Meteorological uncertainties severely limit the usefulness of models of existing microwave propagation in the presence of precipitation. Major uncertainties, including raindrop shape, drop size distribution, and the penetrating distance of precipitation are discussed in this paper in terms of their impact on satellite communications systems. On the basis of more than 30 measurements of raindrop sizes under different meteorological conditions, a new set of distributions is proposed for slant-path transmission in lieu of the conventional Laws and Parsons distribution. Basic propagation parameters, i.e., specific attenuation and specific phase shift, are evaluated, and comparisons with existing models are made. The results provide a yardstick for establishing a dynamic model meaningful for advanced systems.