Fog or mist is essentially supersaturated air in which some of the water has precipitated out to form small droplets of water, usually < 0.1 mm in diameter (19). Fog is mainly formed from two processes—radiation and advection (6)—and rarely extends more than 100 m above the ground. For this reason, fog is usually ignored in satellite-to-ground link design for frequencies below about 100 GHz. Clouds, however, are much more complicated.
Several models are available for the prediction of cloud attenuation (19-23), but there remain two major difficulties in turning these into link design tools: (1) an accurate cumulative distribution of the vertical and horizontal extent, on a global basis, of those clouds that contribute to slant-path attenuation; and (2) separating this attenuation contribution from that due to rain formed in some of the cloud processes. Many measurements of rain attenuation implicitly include attenuation due to clouds. Understanding, and accurately predicting, the contribution of clouds to path attenuation is a key element in the modeling of the proposed V-band (50/40 GHz) satellite services. Of equal importance will be the development of modeling procedures that can predict the combined effects of all the attenuating phenomena on an earth-space link (24,25).