A great variety of atmospheric phenomena such as absorption, dispersion and turbulence can cause big variations in the attenuation of the beam of light of a laser. The absorption and the turbulence can be measured by means of a system free space optics (FSO) designed appropriately. In a continental typical region, where phenomena as rain, snow or fog are presented, the most important factor that produces losses of power is the dispersion of the light, though others exist as the presence of impurities (due to the air pollution) and the absorption. The attenuation due to the dispersion can be expressed as a function of the distance of the link, the wavelength and of the birefringence of the air with disturbances. This work describes the results obtained from the analysis of the propagation of the light in a contaminated atmosphere and of the availability of the link FSO, based on the measure of the ellipticity of a beam transmitted with circular polarization and of the calculation of the birefringence of the atmosphere, as a function of the distance of the link, the wavelength and the level of power received. It is presented a method to estimate the availability of a system FSO knowing the distance of the link and some physical characteristics of the atmosphere.