Variability of atmospheric water-vapour concentration and its implications for microwave-radio-link planning

The amount of water-vapour present in the air has considerable importance at radio frequencies greater than 10 GHz, both in respect of the attenuation and thermal sky-noise emission which it produces. The statistical distribution of atmospheric water vapour, as derived from 10 years of data collected at London´s Heathrow Airport, is described, together with how the spread of the statistical distribution is limited if a restricted range of air temperatures to be expected can be specified.