An exact solution was got for heat conductivity of gas in the gravitational field with the use of model in that vector of speeds of molecules equiprobable distributions for everything to directions as a result of collision in an elementary volume dV. The thermal stream in horizontal direction is described by the law of Fourier: =-λ , and in vertical - = − − . Coefficient of heat conductivity = − 1 3 < ˃ < > coincides with the coefficient got in the model of discrete equiprobable distribution of vectors of speeds of molecules in parallel and антипараллельно to the cartesian axes of coordinates 0x, 0y, 0z and discrete dispersion of molecules in select planes. At the same time number of molecules flying from one side through an arbitrary single ground (n) /4 in 1,5 time exceeds a value (n) /6, given by the classic theory of distribution of speeds of molecules along Cartesian axes of coordinates.