisms respond directly to the heating and cooling of the air, water, or soil that surrounds them. These temperature changes result from the gain or loss of energy by the absorption or emission of radiant energy. When a substance absorbs radiant energy, the surface temperature of that substance is raised. This process represents a transformation of radiant energy into the energy of sensible heat, a physical property measured by the thermometer. Sensible heat, in the case of a gas, represents the kinetic energy of motion of the gas molecules, which are in constant high-speed flight and endlessly colliding with one another. A rise in temperature of the gas represents an increase in the average velocity of the molecules and in the frequency of their collisions.
There are systematic average change in air temperatures from equatorial to polar latitudes as well as from oceanic to continental surfaces. Correspondingly, the lower atmosphere and the surfaces of the lands and oceans must be receiving and giving up heat energy in daily and seasonal cycles.
Despite the existence of thermal cycles and latitudinal contrasts in temperature, human history as well as the, geologic record indicate an overall uniformity of the global thermal environment through time. It is apparent that the earth as a planet maintains within fairly narrow limits a certain average planetary temperature, which has depended on maintenance of approximately the same distance from the sun and approximately the same planetary surface properties. If this were not the case, the gradual drift toward either increasing heat or increasing cold would ultimately render the earth's surface too hot or too cold to support life.
Solar energy is intercepted by our spherical planet and the level of heat energy tends to be raised. At the same time, our planet radiates heat into outer space, a process that tends to diminish the level of heat energy. Incoming and outgoing radiation processes are simultaneously in action. In one place and time more heat is being gained than lost; in another place and time more heat is being lost than gained.
There exists in combination with the global radiation balance a global heat balance, and the two together constitute the earth's total energy balance. Equatorial regions receive through solar radiation much more heat than is lost directly to space, whereas polar regions lose by radiation into space much more heat than is received. So there must be included in the energy system mechanisms of heat transfer adequate to export heat from a region of excess and to carry that heat into a region of deficiency. On our planet, motions of the atmosphere and oceans act as heat-transfer mechanism. Water in its three states-as water vapour in the atmosphere and as liquid and solid water in the oceans and over the land surfaces-absorbs and liberates heat as it changes from one state to another. Consequently, a study of the earth's heat balance will not be complete until the processes of change of state of water in the atmosphere and hydrosphere are examined.
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