DROPLET-ELECTRIFICATION PROCESSES AND COAGULATION IN STABLE AND UNSTABLE CLOUDS

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  • 1 U. S. Weather Bureau
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Abstract

The fundamental electromechanics of droplet electrification and coagulation within stable and unstable clouds is investigated. The analysis shows that atmospheric ions formed by cosmic rays or other means normally diffuse onto cloud droplets and electrify them. A nearly Gaussian distribution is established in which about half of the droplets in any selected volume of a stable cloud acquire a positive charge that is typically eleven electronic units, while the other half is negative. More than 9.5 per cent of the droplets of a typical cloud are electrified. When droplet association is negligible, an equipartition is established between the thermal kinetic energy of the droplets and their electrical potential energy.

In an unstable cloud, these electrified droplets mechanically associate by relative motion in the gravitational field. Thus, the growing droplets accumulate charge and a statistical distribution of highly charged droplets is established. Expressions are derived for the distribution and for the mean statistical charge on the associated droplets. Equal numbers of positive and negative droplets are normally produced, but systematic charging due to unequal ionic conductivities sometimes results in marked electrification of a single sign.

The influence of droplet electrification upon cloud stability is considered. The droplet charges normally have a detectable, but not important, direct effect upon the coagulation rate of the cloud. The indirect effects, however, may be large. Electric fields of appreciable magnitude always accompany precipitation as a result of drop-charge separation. Expressions are derived for the electrical coagulation of clouds immersed in an electrical field. It is shown that electrical coagulation may exceed that due to gravitation whenever the environmental electric field is 2 statvolts per centimeter or larger. Since fields this large are commonly observed in thunderstorms, the resulting precipitation rates are correspondingly large.

Abstract

The fundamental electromechanics of droplet electrification and coagulation within stable and unstable clouds is investigated. The analysis shows that atmospheric ions formed by cosmic rays or other means normally diffuse onto cloud droplets and electrify them. A nearly Gaussian distribution is established in which about half of the droplets in any selected volume of a stable cloud acquire a positive charge that is typically eleven electronic units, while the other half is negative. More than 9.5 per cent of the droplets of a typical cloud are electrified. When droplet association is negligible, an equipartition is established between the thermal kinetic energy of the droplets and their electrical potential energy.

In an unstable cloud, these electrified droplets mechanically associate by relative motion in the gravitational field. Thus, the growing droplets accumulate charge and a statistical distribution of highly charged droplets is established. Expressions are derived for the distribution and for the mean statistical charge on the associated droplets. Equal numbers of positive and negative droplets are normally produced, but systematic charging due to unequal ionic conductivities sometimes results in marked electrification of a single sign.

The influence of droplet electrification upon cloud stability is considered. The droplet charges normally have a detectable, but not important, direct effect upon the coagulation rate of the cloud. The indirect effects, however, may be large. Electric fields of appreciable magnitude always accompany precipitation as a result of drop-charge separation. Expressions are derived for the electrical coagulation of clouds immersed in an electrical field. It is shown that electrical coagulation may exceed that due to gravitation whenever the environmental electric field is 2 statvolts per centimeter or larger. Since fields this large are commonly observed in thunderstorms, the resulting precipitation rates are correspondingly large.

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