SCIENTIST and SCIENCE
- VOL.51, NO.765, AUGUST 1971
The exact mechanism of thunderstorm
electrification and the glaciations processes in ice clouds that
lead up to it. Remain elusive. Earlier this year Monitor reported
some work showing how freezing super cooled drops eject tiny positively
charged droplets to produce primary electric charge separation
(see New Scientist. viol 49. p 54-1970), Roger J. Cheng
who discovered this effect, and his colleague Vincent Schemer
at the State University of New York. Have now pointed out some
related effects in the growth of ice crystals which may account
for the observed very rapid glaciations process that typifies
certain types of super cooled concessive clouds (Journal de
Recherché Atmosphéngues. vol V. no I.-1971).
clouds often cause thunderstorms and blizzards. In the laboratory
it is possible to generate the kind of small ice crystals seen
in Figure d, which closely resemble those found by sampling actual
clouds of this type, by forming a layer of frost on a cold piece
of ice. Schaefer and Cheng say the effect can be demonstrated
by putting some dry ice into a polythene bag and sticking the
bag in warm air. The bag soon becomes coated with a dust-like
frost layer which grows and eventually sprouts minute tree-like
growths of ice crystals. Under a low-power microscope these particles
arc seen to be intensely mobile, twisting and writing, breaking
off from the surface, and sometimes exploding into masses of even
smaller fragments. the photograph shows, the ice particles are
often very tenuously attached to the surface. It seems obvious
that their motions must be a response to static electrical forces.
two cloud physicists propose that a similar mechanism is at work
on the surfaces of hailstones which form in Convective clouds
at an early stage when there are plenty of droplets and moisture
for their growth. As they get larger they fall into regions of
warmer air which, if it is super-saturated. Coats them with a
delicate layer of frost. The disintegration of the frost particles
leads to further nuclei for the development of new ice crystals.
Thus providing a chain reaction of glaciations. Figure C shows
an experimental hailstone-in a moist environment repelling frost
particles electrically from ice dendrites. Under the microscope
the ice particles can be seen to move vigorously in response to
an electrically charged object. The stems” of the ‘trees” are,
in many cases, very thin (often less than five micrometers) that
they would be expected to evaporate easily, adding to the fragility
of this frost layer. And. In fact, Schaefer and Cheng have observed
progressive thinning of the particle’s attachments in this manner.
recently wrote to Monitor pointing out that their team has now
determined that the ejected ice particles from frosty surfaces
mostly carry negative electrical charges. Presumably leaving
the surface in question with a net positive charge. Once a basic
charge-separation mechanism of this kind has been proven it becomes
possible to suggest means by which larger and smaller components
are physically separated and thus build up the high voltages
characteristic of thunder clouds. It appears that protons travel
more easily through ice and tend to congregate at the colder parts.
In Cheng’s earlier experiment the warmer water droplets were positively
charged and the residual freezing parent drop negatively charged.
On disintegrating frosty surfaces this situation is apparently
later discovery is that ejected ice fragments certainly can be
grown to produce perfect ice crystals.