An analysis of the atmospheric radiative transfer processes suggests that the diffuse radiance emerging at the top of the Earth's atmosphere in the position of a satellite can be approximated by a linear relationship with the optical depth. It is found that the variation of the optical depth is associated with the changing of aerosol size distribution. The optical depth increases with the presence of large aerosol particles. Thus, inhomogeneities in aerosol size distribution can affect the high-altitude atmospheric radiance in the visible wavelengths (400–800 nm). To demonstrate an application of ocean colour remote sensing for atmospheric studies, computations of diffusely reflected radiance at the top of the atmosphere, using the radiative transfer equation, have been made for known aerosol size distribution and oceanic surface albedo. The condition of a clear sky over clear water has been considered in the study. The computed radiance was compared with the radiance detected by the Coastal Zone Colour Scanner (CZCS) aboard the NIMBUS-7 satellite. The CZCS radiance was collected within 1–2 hours after the aerosol measurements from the same area (Monterey Bay, California). An optical depth of the Earth's atmosphere is inferred from the comparison. The procedure was carried out for wavelengths corresponding to the CZCS channels (443, 520, 550,670 and 750 nm). Values of optical depth are plotted against the wavelengths to provide an optical depth spectrum. The results indicate that there is a strong dependence of optical depth on the wavelength. The dependence, however, does not agree well with the relationship proposed by Angstrom (1964) for the aerosol optical depth in the atmosphere. Furthermore, the resulting optical depth ratio cannot be estimated by the approximation suggested by Gordon and Clark (1981) and Gordon et al. (1983).