Atmospheric oscillations

The early researches of Laplace* on the problem of atmospheric oscillations were aimed at an explanation of the semidiurnal variation of the barometer. It was known to him that the amplitude of the solar component of the semidiurnal barometric oscillation is larger than the corresponding lunar component, although the tidal force of the moon is more than twice as large as that of the sun. His explanation was that the barometric oscillation is not of tidal but of thermal origin. It was pointed out, however, by Sir William Thomson! that if the pressure oscillation were a thermal effect, the diurnal term should be larger than the semidiurnal term—in direct contradiction to the facts. As an alternative explanation, Kelvin made the suggestion that the atmosphere has a period of free oscillation of nearly 12 hours, so that the solar semidiurnal tide is magnified by resonance. The question whether our atmosphere actually has a free oscillation of a period of 12 hours has since been studied extensively, but on account of the various simplifying assumption common to all these investigations concerning the physical nature of the oscillations and the distribution of temperature in the atmosphere, it st 11 remains open.* Further collection and analysis of the observational data of the barometric oscillations have tended to support the resonance theory, but evidence has also come from other geophysical phenomena which, apparently, cannot be reconciled with it. According to Simpson, the solar semidiurnal barometric oscillation can be represented by p2 = 0*937 sin3θ .sin 2t + 154°) + 0·137 (cos2θ — 1/3) sin (2t— 2ϕ + 105°), where θ denotes the colatitude, t local time, and ϕ longitude, and the unit is 1 mm. of mercury.