An analytic expression is derived to estimate the zenith angle distribution of the downward longwave radiation from sky elements in terms of the surface air temperature, surface water vapor pressure and zenith angle, taking into account the anisotropy of the downward longwave radiation. In addition, a formula for estimating the sky view-factor is also derived for the special case in which the zenith angle of the skyline is constant. It is shown, on the basis of this formula, that the additional downward longwave radiation is a function of the surface air temperature, surface water vapor pressure, surface temperature of obstructions, and the zenith angle of the skyline. The values estimated by use of this function fit well with Kondo's (1982) screening factor F₂, assuming that the surface air temperature is equal to the surface temperature of the obstructions. The relative deviation of the screening factor for isotropic radiation from that for anisotropic radiation is larger with respect to increasing surface water vapor pressure and decreasing skyline elevation. Applying this formula, a new method is proposed to calculate the view-factors of visible and obstructed skies from the fish-eye lens image using a digitizer in conjunction with a computer.