Free space Optical links (FSO) in visible and infrared wavelengths constitute an interesting alternative to new transmission channels for cordless phone, data-processing networks and high definition television. One finds a choice of varied manufacturers and they propose products whose performances are characterized by a raised rate of transmission, from 2 Mbps to 10 Gbps. But the announced ranges are very important, from 100 to 10 000 meters, in spite of the fact that many manufacturers try to indicate the possible ranges according to time, these indications completely miss standardization and are hardly exploitable because, generally, it is very difficult to know the percentage of time during which a value is reached or exceeded. Availability and reliability of a FSO link depend on used systems but also on climatic and atmospheric parameters such as rain, snow or fog. It is the purpose of our study. Its finality is software which integrates: - Results of a bibliographical search (geometrical, aerosols, rain, snow, fog, scintillation, and environment solar light attenuation), - European integrated surface weather data, hour per hour, over several years (1995-1999). The result is the presentation of the Quality of Service Light version software which allows predicting; starting from the data of equipment (power, wavelength, receiver sensibility), geographical situation of a site in Europe (geographical coordinates, altitude, height/ground) and climatic and atmospheric parameter (relative humidity, ground rugosity, albedo, solar radiation, etc) the availability of a FSO link for the following period (year, the most unfavourable month, 8am to 8pm period and 8 pm to 8 am period. The interruption probabilities for each type of attenuation are also mentioned (aerosols, scintillation, ambient solar light, rain, snow, etc).