interference to services sharing the same frequencies at some considerable distance away. However, they are made use of for some purposes, such as over-the-horizon radar and com- munications systems. Fig. 7.23 Angle-of-scatter calculations. Atmospheric Absorption Electromagnetic waves propagating in the nonionized region of the atmosphere around the Earth are subject to attenuation due to absorption and scattering by hydrometeors (rain, snow, ice particles, fog, and clouds) and absorption by atmospheric gases. Curves showing the variation of specific attenuation with frequency are published by the CCIR (1990f). For The Space Wave most practical purposes, this source of attenuation can be ignored for frequencies below 1 GHz. Waves passing through rain precipitation are subject to attenuation due to absorption and scattering, the attenuation being a function of frequency, polarization, the microstruc- ture of the precipitation, and the distance the wave travels through the precipitation cell. In practice, because of air resistance, falling raindrops, other than the very small drop- lets, assume an approximately oblate spheroidal shape and this causes the specific attenua- tion coefficient to be higher for horizontal polarization than for vertical polarization. The calculation of the total rain attenuation along a radio path requires a knowledge of the rainfall rate distribution along the path and an allowance for inclined paths. The total attenu- ation is obtained by integrating the specific attenuation coefficient over the path length. In most practical cases, predictions are required for the attenuation likely to be exceeded for small percentages of time, and this therefore requires a knowledge of the cumulative long- term statistics for rainfall in the geographic area concerned. In the absence of such measured rainfall data for the area concerned, the rainfall data presented by the CCIR, for various geographic areas throughout the world, can be used. The attenuation, in dB/km, due to a cloud of small droplets of