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surfaces and deserts have high albedo values. On the other hand, the surface albedo is also a function of the spectral reflectivity of the surface.

The planetary radiation is dominated by emission from the lower troposphere. It shows a decrease with latitude and such a decrease is at a slower rate than the decrease in the absorbed solar radiation energy. At latitudes less than 30° the planetary albedo is relatively constant at 25% and, consequently, there are large amounts of solar radiation for solar energy activities and benefits in these regions of the earth. However, the solar absorption exceeds the planetary emission between 40° N and 40° S latitudes, and therefore, there is a net excess in low latitudes and a net deficit in high latitudes. Consequently, such an imbalance in the solar radiation energy implies heat transfer from low to high latitudes by the circulations within the atmosphere. Accordingly, the solar energy facilities decrease steadily from the equatorial region toward the polar regions. It is possible to state that the natural atmospheric circulations at planetary scales are due to solar energy input into the planetary atmosphere. In order to appreciate the heat transfer by the atmosphere, the difference between the absorbed and emitted planetary solar radiation amounts can be integrated from one pole to other, which gives rise to radiation change as in Fig. 2.1. It can be noted that the maximum transfer of heat occurs between 30° and 40° of latitude and it is equal to 4 x 1015W.

The regional change of net solar radiation budget is shown in Fig. 2.2, which indicates substantial seasonal variation.

Increased cloudiness can reduce solar energy production. For many reasons, clouds are critical ingredients of climate and affect the availability of many renewable energy resources at a location (Monteith 1962). About half of the earth is covered by clouds at any instant. The clouds are highly dynamic in relation to atmospheric circulation. Especially, the irradiative properties of clouds make them a key component of the earth's energy budget and hence solar energy.

Fig. 2.1 a,b. Zonal solar radiation changes
Fig. 2.2 Seasonal solar radiation changes

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