In this paper we analyze the influence of thermal stability of the atmosphere on the refraction of laser beams propagating along surface horizontal paths. It is shown that under unstable atmospheric conditions a laser beam can hit to the same point at the receiving plane along two trajectories. It gives two different refraction angles due to focusing atmospheric lens, which is caused by unstable stratification of temperature. The difference between these trajectories decreases with increasing the turbulent flux of heat. Beginning with the definite critical value of heat flux, a laser beam on horizontal path does not hit to the given point. The estimates of maximal length of path between two points where steady optical connection can occur at unstable atmospheric stratification are presented in the paper.