Abstract: An updated numerical model of the propagation of a set of spectral harmonics of internal gravity waves (IGWs) in the inhomogeneous atmosphere from the earth’s surface to the lower thermosphere is described. IGW dissipation due to turbulent viscosity and thermal conductivity, radiative heat exchange, and ion drag in the lower ionosphere is taken into account. A numerical simulation of the propagation of the IGW spectrum is performed for the background fields of wind and temperature corresponding to the coordinates of the Zvenigorod (56° N, 37° E), Maymaga (63° N, 130° E), and Tory (52° N, 103° E) stations, where systematic observations of OH nightglow are carried out. Seasonal variations of standard deviations of perturbations of the horizontal velocity components created by the model of IGW spectrum are calculated. A general similarity exists between seasonal variations of the model IGW amplitudes and observations of the variance of mesoscale disturbances of the OH rotational temperature at Zvenigorod and Tory. This gives evidence that the intensity of mesoscale temperature disturbances near the mesopause may depend on the intensity of IGWs propagating from the lower atmosphere and on the profiles of the background characteristics of the middle atmosphere along the path of wave packages propagation in different seasons and at different geographical locations.