CLEARING THE AIR

Southland Times, Issue 21097, 31 May 1930, Page 4

CLEARING THE AIR

CONFUSION. OVER LIGHTNING.

SCIENTIFIC THEORIES.

Since the startling and damaging flash of lightning at Gladstone on Wednesday there has been no little talk in Invercargill concerning atmospheric electricity. Frequently the words “thunderbolt” and "meteorite” have been used in connection with the occurrence, but these are quite erroneous, for it was nothing more than lightning which wrought the damage. Southland has fortunately been fairly free from bad thunderstorms, though many years ago a man was killed at the Elies Road station by a flash of lightning, another branch of which struck the Boys’ and Girls’ High School. It is only in comparatively recent years that thunderstorms and the phenomena associated with them have been scientifically investigated and theories advanced as to their cause.

The following article from the latest edition of the Encyclopaedia Britannica on the subject should prove of interest:— "It is a universally accepted belief that thunderstorms and lightning are a secondary phenomena brought, about by the abnormal meteorological conditions that are always found associated with them. Nearly all the theories of thunderstorms invoke, some mechanism by which falling rain drops, snow or hail become charged in some manner with one sign of electricity, while the air and smaller drops capable of being carried up by the rising columns of air associated with the storm are charged in the opposite sense, ihe separation of electricity proceeds until differences of potential are produced sufficient to result in a lightning flash. Accord- 1 ing to one of the earliest theories the rain drops were polarized by the potential gradient so that they became positively charged below and negatively charged above. The smaller spray associated with the rain, which is carried up in the rising air currents, was supposed to strike the larger descending drops on the underside and remove from them some of the positive charge, so that they fell to earth negatively charged while the smaller drops were carried up with the positive charge. On this view the electrical separation would act to increase the electric field formerly existing, a result not in harmony with the usual facts. There is a certain amount of evidence that the smaller drops may rebound from the larger drops in such a way as to part from them at the top; and on such a view the possibility of positive precipitation becomes realized. However, the theory has suffered criticism in connection with the possibility of the existence of the' necessary contact between drops at the moment of rebound to secure a separation of charge without a coalescence of ,tbe drops. “G. C. Simpson, after a long series of measurements made at Simla, India, in 1909, evolved a theory which rests upon the experimental fact that, when falling water drops are caused to break on a rising stream of air the water drops become positively charged while the air of lighter spray becomes negatively charged. In the period immediately nreceding the thunderstorm we picture a sort of hour-glass structure to the air motion in the vicinity of the thundercloud, the air feeding in towards the axis of the hour-glass below, then rising vertically and finally fanning out at the top. As the air rises into colder regions precipitation forms and the drops grow as they fall. However, it is contended that drops large enough to fall in air which is rising with a velocity up to eight metres per second become unstable and break up into fine drops. In this process they become positively charged while the air around them is negatively charged. It is thus contended that when the drops in their fall reach the narrowest part of the hour-glass where the velocity is high they break up and become positively charged.

“Having been reduced in size by the disruption they start to rise in the air current, but not as fast as the negatively charged air. As they rise they start to coalesce on account of their charge and eventually attain such a size that they once more start to descend. The process continues and as it does so the rain becomes charged more and more posi tively at each successive descent, while the rising air carries negative electricity to the ' top of the thundercloud. Eventually the potentials attained become sufficiently high to result in a flash between the top and bottom of the cloud, or occasionally between one part of the cloud and another. On Simpson’s views the rain which falls in the centre of the thundercloud should be positively charged, while a negative charge should appear on the rain which falls from the more remote regions and has received its charge from the negatively charged air which has risen to the top of the clouds and fanned out at the. aides Simpson's theory does not lead immediately in its details to an explanation of the origin, of lightning flashes in cases where all the water concerned is in the form of ice. However H. Nounder has shown that large separations of charges may be produced by the impact of ice particles with each other, the small particles becoming charged with a eign opposite to that which the larger particles acquired. “By measuring the changes of electrical potential of an exposed insulated conductor, scientists have made estimates of the electric fields associated with thunderstorms, and from these have deduced the orders of magnitude of the charges nn thunderclouds and the amount of electricity transferred when a lightning flash takes place. The fields developed in the vicinity of thunderclouds are of the order of 100,009 volts per metre. Data collected by the British Meteorological Office show that at any one time about 1,800 thunderstorms are in progress in different parts of the world, and that the average frequency of lightning, flashes during a etorm is about 200 per hour. This gives about 100 flashes per second as representative of the average frequency of lighting discharges for the whole earth.

“It has been supposed by one scientist, W. J. Humphreys, that a lightning flash progresses by a sort of burrowing action rather than by a simultaneous breakdown of the whole lightning part. Humphreys supposes that at any instant during the formation of a flash the field at the end of the lightning streak ionizes the air immediately around it, permitting of its further extension, and so on until the flash is completed. While there is some doubt as to the universality of this origin of the flash the process undoubtedly occurs in certain instances and to special forms of this type of development Humphreys has attributed the occurrence of ‘rocket’ lightning and ‘ball’ lightning. The former is seen as a progressive burrowing of the flash through the eky and is to be attributed to conditions in which the flash grows at a slow rather than at its customary rate Ball lightning, in which a ball of fire may be seen to hang in the sky, may be regarded as a form of rocket lightning in which the progress of the flash is very slow and the conditions are such that the ionization at the end of the discharge is just able to maintain itself. ‘Sheet’ lightning is merely the reflection of ordinary lightning from the clouds.”