Clear air attenuation on FSO links

The FSO link attenuation can be caused not only by fog, rain and atmospheric turbulences but also by water vapor (clear air attenuation). The measure of water vapor can be also the sonic temperature. This contribution compares computed sonic temperature from relative humidity, temperature and air pressure with sonic temperature observed by sonic anemometer. We found that the sonic temperature (measured by anemometers) can be replaced by computed sonic temperature being more available in practice. Finally we introduced the Sonic Reduction Factor (SRF) defined as ratio of FSO attenuation computed from sonic temperature to attenuation estimated from meteorological temperature. It could be used to correct the clear air attenuation computed from meteorological temperature instead of sonic temperature.