Free Space Optics technology has gained acceptance in telecommunication industry mostly in enterprise
campus network. However in tropical regions, rainfall is the dominant factor that degrades the FSO link performance and
its availability. In this paper, a method is proposed to estimate the total path attenuation due to rain for Free Space Optical
communication links for all percentage of availabilities and any path length. Path length reduction factor for FSO has been
derived using reduction factors models developed for microwave under measurement in tropical climate. Regression
analysis has been applied and model is derived using best fit curve. Comparison between derived and measured reduction
factors has been made for validation. Derived reduction factor seems to have strong agreement with both models which
were derived based on measurements. This method can provide estimation of total path rain attenuation for FSO for all
percentage of time with any path lengths
