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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.

This might be caused electrically by the kind of orientation of ice-crystal platelets that we have observed in the laboratory, or by the orientation of ice-crystal needles or columns.

Possibly the optical effects might be caused by yet another type of electrical orientation such as that shown in Fig. 3 in which all of the ice-crystal hexagons become aligned in the directino of the electric field. In this case the parhelion might be of extraordinary brilliance when it was viewed from the proper angle.

Figure 1 Ice-crystal platelets fall with their principal axes vertical and they reflect light like little horizontal mirrors


Figure 2 Under the influence of an electric field the ice crystals assume a new orientation and reflect light in a different direction


Figure 3 Under the influence of an electric field induced dipoles can form in crystals that can cause alignment of the hexagonal structures

One might expect that the change of orientation of the ice-crystals would be rather slow while the electric field was building up in the cloud and that it would be very rapid during the sudden change of field caused by the lightning.

In New Mexico during the summer of 1962 I carried out some experiments to see if during a lightning stroke any rapid changes could be observed in the brightness of active thunderclouds illuminated by the sun.. By the use of a simple photo-tube device that responded to light changes occurring in a time of a second or less I found that there were sudden increases and decreases of brightness of the coulds of the order of a few tenths of a per cent that occurred at the same time that lightning was indicated by the noise of a radio station heard on a small portable radio.

It is possible that the observers of the Hertfordshire thundercloud may have been located in just the correct position to view similar but more intense optical effects caused by ice-crystals, such as the sun pillar or the 22-degree halo that were modulated by changes in the electric field of the tundercloud.

It is worth mentioning that I have heard of an apparently similar report from an aeroplane pilot who while flying over the top of a thunderstorm had seen a bright band that suddenly moved across the top of the anvil slowly enough to be observed visually. It is possible that the phenomena of this sort may be fairly common and that they may be useful for providing information concerning the electrical process taking place witin the thundercloud.

REFERENCES

Coons, Richard D and Gunn, Ross 1951 Relation of artificial cloud modification to the production of precipitation. Compendium of Meterology, Mer. Met. Soc., Boston Mass., p. 255
Hale, RB 1950 Unusual lightning Weather, 4, (11) p. 394
Lacy, RE 1950 Unusual lightning Ibid, 4, (11) p. 395
Ludlam, FH 1950 Unusual lightning Ibid, 4, (11) p. 394
Schaefer, VJ 1946 The production of ice crystals in a cloud of supercoolled water droplets. Science, 104, p 457









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