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The Electrification of an Ice Sphere Moving through Natural Clouds

P. V. HobbsUniversity of Washington, Seattle

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D. A. BurrowsUniversity of Washington, Seattle

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Abstract

Measurements have been made of the electric charge acquired by an ice sphere as the result of whirling it through various types of natural cloud and snowfall. The sphere received an appreciable charge only if ice were present in the air. In the case of an ice crystal cloud or a fall of snow crystals, the charge on the sphere was generally negative provided that the air temperature was −4C or less. Simultaneous measurements of the charge on the ice sphere and the number of ice crystals colliding with a sphere of similar size yielded a value for the charge on the sphere of −2 × 10−3 esu per ice crystal collision. When the air temperature was above −4C, the charge on the sphere was erratic but was found to have the same sign as the charge on the particles in the air. If graupel particles were present the ice sphere always received a large positive charge.

The presence of supercooled droplets in the air caused a considerable reduction in the charge accumulated on the ice sphere. When the sphere was whirled through a cloud consisting entirely of supercooled droplets negligible charge was separated. This result is in disagreement with the laboratory measurements of Latham and Mason, and casts doubt on the mechanism of thunderstorm electrification proposed by these workers.

Abstract

Measurements have been made of the electric charge acquired by an ice sphere as the result of whirling it through various types of natural cloud and snowfall. The sphere received an appreciable charge only if ice were present in the air. In the case of an ice crystal cloud or a fall of snow crystals, the charge on the sphere was generally negative provided that the air temperature was −4C or less. Simultaneous measurements of the charge on the ice sphere and the number of ice crystals colliding with a sphere of similar size yielded a value for the charge on the sphere of −2 × 10−3 esu per ice crystal collision. When the air temperature was above −4C, the charge on the sphere was erratic but was found to have the same sign as the charge on the particles in the air. If graupel particles were present the ice sphere always received a large positive charge.

The presence of supercooled droplets in the air caused a considerable reduction in the charge accumulated on the ice sphere. When the sphere was whirled through a cloud consisting entirely of supercooled droplets negligible charge was separated. This result is in disagreement with the laboratory measurements of Latham and Mason, and casts doubt on the mechanism of thunderstorm electrification proposed by these workers.

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