The effective atmospheric emissivity, F↓/(σT04), where F↓ is the downward radiative flux density and σT04 the blackbody flux density at the surface temperature T0, is computed for clear skies and straight temperature and dew-point soundings by means of emissivity integrations. Emissivity data by Jurica and by Staley and Jurica were used, and separate computations made for H2O, CO2, H2O-CO2 overlap, and O3. The effective atmospheric emissivity depends almost entirely on surface vapor pressure, decreases slightly with increasing surface elevation, and is essentially independent of surface temperature. The contribution by CO2 decreases from about 0,19 to about 0.17 as surface elevation increases from sea level to 710 mb. The contribution of overlap is negative and increases rapidly with increasing surface vapor pressure, becoming comparable to the CO2 contribution for very large vapor pressures. Measurements support the computations, but suggest, as has been found before, additional downward flux density from aerosols or from as yet unspecified trace gases.