This study assesses the performance in term of availability of a FSO (Free Space Optics) link for two wavelengths, belonging to atmospheric windows, the standard telecom wavelength 1.55 µm and the mid-infrared wavelength 4 µm. To do so, we compute the transmission rate under various atmospheric conditions, including fog. Using the atmospheric transmission rate from our radiative transfer software MATISSE, the link budget is derived for a simple direct emission and detection system. The source and detector components characteristics, from commercial data-sheet, are considered to compute the reception noise. An estimate of the Bit Error Rate (BER) of the FSO link for the two wavelengths is presented as a function of visibility. Assuming a bit error correction and the corresponding BER value, it is possible to derive the limit of visibility under which the optical link is cut. A weather visibility database has been collected and compiled for a year to obtain the theoretical availability of the FSO system. As an example the availability at Velizy-Villacoublay (France) weather station throughout the year 2017 is used. In this case the theoretical link availability wins 30 hours of operation in January 2017 with the 4 µm optical wavelength, that shows the benefit of using mid-infrared for FSO when fog occurs.