Airborne filters for the measurement of atmospheric space charge

Airplane instrumentation is described for measuring the concentration of atmospheric electric space charge by the utilization of a glass microfiber filter that collects effectively all ions and particulate matter from the air passing through it. This equipment gives space-charge-concentration values that are in good agreement with those estimated independently from the rate of change of the vertical potential gradient with altitude in clear, stratified air. Spurious indications of high concentrations of space charge were found when the filter apparatus was flown into clouds. This difficulty was traced to the undesirable charging of some cloud and precipitation particles that inevitably touched the entrance to the apparatus. The authors' experience leads them to doubt the validity of all airplane measurements—whether made with their devices or with any devices yet reported—of the charge on cloud drops. Outside the clouds the apparatus has been found useful on many flights in the direct measurement of natural and artificial space charge.