ed by reflection back from the layer, to produce attenuation factor, and the condensed water content of the cloud in high fields at large distances. g/m³. In addition, the refractive index changes may be patchy and 'blob' like, in which case energy is returned to earth in the form of 6.4.14 Effect of atmospheric absorption on sky scattered waves. In this case, the resultant received signal level noise temperature changes rapidly with time as the tropospheric discontinuities move. A satisfactory parameter for the prediction of such scatter fields is The non-ionised atmosphere surrounding the Earth, in addition to the angle between the receiver and transmitter horizons, as shown causing attenuation of electromagnetic waves, acts as a source of in Figure 6.23. An empirical relationship can then be determined to radio noise. For many terrestrial communications systems, this is of relate the field strength to this angle. little importance since the receiving antenna noise temperature is The signals resulting from abnormal tropospheric conditions dominated by the noise temperature of the Earth. However, for usually represent unwanted interference to services sharing the satellite and space communications systems using low noise tem- same frequencies at some considerable distance away. However, perature antennas and receivers, the sky noise temperature plays an they are made use of for some purposes, such as over the horizon important role in determining the received carrier to noise power radar and communication systems. ratio. If the temperature of the absorbing medium can be assigned a 6.4.13 Atmospheric absorption mean value, T (degrees Kelvin), and the total atmospheric absorp- tion at a particular frequency for a given radio path is A (dB), the Electromagnetic waves propagating in the non-ionised region of the effective sky noise temperature T. for the ground station receiving atmosphere around the Earth are subject to attenuation due to absorption and scattering by hydrometeors (rain, snow, ice particles, antenna