A theoretical model of an ensemble of harmonics of internal gravity waves (IGWs) propagating from random sources in the atmosphere is used to explain the seasonal variations of IGW intensity observed at different altitudes in the lower and middle atmosphere with the Japanese middle and upper atmosphere radar. Calculations reproduce the seasonal cycles of IGW amplitudes having a maximum in winter and a minimum in summer in the upper troposphere and having maxima in winter and summer and minima at equinoxes in the mesosphere. The seasonal behavior at different altitudes appears to be produced by the seasonal variations of the background wind and temperature, which influence the IGW generation, propagation, and dissipation.