Similarity theory and dimensional analysis are applied to construction of a relationship for the spectral aerosol extinction coefficient in the atmospheric window from 0.48 10-4 to 0.76 10-4 cm. The relationship is justified using the Mie theory. The results allow one to compare the in situ determined humidity dependence of the spectral extinction coefficient with available laboratory data on variation of the aerosol particle radius in the moist air. It is shown that the variations can be efficiently described in terms of the modified Raoult law for concentrated and moderately supersaturated solutions.