The atmospheric aerosol content has the highest influence on the solar irradiance attenuation by the atmosphere. In the field of solar energy modelization, this parameter is mainly extracted from climatological data banks or interpolated maps with a low grid resolution. It can be evaluated from beam irradiance measurements, but these data are rarely acquired by national meteorological institute in a routine basis. This paper presents a method to retrieve the atmospheric turbidity from hourly global irradiance measurements with the help of a clear sky model by back-calculation. Even if the accuracy of the result is varying from one station to the other and is highly dependent of the data quality, it is of great help to solar beam (or direct) irradiance evaluation from geostationary satellites.