g the performance of a microwave Clouds produce an additional loss which depends on their liquid- Earth space link. For a complete solution, it is necessary to consider water content. Layer-type cloud (stratocumulus) will not cause not only direct radiation into the mair. beam but also ground-re- additional attenuation of more than about 2dB, even at 140GHz. On flected radiation, and emission from the ground itself, arriving at the the other hand, cumulonimbus will generally add several decibels receiver via side and back lobes. From the meteorological point of to the total attenuation, the exact value depending on frequency and view, radiometer soundings (from the ground, aircraft, balloons or cloud thickness. satellites) can provide useful information on tropospheric and stra- Absorption in precipitation (see Figure 9.7(a) has already been tospheric structure. Absorption in precipitation becomes severe at mentioned in relation to terrestrial systems. Water drops attenuate frequencies above about 30GHz and scintillation effects also in- microwaves both by scattering and by absorption. If the wavelength crease in importance in the millimetre range. However, for space is appreciably greater than the drop-size then the attenuation is links in or near the vertical direction the system reliability may be caused almost entirely by absorption. For rigorous calculations of sufficient for practical application even at wavelengths as low as absorption it is necessary to specify a drop-size distribution; but 3-4mm. Moreover, spaced receivers in a site-diversity system can this, in practice, is highly variable and consequently an appreciable be used to minimise the effects of heavy rain. scatter about the theoretical value is found in experimental measure- In recent years, extensive studies of propagation effects (attenua- ments. Moreover, statistical information on the vertical distribution tion, scintillation, etc.) have been carried out by direct measure- of rain is very limited. This makes prediction of the reliability 