| 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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