A parameterization of the infrared clear-sky emissivity of the atmosphere at monthly to annual time scales and local to zonal space scales is developed. The total emissivity is the sum of contributions from ozone, assigned a constant value, and from water vapor and carbon dioxide, corrected for their spectral overlap. Emissivity can be adequately estimated from mean annual surface air temperature, annual range of monthly mean temperature and surface station pressure. Over most of the world, emissivity can be determined solely from surface temperature. Longwave counterradiation from the atmosphere to the surface is estimated from the parameterized emissivity and the surface temperature, and compared with fluxes calculated from soundings. This formulation is adaptable to any climate model which produces surface temperature as an output.