In a gas which absorbs radiation an acoutic wave can be unstable. This instability is caused by the fact that the irradiant energy is absorbed preferentially in the high density region of the wave. If in the gas the chemical equilibrium AB⇄A+B is maintained by photo dissociation balancing the reactions due to collisions, the instability increases. This is due to the density dependence of the reaction rate of the reverse reaction. It is argued that this process may explain the excitation or amplification of disturbances in the upper atmosphere. ¿ American Geophysical Union 1990