An investigation of atmospheric electrical phenomena within 22 m of the ground during disturbed weather

Measurements of potential gradient, precipitation current and rate of rainfall were made at ground level and simultaneously measurements of potential gradient and precipitation current were attempted at the top of a 22 m mast. On the mast reliable measurements of precipitation current could not be obtained and the calibration of the field mill proved difficult. An attempt was made to calculate its exposure factor and response to low level space charge by numerical methods. Some success was achieved in this and the mill was found to be about half as sensitive to space charges below it as it was to those above, which halved the experimental arrangement's sensitivity as a space charge detector. Despite this space charges of several hundred picocoulombs per cubic metre were observed when the potential gradient exceeded about 800 Vm(^-1). On one occasion it was possible to show that the space charge was produced by corona discharge from a group of trees. The precipitation current at ground level could be adequately described by the equation I = -2.6R(F - 112) pAm(^-2) where R is in mm min(^-1) and F in Vm(^-1). Although no direct measurements of variations of precipitation current with height could be made it was clear that the rain sometimes did gain charge close to the ground by collecting local space charge.