Equations of the Markov approximation method for the problem of laser beam propagation in the turbulent atmosphere with precipitation have been obtained by replacing precipitation particles with spatially disordered finite black screens and by replacing turbulent inhomogeneities with analogous phase screens. Solution of the equation for the mutual coherence function is represented as a convolution of the three functions using the Wigner function. The results of simultaneous measurements of laser irradiance fluctuation along the 200-m, 2.5-km, and 5-km paths during rain are presented. A qualitative model of irradiance fluctuations at multiple scattering owing to shading of a receiver by an adjacent particle layer is suggested. A saturation phenomenon of the laser radiation scintillation index in the turbulent atmosphere with precipitation is suggested and observed.

The results of theoretical and experimental investigations of intensity fluctuations of optical radiation in model media (suspensions of a great number of macroscopic particles) and in a turbid atmosphere are given. The theoretical calculation is made in two approximations: (1) the particles are on the average in a wave zone of each other; (2) large soft particles are in the near zone of one another. In both approximations all the intensity statistical characteristics including multipoint probability distributions are obtained. The results of the experimental investigation of intensity fluctuations in model media and a comparison of those with the theoretical ones are listed. For a turbid atmosphere the results of the experimental investigation of intensity fluctuations are given.