So far we have discussed separately the downward (i.e. positive) radiation fluxes (comprising global shortwave radiation Rs and longwave sky radiation Rld) and the negative fluxes upward from the ground, consisting of (i) reflected shortwave radiation a.Rs (where a is the surface albedo) and (ii) the emitted terrestrial radiation Rlu. If the combined upward fluxes are subtracted from the combined fluxes downward (Figure 2.6), the result is what we call the net radiation flux Rn at the surface. (Sometimes it is called the 'radiation balance', but that might be confused with the 'energy balance' dealt with in Chapter 5.) The symbolic definition of the net radiation (flux) at the surface is as follows:
It represents the overall input of radiation energy absorbed at the ground, and clearly depends on all the factors which govern the four constituent fluxes—the Sun's elevation, the cloudiness, atmospheric turbidity, surface albedo, the temperature and dryness of the air, and the altitude. Net radiation is important because it comprises the energy available for heating the ground and nearby air (Chapters 3 and 7), and for the evaporation of surface water (Chapter 4).
The difficulty of measuring net radiation accurately means that it is rarely done. Instead, it can be estimated approximately from either measurements or estimates of the solar irradiance Rs (Note 2.M).
The components of the net radiation at a place can be set out in tables, such as Tables 2.5 and 2.6, which show their relative magnitudes. The longwave figures are comparable with those for solar radiation RS, which may seem surprising, since the Sun is so much hotter than the surfaces which generate the longwave fluxes. But those surfaces of the Earth and its atmosphere are much closer than the Sun. Also, longwave fluxes occur throughout each day, whereas shortwave radiation exists only in the daytime.
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