Radiation emitted by Earth is almost entirely long-wave radiation. Most of the terrestrial radiation is absorbed by the water vapor in the atmosphere and some by other gases (about 8 percent is radiated directly to outer space). This radiant energy is reradiated in the atmosphere horizontally and vertically. Horizontal flux (flow or transport) of energy need not be considered due to a lack of horizontal temperature differences. The vertical, upward or downward, flux is of extreme significance.

Some of this radiation is carried aloft by convection and turbulence. Water vapor, under. going the condensation-precipitation-evaporation cycle (hydrological cycle), carries the remainder into the atmosphere.

The atmosphere reradiates to outer space most of the terrestrial radiation (about 43 percent) and insolation (about 13 percent) that it has absorbed. Some of this reradiation is emitted earthward and is known as COUNTERRADIATION. This radiation is of great importance in the greenhouse effect.

Earth does not receive equal radiation at all points as was shown in figure 1-2-4. The east-west rotation of Earth provides equal exposure to sunlight but latitude and dispersion do affect the amount of incident radiation received. The poles receive far less incident radiation than the equator. This uneven heating is called differential insolation.

Due to this differential insolation the tropical atmosphere is constantly being supplied heat and the temperature of the air is thus higher than in areas poleward. Because of the expansion of warm air, this column of air is much thicker and lighter than over the poles. At the poles Earth receives little insolation and the column or air is less thick and heavier. This differential in insolation sets up a circulation that transports warm air from the Tropics poleward aloft and cold air from the poles equatorward on the surface. (See fig. 1-2-8.) Modifications to this general circulation are discussed in detail in Unit 3.

This is the account of the TOTAL radiation. Some of the radiation makes several trips, being absorbed, reflected, or reradiated by Earth or the atmosphere. Insolation comes into the atmosphere and all of it is reradiated. How many trips it makes while in our atmosphere does not matter. The direct absorption of radiation by Earth and the atmosphere and the reradiations into space balance. If the BALANCE did not exist, Earth and its atmosphere, over a period of time, would steadily gain or lose heat.

Although radiation is considered the most important means of heat transfer, it is not the only method. There are others such as conduction, convection, and advection that also play an important part in meteorological processes. These will be discussed in more detail later-in this unit .

Figure 1-2-8.Beginning of a circulation.

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