LIGHTNING AND THUNDERSTORMS

Hawera Star, Volume LIII, 17 June 1933, Page 14

LIGHTNING AND THUNDERSTORMS

1? N reviewing the history of our knowledge I of atmospheric electricity, we find it can he _L divided into three well-marked periods, said the lecturer. The first of these periods opens with the suggestion made, by a number of philosophers, between 1730 arid 1750, that the phenomenon of lightning was nothing more than an exhibition, on a grand scale, of the sparks which could be drawn from the primitive electrical machines of that time. Dalibard in France and Franklin in America were the first to show in 1752, by actual experiments, that electricity can be drawn from clouds during thunderstorms.

when a charge passes to or from the ground. We know that the mobility of a water particle —no matter how highly charged—is so small that no electrical field, short of that which will break down air, can produce an appreciable current. In other words, there can be no discharge until there is an electrical breakdown of the air. Electrical breakdown in air, at normal temperature and pressure, occurs when the field strength reaches 30,000 volts per c.m. In this field pressure an electron attains such a high velocity during the short time that it is free, that when it impinges on a neutral molecule at the end of its free path it has sufficient energy to drive one or more electrons out of the neutral molecule. These released electrons are in the same intense field and they repeat the process. The effect is obviously progressive, and in a very short time there arc great quantities of free electrons all moving with high velocity along the lines of force; The rapidity of motion of an electron is of an entirely different order of magnitude from that of an ion in the same field; in fact, the difference is so great that we may consider that both positive and negative ions are, when compared with electrons, quite immobile, even in the strongest fields. Positive electricity is always carried by ions. Protons, the elementary units of electricity, are themselves atoms of matter and fherefore are ions. As soon, therefore, as air breaks down large quantities of negative electricity can move as electrons, while the .positive electricity is held immobile on the positive ions. In short, the progressive release of electrons forms a highly conductive path along the lines of force or stress, and tlie result is a discharge.towards the seat of negative electricity. It is of importance in all practical problems connected with'lightning to know the order of magnitude of the electrical factors involved. We owe to C. T. R. Wilson the best knowledge we have on these points. Wilson finds that the quantity of electricity discharged in an average lio-htning flash varies between 10 and 50 coulombs, and he takes 20 coulombs as being typical. This is a surprisingly small quantity of electricity, being merely 20 ampere seconds,, a quantity which would appear to be too insignificant to call for consideration by electrical engineers. Wilson was the first to fix bv direct observation the quantity of electricity involved in a lightning flash, and that as recently as 1920. With regard to the potential or pressure associated with a lightning flash, Wilson estimates that the potential reached in a thunder cloud, before the passage of a discharge of 20 coulombs, is of the order of a thousand million volts. Of course, potentials of this order of magnitude are never fully reached at the earth’s surface, when a body is struck, because of the resistance of the air channel, but. there is no reason why potentials of tens of millions of volts should not be reached, when conductors such as transmission lines, which are not directly connected to earth, receive a direct stroke. Expressed in units, the energy dissipated in an average lightning discharge is o± the order of 3000 kilowatt hours. As electricity of one sign is accumulated durino- a thunderstorm in one part of a ctouct, and the opposite electricity in another pai . the cloud, the field between them locreases intensity. This' is represented by the familia positive and negative. As the charge i. uniformly distributed, the lines of foiee aie not parallel, but approach one another m the region of the strongest field at the .junction o the two charges. If the field strength increases a break-down of the air will occur. As soon as this happens, a small region becomes a »ood conductor—and the effect is exactly the same as if a piece of wire were introduced into trie electrostatic field. The lines of force assume a new distribution, and the field becomes great]v intensified at the ends of the conducting region, Where the lines crowd into the conductor. The electrons have moved rapidly towards the positive charge on the left, leaving the right-hand half of the conducting region full of positive ions which, as already stated, may be considered to be immobile. These ions have little mobility and remain in a diffused region around the end of the conducting channel. The initial intense field no longer exists at the end of the conducting region, because this region has been greatly enlarged and many of the lines of force find tlieir ends on the negative electricity bound on the new negative ions. At the other end of the conducting region conditions are entirely different. The channel is full of positive ions, but although the field tends to drag them out, they are too massiv c to move appreciably, and the shape of the channel remains unchanged. Owing, however, to the concentration of the lines of force which took place when the air broke down, the field at the end of the conducting region is very intense, and within a space around the end of the channel air cannot withstand the stress and breaks down still further, with the liberation of further large quantities of free electrons. These electrons move at once into the conducting channel and, passing along it, keep the air within the channel highly ionized by their collisions with the air molecules. Ultimately they find their way into the cloud of negative ions at the other end of the channel. The transfer of the electrons from the newly ionised region at the end of the channel, leaves that region full of positive ions; in other words, the conducting channel is simply prolonged. The process, however, cannot stop here, for the end of the growing channel remains sharp and the field at the tip in consequence still sufficiently intense to ionise the air. Thus the channel prolongs itself rapidly in the direction opposite to the flow of the electrons, and far into regions where, before the discharge, the field was much too weak to cause the break-down of the air. The discharge, once started, makes a channel only in one direction, that direction being

During the century which followed it was shown that the atmosphere is electrified, not only during thunderstorms, but also during fine weather, even when there is not a cloud in the sky. It was found that' the electricity of the air undergoes a regular daily and yearly variation, and that the state of the weather plays a predominating part in determining the electrical state of the atmosphere. The results, however, were very vague and it was impossible to formulate any clear ideas or even to describe the results in any satisfactory way. During the latter half of this first period, that is, during the first half of the 19th century, dittle progress was made, and observations gave way to speculative theory. About this time the foundations of the modern theory of mathematical electricity were being laid on the classical work of Coulomb and Faraday. Amongst the pioneers of this new study of electricity was William Thomson, afterwards Lord Kelvin. Among the most important of his suggestions was the idea of an electrical potential of a point in the air, and he showed how it could be: measured and recorded. Electrical potential gradient in the atmosphere now became recognised as an important meteorological factor with a physical meaning and one which could be recorded continuously. In 1861 the first recording electrometer was installed at Kew under Thompson’s direction, and these recordings have continued almost continuously till this day. For the next forty years these measurements of the potential gradient were made iri many parts of the world.. During this period our knowledge of the electrical field of the earth’s atmosphere was greatly extended, but little advance, was made in our knowledge of the' cause of the electrification.. The third period opened in 1900-1901 by the discovery of radio activity and of the existence of ions in the atmosphere. It has been known for a long period that air is not a complete non-conductor of electricity. Charged bodies were known to. lose their charge when there was no other possible source of leak. Linss showed in 1887 that this loss was greatest when the air was dry, during fine weather. There was, however, great doubt as to the way the charge was lost. It was. generally supposed that the molecules of the air striking the charged body took on a small charge which they carried away with them. It was impossible to disprove this suggestion, and it did offer a reasonable explanation. The discovery of the Rontgen rays led to a rapid expansion of our knowledge of the passage of electricity through gases. It was found that molecules of gases could be ionised; an electron could be driven out of one molecule, leaving it positively charged, while a neutral molecule could capture a free electron and become negatively charged. A molecule charged in this way is called an ion. In 1900-1901 Elster and Geitel in Germany and ‘Wilson in England showed that clear, dustfree air always contains a number of positive, and negative ions, and it is the movement of these ions in an electrical field which makes air a conductor. The conductivity depends on the number of ions present and their mobility. Subsequent investigation has shown that in the lower atmosphere there are generally about 500 ions of each sign in a cubic centimetre, and that they move with a velocity of about 1 c.m. per second, in a field strength of one volt per c.m. A column of air one inch long offers as much resist/ince to the passage of electricity as a copper cable 30,000 million million miles long of the' same cross section, or as much resistance as that of a copper cable long enough to reach from here to Areturus and back twenty, times. This conductivity, when expressed in this way, may sound very small —but it is far from being insignificant. It is so large, in fact, that a charged insulated body exposed to the atmosphere loses some 3 per cent, of its charge in a minute, and the bulk of its charge will have leaked away through the air in half an hour. Within a cloud there are practically no ions, for if ions are formed within a cloud as rapidly as they are formed in clear air, they cannot persist, for they are immediately absorbed by the cloud particles and lose their mobility. Thus there' can be little conduction of electricity within a cloud, and a cloud is one of the best insulators we have. We must obviously change our picture of a thunderstorm, for in place of conducting clouds in a non-conducting atmosphere we have to think of non-conducting clouds floating in a conducting atmosphere. Any electricity within a cloud must be carried by the water in the cloud, either on the cloud particles themselves of on the rain, hail or snow contained within the cloud. These charges can be very great, so great, in fact, that in the strong electrical forces associated with a thunderstorm, the electrical forces on the raindrops may be actually greater than the force of gravity. As the charge accumulates in a certain part of the cloud it must ultimately discharge, either to an accumulation of the opposite sign in another part of the cloud or to the induced charge on the earth below. Very little is known as to how this charge is initiated and then propagated, but a great deal is known about the discharge of electricity through air from one conducting electrode to another. But then again there are no electrodes in a thunderstorm or, at most, only one:

THEORETICAL CAUSES OF PHENOMENA ELECTRIFICATION OF ATMOSPHERE (Lecture given before the Hawera Astronomical Society by Mr. W. W. Davy.)

towards the seat of the negative electricity, while in the other direction there is no channel only a diffuse cloud of negative ions. All the branches are in the same direction, that is, pointed away from the seat of the positive electricity. This is a most important point, for it enables us to identify the positive end of a lightning flash whenever branches can be seen. A series of laboratory experiments made hy Dr. Simpson support the theory in all particulars. One of these experiments consisted of two copper discs to represent the charged region of a cloud, one positive and one negative, were placed on a photographic plate. Each disc had a small wire protuberance to concentrate the field. The discs were connected to a Wimhurst machine and a discharge was passed. Long, thin channels, each sharply pointed, passed out of the positively charged electrode, while from the negatively charged electrode there were no channels, but only a small cloud in the immediate neighbourhood of the wire point.

There can be little doubt that the formation of the cloud of negative ions around the end of the discharge plays an important role in the character of the discharge, for it reduces the field at the root of the channel and tends to prevent the flow of electricity along the channel. The channel may be actually blocked by the cloud of negative ions before sufficient negative electricity has been passed along the channel to neutralise the positive charge in the cloud. The discharge then ceases until either the block is removed or a side discharge opens a new passage between the end of the channel and another part of the cloud. In this way the discharge down the main channel may be intermittant, for the channel remains ionised for an appreciable time after each partial discharge has ceased. It is a great help towards understanding the varied phenomena of lightning to have some idea of the processes which build up the intense electrical fields associated with., thunderstorms. Any theory of the electrical processes in a thunderstorm has to account for the initial separation of positive and negative electricity (or, shall we say the generation of the electricity) and then for the transfer of the separated electricities into widely different regions of the cloud. In the theory which Dr. Simpson describes the generation of electricity is a consequence of the disruption of raindrops, and is therefore called “the breaking drop theory,” while the separation of the electricity into different regions of the cloud results from the different velocities of cloud particles and raindrops —relative to the vertical air currents which are such a marked feature of all thunderstorms. So long ago as 1892 Professor Lenard showed that when pure water splashes against a solid obstacle electrification results, the water becoming charged with positive electricity, while the negative electricity remains suspended in the air. In 1908 Dr. Simpson made experiments in Simla extending Professor Lenard’s work and showed that the same separation of electricity takes place when a drop ot water is broken up in the air without striking a solid obstacle. It. was natural to conclude that we have here the source of electricity in thunderstorms. The next step was to consider the conditions under which drops are disrupted m thunderstorm, and here again the P ceeded from previous work by Lenaid. A d p of water falling through still an iW tains an “end velocity” which obviously wil be different for drops of different sizes. It was found that the end velocity reaches a limiting value as the size of the drop increases very nearly equal to eight meters per second, above which it does not increase; it even decreases a little as the drop grows still greater. Lenard showed that this apparent anomaly is due to the drops becoming deformed, so that instead of retaining the shape of spheres, they become flattened out, thus presenting an increased resistance to the air through which they fail. Owin «■ to this deformation, drops larger than lout .B cm. in diameter are unstable and quickly break up mto a trumbei of small drops. The practical application of this‘ x ®® is that water cannot fall—relative ° at a greater rate than about eight meteis P second, and as a consequence no ram can fa through an ascending current having a veitical competent greater than eight meters per second It is a suggestive fact that violent air currents much greater than eight meteis pei o.nd are a characteristic of all thunderstorms. The region in the cloud where these violei , vertical air curroits cause the disruption of rain drops may well be called the “region of separation.” The chief point of origin of lightning discharges will obviously be the region of separation, for here one can have unlimited concentration of positive charge carried on the accumulated water. From this positive charge discharges may pass towards the negat electricity in the cloud, but the most frequent discharges will be downwards towards tiie ground, some of which will reach the ground, and some will end in the air, the latter type being the more frequent in tropical storms. There will also be occasional discharges from the ground towards the main cloud, which is negatively charged. These will branch upwards, while those from the region of separation will branch downwards. This, then, is the mechanism of a thunderstorm according to the “breaking-drop” theory; but of course, in nature, the conditions are much more complicated. It is therefore not surprising that observations are occasionally made which it is difficult to fit into the scheme; but there can be no doubt whatever that it does explain all the chief facts of observation—in particular the distribution of positively and negatively charged rain—and the frequency of the different forms of lightning discharge.