Radio signals undergo significant levels of attenuation from humidity and precipitation around 8GHz and higher. Therefore, it's always recommended to consider humidity and rainfall as factors in link planning.
Due to their acute wavelengths, signals in the 24GHz band encounter additional atmospheric attenuation beyond that which is expected from free space path loss due to water absorption, including rain fade. A general guideline is an additional loss of 1-3 dB per kilometer. The amount of additional atmospheric attenuation will vary, depending on your specific installation environment (the world has a variety of rainfall zones).
For example, links exposed to 25 mm/hr of rainfall could undergo additional 4dB of attenuation per km! The table below illustrates the estimated additional attenuation at various rainfall rates and distances, based on information from the FCC bulletin.
|Estimated rainfall attenuation rates at 24GHz.|