The method of estimating optical instability of the earth`s atmosphere, which is typical for small-scale fluctuations of meteorological parameters, is important problem of astroclimatic investigations. The energy spectrum of atmospheric turbulence has been studied on a large range of wavenumbers. Using the fluctuation characteristics in some range and well-known shape of the energy spectrum, it is possible to make the calculating method of the numerical characteristics in some range of wavenumbers using calculated characteristics. The method based on calculating the structural characteristic of air refraction index, which is typical for 0,3 m spectral range of turbulence, using meteorological data.