In such an instrument the receiving surface must exchange radiation freely with the sky above. If measurements of the atmospheric radiation only are required, the observations must be made at night because there is no way of removing the unwanted solar radiation. The Calculation of Atmospheric Radiation Measurements of atmospheric radiation are rare, especially in developing countries. However, the intensity can be calculated from routine meteorological data with an accuracy comparable with that of the measurements. A simple method of calculation is outlined in this section.
T is the absolute screen-level air temperature in kelvins.
Table 1 gives values of the blackbody irradiance sigma .T 4 and the downward atmospheric irradiance L 0 calculated from the Idso-Jackson formula for various surface air temperatures. In using the Idso-Jackson formula to estimate the downward atmospheric irradiance two corrections are necessary. The first is a diurnal correction for variations in the air temperature lapse rate near the surface. Between these times during the forenoon and in the evening the Idso-Jackson formula gives good estimates of L 0 . The second correction necessary in using the Idso-Jackson formula is for the effect of thermal radiation from clouds, which always increases the downward atmospheric irradiance.