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ORIENTATION OF ICE CRYSTALS IN THE
ELECTRIC FIELD OF A THUNDERSTORM

B. Vonnegut
WEATHER v20, no.10, pp310-312 (1965)
doi: 10.1002 / j.1477-8696. 1965. tb02740.x

An interesting and unusual optical phenomenon associated with a thunder- cloud is reported and discussed by Hale(1950), Ludlam(1950), and Lacy(1950). Early in the afternoon a thundercloud viewed from the north was observed to form a 'bright streamer apparently of cloud projecting northwards from the anvil.' This streamer repeatedly built up slowly and then suddenly disappeared apparently at the same time as a lightning discharge took place. When I recently came across the report of these observations, I was struck by their similarity to an interesting electrical phenomenon that can readily be demonstrated in the laboratory using the elegant cold-box technique devised by Schaefer(1946). It appears worth describing this possible laboratory analogue to the thunderstorm phenomenon for it adds yet another possible interpretation to those that have already been suggested by Ludlam and Lacy.

If a supercooled cloud in a Schaefer cold-box is seeded (either with dry ice or silver iodide) and illuminated with a beam of light, it can be observed that the ice crystals reflect the light like little mirrors. As the ice crystal platelets slowly fall, they all become orientated with their princepal axes in the vertical direction so that the light is reflected from their horizontal surfaqces and the cloud appears quite bright in the region where the light is reflected toward the observer (see Fig. 1). This phenomenon is apparently identical with the sun pillar and sun reflections sometimes observed in the atmosphere (Coons and Gunn 1951).

If one performs the experiment of creating a strong electric field in the ice-crystal cloud by the introduction of an electrically-charged object such as an ebonite rod, it will be observed that the position where the cloud appears bright because of reflections from the ice crystals can be made to change and move about by changing the position of the electrified object.

The electric field apparently causes this effect because it induces electrical dipoles in the ice crystals giving rise to forces that tilt them as is illustrated in Fig. 2.

According to the descriptions of Hale and Lacy, ice crystals were present and it appears possible that the unusual optical effects that they observed might have been caused by changes in the orientation of ice crystals produced by the strong electric field of the thundercloud.

[Fig. 1]

[Fig. 2]








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