THE HYPERELECTRIFICATION OF RAINDROPS BY ATMOSPHERIC ELECTRIC FIELDS

Abstract

The role of atmospheric electric fields in selectively charging raindrops is analyzed and compared with direct measurements made inside an active thunderstorm. Expressions are developed for the free charge transferred to conducting spheres polarized by an electric field and immersed in an environment in which the positive and negative ionic conductivities are different. Measurements in a wind tunnel operating with ionized air confirm the analysis. Because the electrical conductivities inside a cloud and in the clear air immediately outside are so very different, electric fields acting across such a cloud boundary selectively concentrate ions of a single sign in a space-charge layer. Extensive regions are thereby established wherein the positive and negative ionic conductivities are notably different. Cloud and raindrops in this transition layer become hyperelectrified. Expressions for the mean space charge in the transition layer are developed. The estimated drop charges and space-charge densities are found to agree with earlier measurements made at various levels inside an active thunderstorm. It is found that a large fraction of the free space charge inside a thundercloud is carried by the precipitation and is roughly proportional to the electric field intensity within the cloud-boundary transition layer. Since transition layers near the cloud boundaries play an important part in the electrification process, precipitating cumulus-type clouds developing in a clear-air environment are most likely to produce active lightning.