Origin of the Semi-diurnal Pressure Wave in the Earth's Atmosphere

Abstract

THE small semi-diurnal wave of atmospheric pressure has long been recognised as a world-wide phenomenon. Simpson (Q. J. Roy. Met. Soc., 44, pp. 1–18; 1918) showed ten years ago how closely its vahie at individual places corresponds with the resultant of two twelve-hour vibrations, one parallel to the circles of latitude and one parallel to the meridians, as suggested originally by Schmidt (Meteorologische Zeitschrift, 7, p. 182 ; 1890). The mathematical investigations of Laplace, Kelvin, Margules, and Lamb have led to the conclusion that the atmosphere has a natural period of vibration of about twelve hours. The general opinion during recent years has in consequence been that the semidiurnal wave of pressure is a forced oscillation of thermal origin, with the reservation that a considerable degree of mystery attaches to the precise way in which such a resonance effect can take place in an atmosphere complicated and changeable as that of the earth. Some meteorologists apparently go even further, and reject this theory completely. Goldie, for example (Proc. Royal Soc. Edinburgh, 47, part 4, No. 25), has been led by a critical examination of many autographic records of meteorological elements, to take up a point of view the essence of which, to quote his own summary, is as follows: