Large-Scale Vertical Motion in the Atmosphere

Abstract

AT a meeting of the Royal Meteorological Society on February 16, the subject for discussion was large-scale vertical atmospheric motion. In opening, Mr. J.K. Bannon stressed the importance of the rising and sinking of large masses of air in atmospheric dynamics and explained the relatively small attention given to vertial motion by meteorologists by the fact that so far it has been impossible to measure directly the vertical velocities, which can only be inferred, and that with difficulty. He went on to describe a method of estimating up-currents from the of rainfall1, depending on the approximate equality of the rate of release of precipitation from a mass of rising air and the rate at which the saturation vapour content of the same air decreases as the temperature falls due to adiabatic expansion (that is, neglecting the changing water content of the cloud particles, which is small except in convection clouds). This method can be applied in cases of steady rising currents over a wide area (thirty miles square or more), and examples of results obtained from rainfall records in the Hebrides show that up-currents in the lower and middle troposphere are of 'the order of 10-20 cm./sec. in typical disturbances of the North Atlantic.