I

Outgoing long-wave radiation

Surface area receiving insolation

Sun's rays (Arrive parallel at Earth)

Figure 1.1. Top: The incoming solar radiation impinges on a disk of area na2 but is on average spread out over a sphere of area 4ua2. Bottom: Variation of incoming solar radiation with latitude. A given amount of radiation is spread over a larger area at high latitudes than at low latitudes, so the intensity of the radiation is diminished, and thus high latitudes are colder than low latitudes.

radius a, and the area of a sphere is Ana2. Thus, the average amount of radiation that Earth receives per unit area may be calculated as follows:

Total radiation received = Sua2. (1.1)

That is, the average rate at which radiation is received at the top of Earth's atmosphere is 1,366/4 « 342 W/m2, and we denote this S0; that is, S0 = S/4 « 342 W/m2.

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