A method of calculating the self-radiance of the sky based on the LOWTRAN-4 computer code is described. The method is a significant improvement on that included as an option in LOWTRAN-4 itself as a result of two modifications: a) Our method, unlike the one included in LOWTRAN-4, takes into account the radiance scattered into the line of sight. b) The model of the atmosphere in the computer code must be divided into a certain number of layers, each one having approximately uniform conditions. Our method uses a correct optimal division. Consequently, our method gives a better agreement with the experimental data, especially for long optical paths in the lower layers of the atmosphere.