Observations on the structure of convection currents; meteorological aspects of some South African gliding flights

Abstract

Abstract(1) During the early stages of convection, the upper limit of air currents ascending from the surface, as recorded by sailplane, is found to agree closely with the level of the intersection on the tephigram of the path curve through the surface temperature at the time and the environment curve given by the corresponding upper‐air temperature observations. Details are given of one series of observations showing clearly successive stages in the breakdown of a morning inversion; and the good and bad thermal soaring conditions encountered on different days are shown to be fully explained by differences in vertical temperature distribution.(2) In the absence of appreciable convection effects, the vertical extent of orographic up‐currents over a hill is shown to be correlated with the component of the surface wind perpendicular to the line of the ridge.(3) Examples of the distribution of convective up‐currents in relation to the surface topography and to the structure of the wind, together with the magnitude of these currents, are derived from detailed analyses of barographic records of cross‐country sailplane flights on which a suitable flight‐plan has been followed sufficiently rigorously. Observations are quoted indicating convective cells of horizontal extent about twice their depth and 9,000 to 12,000 ft. long in the direction of the wind, and providing an approximate value of the ratio of the masses of ascending and descending air within a given isobaric slice.(4) An analysis of sailplane flights made at a well‐marked cold front demonstrates the existence of a narrow but apparently continuous zone of rising air immediately ahead of the front, with a vertical velocity greater than can be accounted for by passive lifting of the warm air at the frontal surface. Surface and upper‐air temperatures in the warm air ahead of the front show that this effect may be explained in terms of the localisation of previously scattered convection currents in the highly unstable warm air into a narrow zone just ahead of the advancing cold air.