THE DEVELOPMENT AND MASKING OF CHARGES IN THUNDERSTORMS

Abstract

It has been generally accepted that segregation of charges in thunderstorms is accomplished by gravitational sedimentation. In contrast to the much-studied phenomena of charge generation and charge distribution, the mechanism of charge segregation has commanded little attention so far although it is the connecting link between the generation and distribution of charges. As a consequence, conclusions as to the ?proper? polarity of charging effects have been based on relatively weak reasoning. A more thorough investigation of the sedimentation process reveals that the mechanism is more complicated and tends to ?mask? rather than to segregate charges. Only the net space charge is perceptible through the electric field gradient. The masking process is illustrated by studying the development of charges on solid precipitation in a supercooled water cloud. During the charging process, a volume of cloud air obviously loses the charge withdrawn by precipitation particles falling through it. The theory shows that it takes only about one minute from the onset of precipitation until the opposite charges on precipitation and cloud particles compensate each other completely in a given space. After this ?compensation period?, the cloud charge begins to predominate over the masked precipitation charge and determines the polarity of the (net) space charge. However, it does not grow indefinitely. Subsequently air conductivity enforces an equilibrium in which the two opposite charges mask each other in a fixed ratio expressed by the ?masking factor?. The rate at which the equilibrium is approached is controlled by the ionic relaxation period. The masking factor is given simply by the ratio of compensation period to relaxation period. It is further shown that the result is different if updrafts exceed the falling speed of precipitation. The precipitation charge then predominates over the cloud charge. In this way, the distribution of vertical motions in the mature thunderstorm cell tends to favor the development of a charge dipole. A positive ice phase is tentatively concluded. At certain ?stop levels? (for example near the freezing level), an ?unmasking process? sets in which is capable of exposing the hidden precipitation charge. Quantitative estimates of the masking process are based on precipitation mechanisms. The resulting field gradients seem to bear out the essential properties of observed thunderstorm fields. Especially the discontinuity in the freezing level is confirmed. It is pointed out that the masking process works in every type of charge segregation, regardless of whether or not precipitation is an essential participant. Some implications of the masking process indicate a need for a critical review of generally accepted concepts in the field of thunderstorm electricity.