Thunderstorms and Related Phenomena

THUNDERSTORMS AND RELATED PHENOMENA The first observations on the charge carried down by rain were made by Elster and Geitel (1899) and by H. Gerdien (19o3 ), and led to the conclusion that more negative than positive electricity was precipitated by this cause. However, G. C. Simp son (1909), in a long series of measurements made at Simla, India, concluded that a much larger precipitation of positive than of negative occurs, and his conclusions have been supported by many observers in various parts of the world. The currents car ried to ground by rain are usually of the order i o 12 amperes per sq.cm., and the amount of electricity per cu.cm. of the precipitated water is usually less than one electrostatic unit.

Thunderstorms.

It is a universally accepted belief that thunderstorms and lightning are a secondary phenomena brought about by the abnormal meteorological conditions we always find associated with them, and nearly all 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. The separation of electricity proceeds until dif ferences of potential are produced sufficient to result in a light ning flash.

According to one of the earliest theories, that of Elster and Ceitel (1885, 1913), the rain drops were polarized by the potential gradient, so that they became positively charged below and nega tively charged above. The smaller spray associated with the rain, and 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 upwards 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 be tween drops at the moment of rebound to secure a separation of charge without a coalescence of the drops.

G. C. Simpson's theory 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 or lighter spray becomes negatively charged. In the period im mediately preceding the thunderstorm we picture a sort of hour glass structure to the air motion, in the vicinity of the thunder cloud, 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 fail in air which is rising with a velocity up to 8 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 dis ruption, 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 positively 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 re ceived its charge from the negatively charged air which has risen to the top of the cloud and fanned out at the sides.

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. Norinder 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 sign opposite to that which the larger particles acquire. Again, a separation of the two comes about through the greater supporting power of the rising currents for the smaller as compared with the larger particles.

The Electron Currents and Fields Developed in Light ning Flashes and Thunderstorms.—By measuring the changes of electrical potential of an exposed insulated conductor, C. T. R. Wilson (192o), H. Norinder and others have made estimates of the electric fields associated with thunderstorms, and from these have deduced the orders of magnitude of the charges on thunder clouds, and the amount of electricity transferred when a lightning flash takes place. Estimates of the last named quantity have also been made in a novel manner by W. J. Humphreys from an estimation of the pinch effect produced in a lightning rod which had been struck and crushed by the attraction of the current elements on each other during the discharge.

The fields developed at the earth's surface in the vicinity of thunderclouds are of the order of ioo,000 volts per metre, and an average lightning flash may discharge about 20 coulombs through a length of two kilometres.

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 storm is about 200 per hour. This gives about loo flashes per second as representative of the average frequency of lightning discharges for the whole earth.

Special Forms of Lightning.

It has been supposed by W. J. Humphreys that a lightning flash progresses by a sort of burrow ing action rather than by a simultaneous breakdown of the whole lightning part. Humphreys supposes that at any instant during the formation of the 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 sky and is to be attributed to conditions in which the flash grows at a slow rather than at its customary rapid 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. Some observers have reported the ap pearance of discontinuous or beaded streaks of lightning. It has been suggested that they arise from successive discharges and that each bead may represent the end-on view of the irregular portions of the lightning path; these remain luminous during the subsequent lesser discharges, while the intermediate sections be come non-luminous because viewed from the side. St. Elmo's fire is simply a brush discharge from objects projecting from the earth's surface and it is caused by intense fields resulting from the presence of thunderclouds.