Measurements of stellar scintillations caused by the atmosphere, conducted on board the space station Mir, are analyzed here. From scintillation spectra we obtained the parameters of spatial spectra of atmospheric density fluctuations (the altitudes 20--70 km) with vertical scales from hundreds of meters to tens of centimeters. The observed characteristic scales of atmospheric inhomogeneities are interpreted with a nonlinear model of internal wave spectrum and a theory of locally homogeneous and isotropic turbulence.