Equations are obtained which describe the trajectory of light ray through the unhomogeneous atmospheric surface layer when the three-dimensional equation of ray is analyzed and simplified. The equations can be used as the base of optical measurement. Characteristics of temperature gradient in the surface layer are discussed. The integral which relates the refraction angle and its variable to the temperature gradient based on the geometry of ray curve and the weighting function of temperature gradient are discussed. It was shown that the area-averaged value can not be obtained by means of unilateral measurement of angle when the distribution of temperature gradient is unhomogeneous and symmetrical. A bidirectional optical system is necessary. It was shown that the area-averaged temperature profile also can not be obtained by means of a system with multi-laser sources in the unhomogeneous atmospheric surface layer.