Wet snowflakes may produce much larger attenuation because a thin coating of water can cause a small ice particle to have a total cross-section nearly the same as an all water particle. The shape of the snowflake and the orientation of its principal axes with respect to the polarization of the microwave beam could also influence the amount of attenuation produced by snow in the melting layer below the 0°C isotherm. Attenuation due to water-coated hail can be quite severe, although infrequent because of the low probability of hail. Very few measurements are available; however, we can get an estimate of the expected loss from computations made by Battan.(4) Table 3-2 lists attenuation coefficients for 10 cm propagation through dry and wet hail. Results are shown for three exponential particle-size distributions with different maximum particle sizes. The attenuation produced by a 1 km path through 25 mm/hr rain is shown for comparison. A one kilometer path through wet hail could result in transmission efficiency at 3 GHz of 0. 87. TABLE 3-2 3 GHz ATTENUATION IN dB/km FOR SPHERICAL HAIL If the receiving array is to be located at a site where hail or snowfall is likely, then closer examination of the transmission loss associated with propagation through these precipitates is needed. Forward scattering by rain and particularly by hail will increase the field intensity outside the main beam. This problem has received recent attention because of the possible interference between satellite downlinks and microwave
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