An initially coherent probing beam propagating in a randomly inhomogeneous channel formed by powerfull heating radiation is theoretically analyzed. The relation is derived for the modulus of a spatial correlation function allowing one to trace the transformation of a probing beam width and of a correlation radius along the propagation path. The effects of asymmetrical defocusing and stochastic transformating a probing beam nonuniform in the transverse cross section as functions of the initial characteristics of beams and their relative position are analyzed.