Electrical Conditions Near the Bases of Thunderclouds in the Southwestern United States

Abstract

Measurements of the electrical conditions near the bases of thunderclouds have been made at Langmuir Laboratory, a mountain top observatory, with instruments carried aloft by captive balloons and with others at the earth’s surface. The electrical properties measured include the local electric field vector, the conductivity of the air charge on individual precipitation particles, and the current density carried by falling precipitation.

The electrical conductivity of cloudy air was found to be about 1/10 that of clear air at the same altitude with a typical electrical relaxation-time-constant of approximately 4000 seconds inside the lower regions of clouds.

The polarity of charge on the precipitation aloft was almost invariably that of the local potential gradient; it did not show the well-known ‘mirror-image’ relation that was observed between the precipitation electricity at the ground and the local potential gradient during periods of point discharge. An analysis indicates that Wilson ion capture in the subcloud region may explain adequately the charge arriving at the mountain top on falling precipitation.

The transport of charge by precipitation aloft that we observed usually acted to decrease the local electric field; no significant increases in the electric energy of the clouds were produced by the fall of precipitation particles that we captured.

Analysis of the precipitation data suggests that the charge on raindrops aloft could have been derived from that residing on the cloud droplets from which the raindrops were formed. The charge carried on both liquid and frozen precipitation particles at cloud base was generally less than 5 pC; the mean specific charges were of the order of 3 mC m^-3 of precipitation water.