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We have analyzed the dependence of the Ångstrom turbidity parameters on wavelength throughout the visible range by three different methods. We have obtained significant results about the ability of these parameters to represent the turbidity of the atmosphere. An important variation of these coefficients with the wavelength is shown, depending on the method used and the spectral resolution of the data. The log–log plot of aerosol optical thickness vs wavelength appears as an adequate method of obtaining the Ångstrom parameters to represent the turbidity of the atmosphere and to model the effect of aerosols on solar radiation.

Unexpected absorption in the real atmosphere in the window from 840 to 890 nm has been clearly observed through measurements of direct solar spectral irradiance under clear, cloud-free skies. This absorption is not predicted by the known theoretical models. The cause of this apparent absorption may be due to the presence of high thin nonvisible cirrus clouds. A quantitative evaluation of this absorption and an improvement of oxygen absorption coefficients has been carried out after a comparison of more than seventy experimental spectra. Moreover, it is necessary to take into account the different behavior of modeled and experimental data at low and high air masses.

Three models of spectral direct irradiance of the sun at the ground are compared with experimental spectral measurements. Satisfactory agreement is obtained although their behavior is quite different mainly in the absorption bands. The turbidity parameters required by the model are obtained from the measured data and the water vapor by simultaneous radiosondage.