At Ka-band frequencies clouds containing liquid water can produce both signal attenuation and amplitude scintillation; ice clouds, in general, do not produce these effects. The small size of cloud particles relative to the wavelength makes cloud attenuation essentially a function of cloud temperature and the integrated liquid water content along the propagation path. Depending on the elevation angle and the cloud climatalogy of the earth station site, cloud attenuation levels of as much as 3 dB can be expected at the upper end of the 20/30 GHz frequency band. It is seen that the cloud attenuation at 27.5 GHz can reach values close to 1.5 dB at the given elevation angle. Several models are available for the prediction of cloud attenuation distribution .
Cloud attenuation at different frequencies:-
Total cloud attenuation in dB is given by
Acl = ( Wt. Kl )/(Sin Esat )
Wt = ILWC(Kg/ sq.m)
Esat = elevation angle[ 630]
Kl = specific attenuation coefficient[(dB/Km)/(gm/m3)}@ 00 c
kl=(f ^a1)*exp(a2*(1+a3*t))(for liquid water)
ks=(f ^b1)*exp(b2*(1+b3*t+b4*t^2))(for solid water)
f=frequency in GHz
t=temperature in 0C.