Measurements of the atmospheric optical thickness at ‘window’ wavelengths in the visible and near infrared region have been obtained from data of direct solar radiation through clear and hazy atmospheres. At these wavelengths, the weak attenuation of the atmosphere is due to Rayleigh scattering, non-selective absorption by water vapour and extinction by particulate matter. Since non-linear dependence may exist between the particulate matter optical thickness and the preci-pitable water vapour, seasonal sets of atmospheric optical thicknesses have been selected, which present linear and direct correlation with the precipitable water vapour, at each wavelength. Thus the slope of the regression line of optical thickness on precipitable water vapour is the sum of (i) the non-selective absorption coefficient of atmospheric water vapour and (ii) the linear variation coefficient of the particulate matter optical thickness as a function of precipitable water vapour. The latter coefficient has been evaluated at the various wavelengths for each set of data, by examining the spectral series of data in terms of Angström's expression and determining linear relationships between atmospheric turbidity parameters and precipitable water vapour. Each monochromatic estimate of the non-selective absorption coefficient of water vapour has been obtained as the difference between the slope of the regression line and the linear variation coefficient of the particulate matter optical thickness. The resulting estimate has been related to the vertical distribution of absolute humidity, in order to determine the mass absorption coefficient of water vapour at unit air pressure.
