A technique is proposed for determining the integral variation of the refractive index in a zone of action of high-power optical radiation by shifts of an image of a sensing beam source, when the beam crosses the active zone at some angle with respect to its axis. Some experimental data on the refractive index measured at a windless part of a path were obtained and compared with the calculational data. It is shown that under certain meteorological condition a nonlinear thermal lens formed at the input part of the path determines the distortions of a high-power radiation in the focusing region at the far end of the path.