Summary The Greene and Whitaker mechanism for the production of an acoustic- gravity wave in the 100-km region of the ionosphere is discussed from a physical point of view. The shock wave from a low altitude detonation propagates upward and, interacting with the atmosphere in the 100-km region, produces a horizontally propagating acoustic-gravity wave. Several aspects of this phenomenon are discussed. The initial period of the generated waves is in the 200–300 s range, the energy associated with the wave is about 0.3 per cent of the yield, and the wave generated appears to result principally from the refraction of the shock at the 100-km level.