Recent oceanic observations of net long-wave radiation examined in light of various empirical formulas for estimating the irradiance. The observations suggest that Efimova's formula is the most realistic one for computing net long-wave radiation under clear skies. The data also indicate that the irradiance decreases in a linear manner with cloud amount: for low stratocumulus a cloud factor of 1-0.9C seems appropriate, and for the higher clouds typical of the tropics the factor is approximately 1-0.7C.