Electric space-charge measurements in convective and other weather conditions

Abstract

Theories of convection below cloud-base are reviewed, together with experimental techniques and evidence. It is concluded that, over land in sunny weather, a forced convection layer is probably overlain by one in which the heat flux is carried by buoyant plumes which may change their form at a few hundred metres. If space charge is considered as carried by air movement, the convection current i(_2) in forced convection is given by where k is von Karman's constant, z the height, u the mean horizontal wind-speed, and σ the space-charge density. In free convection where h is a number equal to about 0.9, H the heat flux, T the absolute temperature, Cp the specific heat and P the density of air, and g the acceleration due to gravity. Buoyant plumes will probably have space-charge density excesses, of magnitude much less than 1 pC m(^-3). Measurements of space-charge density with filtration apparatus show pulses lasting about 40 s, and about 40 pC m(^-3) high: these seem to be associated with free convection, but are probably not coincident with buoyant elements. The hori2sontal diameters and separations of the pulses are proportional to wind speed. The turbulence theory could be used to determine the charge given to the air by melting ice by measuring the space-charge density gradient over melting snow, A calculation from earlier results gives a charge of about 0.16 uC kgm(^-1) melted, similar to values obtained by other methods. Measurements of potential gradient near a small group of deciduous trees in stormy weather are provisionally explained in terms of point discharge, starting at about 1000 V m(^-1) and reaching 0.5 uA at 1650 V m(^-1). Observations in fog show negative space charge originating at power lines, confirming earlier work, and suggest the use of space charge measurements to study atmospheric diffusion from a point source.