Atmospheric tide

(Also called atmospheric oscillation.) Defined in analogy to the oceanic tide as an atmospheric motion of the scale of the earth, in which vertical accelerations are neglected (but compressibility is taken into account).

Both the sun and moon produce atmospheric tides, and there exist both gravitational tides and thermal tides. The harmonic component of greatest amplitude, the 12-hour or semidiurnal solar atmospheric tide, is both gravitational and thermal in origin, the fact that it is greater than the corresponding lunar atmospheric tide being ascribed usually to a resonance in the atmosphere with a free period very close to the tidal period. Other tides of 6, 8, 12, and 24 hours have been observed.
Chapman, S. 1951. Compendium of Meteorology. 510–530.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.

1. The study of the physics, chemistry, and dynamics of the earth's atmosphere, including the related effects at the air–earth boundary over both land and the oceans. Fundamental topics include the composition, structure, and motion of the atmosphere. The goals ascribed to meteorology are the complete understanding and accurate prediction of atmospheric phenomena.