| Recent observations
by Roger J. Cheng of the State University of New York reveal a fascinating
new microscopic process which almost certainly has an important
bearing on how thunderclouds become charged to their high voltages
(Science. vol
170, p 1395, 1970).
About 10 years ago Dr B. J. Mason,
Director of the Meteorological Office (then at Imperial College,
London), advanced a novel theory in which the charge separation
depended, at the atomic level, on the greater mobility of the
protons in water compared with that of the negative OH ions. His
experiments confirmed that protons do, indeed, travel more readily
towards the colder end of a piece of dry ice supporting a temperature
gradient. He established that when a stream of supercooled water
drop froze on being blown past a hailstone in a wind tunnel it
burst, shattering with the ejection of positively charged ice
splinters, and leaving the hailstone negatively charged. Gravity
acting on the 'hailstone, and up-currents on the splinters, could
then give realistic charging rates for a typical thundercloud
with a net charge of 1000 coulombs.
Cheng's
discovery is that a freezing supercooled drop can perform its
own charge separation in an even neater fashion. Starting
with drops of one mm diameter he cooled them until they began
to freeze at about - 15°C. Using cinematography he observed that
at this juncture, the drops began to spray out tiny droplets 1
to 20um in diameter, through the thin, forming ice shell. They
fell on hit micro slide in large numbers for approximately 50
seconds until the drop was completely frozen.
With an electrometer probe, and
also by electrostatic deflection methods, Cheng showed that the
ejected droplets carry a net positive charge, while the frozen
drop retains a net negative charge. However the droplets form-whether
forced out through minute cracks in the ice shell in response
to internal pressure, condensed from water sublimated from the
drop, or as a result of small bursting air bubbles beneath the
ice surface-the charge separation
at the atomic level clearly occurs as in Mason's hypothesis.
The outer surface of the drop is colder than its interior and,
again, concentrates the positive charge.
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