SUMMARY

The apparent emissivity of the atmosphere t', defined as the ratio of incoming long-wave radiation to black-body radiation at screen temperature To, was measured under clear skies in the English Midlands and in the Sudan. At a zenith angle Z the emissivity was given by F(Z) 7 u + bln(usec2) where I I is the reduced depth of precipitable water (cm). For a set of 46 scans in England, the mean values of a and b were 070 i 0.05 and 0.090 k 0.002. Systematic deviations about these mean values could be ascribed to: (i) temperature gradients; (ii) aerosol. The Sudan measurements gave a = 0.67 * 0.03 and b = 0.085 * 0.002 consistent with the English results and observations already published. There is some evidence that minimum values of a have increased over the past 50 years. Integration over the hemisphere gives the flux density (W m-*) of atmospheric radiation as 1.06 uT,,~-119 (Tin K), where IJ is Stefan's constant, or 5.5 T ' + 213 (T in "C). Radiation records by Dines and Dines (1927) for overcast skies are analysed to show that the angular distribution is the same as for cloudless skies; that the mean temperature of cloud base at their site was 11K below screen temperature; and that when the mean fraction of cloud cover is c, the apparent emissivity is pn(c)-- (1- 0.84~)~.(0) + 0.84~. 1