In this paper, the effects of tropical rainfall on the propagation of microwave and millimeter-wave signals on terrestrial paths have been investigated. Quantitative evaluation of spheroidal raindrop scattering parameters such as the scattering amplitude functions, total and scattering cross sections, and so on, were carried out for linear polarization (horizontal and vertical), using the least squares fitting method over the frequency range 1--100 GHz. Incident radio signals are assumed plane and arrive normally on the raindrop axis. For the purpose of application to slant paths, two other incident angles, 50¿ and 70¿, were considered. The computed forward scattering amplitude functions were employed to compute the complex effective propagation constant of the radio signals in four tropical rainfall types: drizzle, widespread, shower, and thunderstorm. The corresponding specific attenuation and specific phase shift were computed. Specific attenuation and phase shift due to the rain types for circularly polarized radio signals were also derived. The parameters of the power law relationship between attenuation, phase shift, and rain rate were computed for each rain type and compared with existing temperate and tropical models. The problem of rain-induced depolarization was also investigated by computing the cross polarization discrimination for the four rainfall types over the frequency range 1--50 GHz, which is of immediate importance to system designers. ¿ 1999 American Geophysical Union