Estimating the impact of rain, haze, and other visibility factors is an important consideration when planning the deployment of wireless point-to-point and point-to-multicast installations.

The International Telecommunication Union (ITU) provides a normalised data set and a robust statistical model which is widely used for link budget planning for wireless communications technologies of all kinds, including Free Space Optics (FSO).

The ITU model divides the Earth into different "rain zones" and calculates what the average rate of expected rainfall in each zone should be, on an annual basis. The model further provides an estimate of the % of the time that the rainfall is expected to exceed this average.

Generally, in regions or rain zones where the rate of rainfall is higher, one can expect a greater impact on the availability performance for wireless communications links.

The table below is based on ITU Recommendation P.837: Characteristics of precipitation for propagation modelling.

We used this data as part of our own model of the worst-case availability vs. distance estimates showing in the following article: What is CENTAURI's availability over distance in different rain zones?