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The electric charge of raindrops
Author(s) -
Smith L. G.
Publication year - 1955
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49708134705
Subject(s) - electric field , drop (telecommunication) , electric charge , charge (physics) , range (aeronautics) , mechanics , physics , materials science , electrical engineering , composite material , quantum mechanics , engineering
The observations which are described and discussed in this paper were obtained using equipment designed to record rapidly, and without ambiguity, the electric charge and size of individual raindrops. The electric charge is measured by electrostatic induction on a metal cylinder as the drop falls through. The size of the smaller drops is determined from the time of fall between two such cylinders, while the size of the larger drops is measured by the change of capacity of a parallel‐plate condenser as the drop passes between the plates. The point‐discharge current, electric‐field strength and rain current were also measured. The observations show that on all occasions the range of charge on a particular size of drop is large, often including both signs, and that this range is not appreciably reduced by considering drops recorded during shorter time intervals. However, it is often found that the average charge and the limits of the range of charge show definite relations to the size of the drop. The variation of average charge is such that there is a certain size for which the average charge is zero; for smaller drops the average charge is opposite in sign to that of the electric field, while the larger drops have average charges of the same sign as that of the electric field. On some occasions the charges on the drops are too small to be recorded. The field is then usually reversed to a small negative value. It is suggested that the drops are falling from electrically neutral or slightly electrified clouds, the reversal of the field resulting from the splashing of drops on the ground. The capture of ions by raindrops falling in the region of the point‐discharge current below an electrified cloud is examined and it is shown that, if reasonable assumptions are made about the initial charges on the drops, the relation between the charge and size of drops arriving at the ground is similar to that found experimentally. Comparison of theory and experiment indicates that the field increased with height owing to the point‐discharge space charge. The range of charge is attributed to the mixing, while falling, of drops leaving the cloud base in regions of different current density. This requires that the regions of high current density are small areas of the cloud base, probably not exceeding 100 m in diameter.