Abstract A LOWTRAN 5 based flux model has been developed to calculate downwelling infrared irradiance from a clear night atmosphere onto a horizontal or tilted surface. This model is based on the transmittance/radiance code LOWTRAN 5 which can calculate the radiance from the atmosphere for user defined paths, atmospheric conditions and spectral intervals. Included in the model is the addition of a zeroth order scattering approximation to the LOWTRAN 5 code, methods of integrating LOWTRAN 5 calculated radiances over the sky hemisphere to obtain the downwelling flux, and a method for calculating the radiance from the atmosphere at wavenumbers outside the range of LOWTRAN 5. The accuracy of this model is verified by comparison of calculations based on radiosonde data with surface flux measurements taken concurrent with radiosonde ascent. Agreement is excellent for both horizontal and tilted surfaces with the deviation between measurements and calculations of the flux on a horizontal surface being less than 4 per cent.