Longwave Radiation

The longwave radiation (LW) emitted by any object at Earth temperatures differs in several ways from shortwave radiation reflected from it. The LW is invisible, it flows day and night, and both the wavelength and amount of LW from an object depend on the object's own temperature, not on that of the Sun. And whilst the albedo of a surface (i.e. its shortwave reflectivity) may be either large or small, the reflectivity of longwave radiation is nearly zero for almost all materials. In other words, LW is almost totally absorbed by the recipient surfaces, e.g. by clouds thicker than about 500 metres, or by the ground. Another difference between shortwave and LW radiations concerns their transmission through the atmosphere, as follows.

Longwave radiation is emitted upwards by the ground and oceans and is called terrestrial radiation. Then water and carbon dioxide in the air intercept much of it (Figure 2.2), becoming warmed by the energy absorbed. Subsequently, the same molecules radiate LW in all directions at wavelengths determined by the molecules' temperatures (Note 2.B).

Cloud droplets are particularly efficient absorbers and emitters of LW, and clouds which are thick enough to hide the Sun are completely opaque to LW. The longwave radiation from such clouds is governed by their temperature, not by any source behind them, whereas SW from clouds is unaffected by cloud temperature.

The downwards LW from clouds and air is called sky radiation (or atmospheric radiation or counter radiation). This is more than offset by the terrestrial radiation upwards because the ground is warmer than the sky. For example, a ground surface at 20°C radiates about 28 per cent more than a sky which is effectively at 0°C (Note 2.C). The difference accounts for the cooling of the ground at night, when there are only longwave fluxes. Latitudinal averages of the longwave fluxes show that the difference between sky and terrestrial radiation is most at low latitudes, i.e. nocturnal cooling is fastest there (Note 2.K).

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