Environmental Engineering Reference
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and, because they are also emitters, more of the radiation will be returned to the ground
as counter-radiation than if the sky were clear. It is absorbed by the ground,
compensating for the emission of longwave radiation and so reducing the rate of cooling
at the ground. Figure 3.4 compares temperatures on clear and cloudy nights to
demonstrate this effect.
Some of the radiation given off by the surface is lost to space but the majority gets
caught up in the two-way exchange between the surface and the atmosphere. Figure 3.3
shows the emission and absorption of long-wave radiation as a proportion of incoming
energy. Radiation from the atmosphere is emitted spacewards as well as down-
Figure 3.2 Earth's eight-year mean annual absorbed short-
wave radiation. Units are W m -2 .
Source: After Kyle et al. (1993).
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