Cloud attenuation at millimeter wavelengths

Total atmospheric attenuation under conditions of complete and partial cloud cover was measured at frequencies of 15 and 35 GHz in the Boston area. The attenuations were actually inferred from extinction measurements using the Sun as a source. Measurements were made at 29 elevation angles from 1° to 20°, and the angular dependence of the attenuation was examined. For most cloud conditions the attenuation was found to be proportional to the slant path distance through the absorbing atmosphere. For elevation angles above about 8°, a flat Earth approximation is valid and the slant path distance is proportional to the cosecant of the elevation angle. For low elevation angles the slant path distance is a function of the effective earth radius and the effective height of the attenuating atmosphere, in addition to the elevation angle. A statistical technique for determining the radius and height is described. A zenith attenuation was extrapolated from each set of data. The humidity and frequency dependence of the attenuation was examined. An algorithm for estimating total atmospheric attenuation as a function of elevation angle, frequency, and surface absolute humidity was derived