This paper considers Free Space Optics (FSO) systems. FSO availability and reliability is taken under the scope. FSO links operate within the infrared wavelength region. Lasers are used as light sources between two nodes to carry informational stream. FSO systems use Line of Sight (LOS) technology in order to maintain connection between two points. Weather conditions have quite significant impact on FSO operation in terms of availability and reliability. It is necessary to evaluate the air quality at the actual geographical location where FSO link is supposed to be placed. It is important to determine the impact of a light scattering, absorption, turbulence as well as a receiving power when dealing with FSO systems. Visibility has one of the most critical impacts on the quality of an optical transmission channel. Moreover, it is essential to monitor and store information about rain, snow and fog conditions. This paper introduces a device which measures all mentioned weather indicators such as a fog density, a relative humidity and a temperature. FSO availability and reliability estimation is based on measured data. These results evaluate weather conditions for Kosice (a campuse of Technical University of Kosice) in terms of FSO operation.