Fog is the most important impairment factor for free space optical links. The detailed data about fog characteristics are very rare. Therefore, fog visibility data obtained at airports are also processed. The characteristics of fog events obtained at the Prague-Ruzyne airport in the Czech Republic over a 13-year period are given. These characteristics were converted to the fog attenuation distributions which can be used for the assessment of both availability performances and error performance parameters of free space optical links.