Abstract: Attention has becn paid to the fact that goouping associatiop between particles exists in clouds as well as in rain.Coherent scattering of radar waves from cloud and rain drops dependent to each other has been evaluated and a new radar equation derived.The new equation involves the old one as its special form in the casc of incohercnt scattering.It is found that the effect of coherent scattering in some cases is not quite small and should not be neglccted.