A spectrum is automatically searched to select monochromatic spectral points, monochromatic transmittances or radiances are automatically calculated at the selected monochromatic spectral points, for one or more viewing geometries and for non-homogeneous atmospheres belonging to an ensemble of non-homogeneous atmospheres, and a weighted sum of the monochromatic calculated transmittances or radiances is automatically determined. The individual spectral points are selected so that the weighted sum is representative of calculations of spectrally averaged transmittances or radiances for the viewing geometries and for any non-homogeneous atmosphere belonging to the ensemble, in spectral bands to which the selected spectral points belong. The weighted sum is used for computing transmittances or radiances in emitting or scattering atmospheres.