 a diameter less than 0.1 mm, is given by the product of the specific attenuation factor and the condensed water content of the cloud in g/m3. Effect of Atmospheric Absorption on Sky Noise Temperature The nonionized atmosphere surrounding the Earth, in addition to causing attenuation of electromagnetic waves, acts as a source of radio noise. For many terrestrial communications systems, this is of little importance because the receiving-antenna noise temperature is dominated by the noise temperature of the Earth. However, for satellite and space commu- nications systems using low noise temperature antennas and receivers, the sky noise tem- perature plays an important role in determining the received carrier-to-noise power ratio. If the temperature of the absorbing medium can be assigned a mean value, Tm (degrees kelvin), and the total atmospheric absorption at a particular frequency for a given radio path is A (dB), the effective sky noise temperature, Ts, for the ground station receiving antenna is given by equation 7.86, where TE is the extraterrestrial noise temperature (usually taken as 2.7°K for the frequency range 2 to 50 GHz, unless the receiving antenna is pointing near the sun). Ts=Tm(1-10-4/10)+Te10-4/10 (7.86) Figure 7.24 shows the variation of effective sky noise temperature for clear air with 7.5 g/m³ water vapor density for various elevation angles (0), for the frequency range 1 to 60 GHz. Noise contribution due to absorption by rain or clouds can also be calculated from equation 7.86, provided that the temperature and specific attenuation variations along the radio path are known. Figure 7.25 shows an example of the theoretical variation in sky noise temperature due to attenuation from clouds on a radio path. Depolarization due to Hydrometeors The use of orthogonal polarization in communications systems is an important technique by which economy of frequency spectrum usage can be improved by reducing mutual interference between systems sharing the same, or adjacent, frequency channels. This Chapter 