LOUD KELVIN1 seems to have been the first to suggest that the equatorial wave of the semi-diurnal oscillation of the atmosphere of Earth may be explained by the resonant tidal effect of the Sun on an atmosphere with a natural period of oscillation close to 12 h. In the thirties, Taylor2 and then Pekeris3 were able to support the concept of a natural period of 12 h by consideration of the then known vertical distribution of temperature in the atmosphere. This theory is no longer satisfactory for the following reasons, (i) Recent measurements of atmospheric temperature do not lead to a period of 12 h. The work of Lindzen4 on the classical theory, however, suggests that the thermal excitation provided by the ozone and water vapour absorptions in the atmosphere may be adequate to yield the proper magnitude for amplitude of the migratory wave at the equator without a critical dependence on the vertical profile of temperature, (ii) The predicted variation of amplitude from equator to pole is substantially different from the observed one. (iii) There is nothing in the theory to account for the polar semi-diurnal oscillation observed at high latitude
