Sky radiation comes partly from the carbon dioxide in the air, but mostly from the water which is confined to the lower levels of the atmosphere (Chapter 6). The downwards sky radiation received at the ground comes from a layer of the lower atmosphere whose thickness is sufficient to contain enough water to be opaque to longwave radiation. Such a layer of cloudless air is commonly only some hundreds of metres thick, depending on the humidity. Consequently, the effective temperature of the layer radiating to the ground is only a few degrees below that at the surface. Also, the layer responsible for sky radiation is thinner (and therefore lower, warmer and emitting more) if the air is humid, as occurs over low-latitude seas.
A result of the effect of atmospheric moisture on sky radiation is that the surface cools more rapidly at night in winter, as winter air holds less water vapour (Chapter 6). The effect also explains why particularly cold nights are experienced in dry climates, with a consequent large diurnal range of temperature (Chapter 3).
Clouds increase sky radiation, depending on the elevation of cloud base: if the base is low, it is relatively warm, and therefore emits more sky radiation. The increased sky radiation due to cloud offsets more of the upwards terrestrial radiation, so that surface cooling at night is slower with an overcast sky. As an example, measurements at Sydney showed cooling by 6.2 K between 6-9 p.m., when the sky was clear, but only 2.3 K when totally cloudy.
The lowest kind of cloud is a thick fog (Chapter 8), so it is a most efficient blanket, its temperature and hence its longwave radiation being about the same as that of the underlying surface.
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