Observations of hydrometeor charge evolution in thunderstorms

Systematic observations were made of hydrometeor charges associated with thunderstorms at Flagstaff, Arizona. The measurements were made from a light aircraft by the standard induced‐charge method, in and below clouds which were entirely supercooled. The aircraft would spiral within the central upcurrent up into the cloud and later spiral down through the precipitation shafts. From these limited cases, a rather consistent picture emerged: (a) generally, charges on graupel and hail within the supercooled clouds were positive (measurements made as high as the −17.5°C level), (b) below the cloud at the + 2° to + 8°C level, just where the melting of the hydrometeors was apparently complete, the hydrometeors became abruptly negative, (c) at still lower, warmer levels the charge magnitudes were consistently smaller, with both signs represented but predominantly negative. The data suggest that a strong hydrometeor charging mechanism is associated with the melting of ice hydrometeors outside the cloud. The maximum charge magnitudes appeared comparable to the amounts which would be limited by breakdown gradients at the hydrometeor surface.