11.3.8 Atmospheric Losses

The effect of rain, fog, and clouds must be considered on the propagation of radio waves since a wave passing over a single spherical droplet has part of its energy absorbed by the droplet and part scattered in all directions. The wave is therefore attenuated with the attenuation being a function of frequency, drop diameter, and the drop dielectric constant. For a concentration of drops the attenuation is proportional to the number of drops per unit volume.

Since the drop size distribution varies in a known manner with the intensity of precipitation it is possible to calculate attenuation conveniently in terms of the precipitation rate. It is found that the attenuation is only appreciable in the GHz frequencies and its value there is roughly proportional to the precipitation rate. As a general rule of thumb, most link designers only consider attenuations due to rainfall to be significant at frequencies above 10 GHz. While rainfall greater than 20 mm hr^–1 seldom occurs outside tropical areas, tropical rainstorms can reach up to several hundred mm hr^–1.

In the case of cloud and fog the drop sizes are smaller than for rain and the attenuation is proportional to the mass per unit volume. Again, attenuation is only significant in the GHz range.