THE MECHANISM' OF LIGHTNING

The dramatic phenomenon of nature is closely

studied in the laboratory and in the field. It is

found to be an intricate series of physical events

TIKE many other commonplace natu­

L ral phenomena, lightning has been uncommonly difficult to explain.

Although it is two centuries since Ben­jamin Franklin made a start by showing that lightning was an electrical dis­charge, not until the present decade have we begun to understand the extraordi-. narily intricate mechanisms involved in the origin and development of a light­ning Rash. Recent researches, however, appear to have removed much of the mystery from this awesome spectacle.

Let us begin by considering the atmos­pheric situation that leads to a lightning storm. \Vhenever weather conditions produce rapid updrafts of warm, mois­ture-laden air that rise well above the freezing level in the atmosphere, the region involved becomes a huge genera­tor of static electricity. The water drop­lets of this thundercloud, in which the updrafts may reach a velocity of 160 miles an hour or more, become electri­fied. Just how the charges are generated is still a matter of conjecture. It may be that the wind currents tear at the sur­face of the droplets, producing a fine, negatively-charged spray and leaving the larger droplets positively charged. Another possibility is that electrical fields already existing in the clouds induce charges on falling droplets. Still another is that ice crystals at the upper levels are electrified by friction or by some process that accompanies freezing.

In any case, large masses of charged water droplets and ice crystals become segregated in positively- and negatively­charged groups and collect at different localities within the thunderhead (see diagram on opposite page). Between these huge groups of opposite charge very high potential differences and elec­trical fields develop. It is these highly charged regions of the cloud that ac­count for the split-second 'electrical dis­charges called lightning.

As the charged, wind-driven thunder-

22

by Leonard B. Loeb

head approaches a given ground area, electrical fields gradually build up be­tween the earth and the cloud. Near the earth the fields rarely exceed 2,700 volts per centimeter of their length, but even at such a field a vertical conducting rod from the earth only 10 feet long (about 305 centimeters) would have a potential difference of more than 800,000 volts (305 X 2,700) with the uncharged sur­rounding atmosphere. Such a field would make the hair of a person seated on the ground literally stand on end. It accounts for the corona discharge, popu­larly called St. Elmo's Fire, which is sometimes seen issuing from a church steeple or from the wingtips of an air­plane during a storm.

In a cloud that produces lightning, the area of the charged region generally has linear dimensions of some 1,000 feet. At nearby pOints the charged region devel­ops fields of 30,000 volts per centimeter, so the field of the cloud is vastly greater than the one at the ground. Thus light­ning discharges usually originate at the cloud and work downward. On the other hand, a very tall grounded conductor such as the Empire State Building may develop a potential high enough to ini­tiate a lightning discharge from the earth. Be this as it may, however, the distribution of charges in a thunder­cloud is such that there are many more discharges within the cloud than be­tween the cloud and the ground. These discharges, largely concealed by the cloud, account for the so-called sheet lightning sometimes seen in distant thun­derclouds on dark nights.

Lightning strokes vary in length from 500 feet to two miles or more. Calcula­tions based on the length of these paths and on the fields at the earth indicate that the electrical potential between a thundercloud and the earth is of the order of hunch'eds of millions to billions of volts. If the space between the cloud and the earth were a vacuum, these huge

potentials would accelerate electrons and ions to a speed sufficient to smash the nuclei of atoms. In the air, the ac­celerated particles cannot attain any such energies, for they are slowed by countless impacts with the air molecules. Nevertheless the power of a lightning stroke remains impressive. The brief currents it generates will vaporize No. 12 copper wire; the magnetic fields pro­duced by a relatively short stroke down a copper tube one centimeter in diameter with walls one and a half millimeters thick will collapse the tube. A lightning stroke which travels down the moist in­terior of a tree in 30 feet liberates enough heat and steam literally to blow the trunk open.

THE visible flash of lightning is pro­duced by the heating of air mole­

cules in the path of the stroke, which may reach a temperature of 30,000 de­grees Centigrade. Camera studies show that the channel of the stroke remains luminous for some 100 millionths of a second after the stroke itself. Owing to the enormous power of the stroke, the channel expands explosively, and this accounts for thunder, which comes from the shock waves produced by the chan­nel's expansion. The explOSion of seg­ments of the channel near an observer is heard as a sharp crack; rumbling thun­der comes from more distant segments, from repeated strokes and from echoes. These rumbles, it may be noted, cannot be heard as far as those from major gun­fire, which indicates that lightning dis­charges, impressive as they are, do not yield as much power as ordinary man­made explosives, to say nothing of an atomic bomb. The noise of big guns can be heard for a distance of some 15 miles; the audible limit for thunder is only about seven miles.

Analysis of the lightning stroke shows that it is very similar to the long sparks that jump the gap between two widely

© 1949 SCIENTIFIC AMERICAN, INC

separated condensers of high potential. Like these sparks, a lightning stroke fol­lows a crooked path and develops branches or forks that advance in the direction of the stroke, so one can al­ways deduce from its branches the direc­tion in which a stroke is traveling. Unlike condenser sparks, lightning does not os­cillate back and forth. Damping due to the high resistance of the electrical feeders from which the stroke originates permits it to discharge in only one direc­tion. A lightning stroke may come from a positive or negative cloud, but most strokes, except in mountain storms, are from negative clouds.

The speed of lightning is no idle meta­phor: a lightning stroke travels at a velocity of approximately one billion centimeters per second. It lasts no longer than five to 500 microseconds (millionths of a second), the median being some 30 microseconds. The quantities of elec-

tricity involved, however, are huge: a single discharge may transfer 200 cou­lombs of electricity (a coulomb being the quantity of electricity transferred by one ampere of current in one second); in terms of current a stroke may carry as much as 500,000 amperes. A stroke of 200 coulombs and one billion volts which lasts 200 microseconds produces a thou­sand billion kilowatts of power.

The spark channel down which this huge packet of energy travels at first is tiny-an inch or less in diameter-but the power of the completed sh'oke expands the channel at the explosive rate of 50,000 inches per second. Thus it is dif­ficult to define the diameter of a light­ning stroke, either as it appears to the eye or in a photograph. The lightning channel loses much of its luminosity after it expands to a diameter of a foot or more, so that it appears reasonable to estimate that the visible lightning

stroke ranges from an inch to a foot in width.

THESE observations serve to describe the phenomenon, but they do not ex­

plain the mechanics of the lightning stroke itself. The basis for an explanation of that fundamental question was laid by several independent studies made just before the recent war. One group of stud­ies, conducted by the writer and J. M_ Meek in the U. S. and independently by H. Raether in Germany, analyzed the mechanism of ordinary electrical sparks in air. Other basic information was provided by photographic analyses of the progress of electrical strokes on a

microsecond time scale. These were made in South Africa by B. J. F. Schon­land and his associates, who used a

camera with a rapidly revolving lens to photograph actual lightning sh'okes, and in England by T. E. Allibone and J. M.

THUNDERCLOUD is a mighty generator of static elec­tricity. The lightning flashes in this drawing are massive discharges between rel?ions of differing potential. Some flashes are within the cloud. Othel's play hetween the cloud and the ground. The latter is indicated by A. B is the hase of the cloud. C is the wind that drives the cloud.

D is the ascending, moisture-laden air current. E is the descending ail' current. F is the roll or scud cloud. G indicates up and down drafts. H is the region where hail is generated. I is the highest region of ice formation. J i rain falling from the cloud. A scale of height in miles and temperature in degrees Centigrade is at the le.ft.

21

© 1949 SCIENTIFIC AMERICAN, INC

+

G) F ;1"+++ + " r+++ ++ {! .. E&e�/ + .. ++ . \

• :t' + + ""\.. ....<'\ $ '1'+ + 1"+ - '-A..­+ + + �I;:!;I + + ... " +++l .. \. , ++++ . , + + +++�t%l + + . v.' . .,.:+ 1' " .

. .. + � . + pO .. , + ++

E&�+ .. . � /' \ . $

. + +r ION PAIR BY . + : + 'PHOTOrONIZATI0N, ... ++ ++ +

� :+� �+ . . '�HOTON + E

SEQUENCE OF EVENTS in a flash of lightning is out­lined by analyzing a discharge between two parallel plates. The upper plate is positively charged; the lower plate, negatively. The field between them (A) has a

strength of 30,000 volts per centimeter. At the lower left a random photon knocks a single electron from an

atom. Moving towards the posItIve plate, the electron collides with other atoms to liberate an avalanche of electrons. In the wake of the electrons remain positively­charged atoms, or ions (C and D). These ions reinforce the charge of the positive plate, thus attracting new electrons (F) that have been liberated by radiation (E)

Meek, who made similar photographs of long sparks with a revolving film camera.

The investigation of ordinary sparks showed that the path they follow in the air is created by a so-called "streamer" mechanism. This process begins when air at atmospheric pressure is placed in a field of about 30,000 volts per centi­meter, or as little as 10,000 volts if water droplets are present. A single electron starts the process, and there are always stray electrons, liberated by cosmic rays or radioactivity in the air, on hand to start it. The elech'on, advancing from the negative end of the field towards the positive end, gains energy from the field despite its billions of collisions per sec­ond with gas molecules. When it gains enough energy, it begins to knock elec­trons out of some of the molecules it

24

strikes. These in turn repeat the process, so that the liberated electrons soon be­come an avalanche. After a run of one centimeter in a field of 30,000 volts per centimeter, the single initial electron produces an avalanche of 10 million free electrons. Raether has photographed such avalanches in a Wilson cloud cham­ber by condensing water drops on the ions left in the avalanche h·acks.

In their wake the freed electrons of course leave large numbers of positively­charged ions, for each molecule from which an electron escapes is ionized. The ions create a positive electrical charge throughout the space in the path of the electrons. When the ions left by the electron avalanche are deserted by the electrons at the positive end of the field, they add to its positive potential.

The augmented field soon becomes strong enough to draw photoelectrons, created as an accompaniment to the avalanche process, from the negative side of the field. The photoelectrons, feeding into the ion space-charge left by the initial avalanche, produce more ionization and enlarge the space-charge so that it ex­pands backward towards the negative end of the field. The situation can be pictured by imagining a magnet that by its field draws iron filings from a distant pile in such a way that the filings, build­ing on to one another from the magnet backward, form a path back to the pile.

THE entire chain of events, illustrated in the series of diagrams above, takes

place in a matter of microseconds; the avalanche of electrons, for example, ad-

© 1949 SCIENTIFIC AMERICAN, INC

J

during the previous events. The electrons, in turn, ionize more atoms so that a heavily ionized region (G) begins to extend towards the negative plate. This process con­tinues until there is a bridge of ions (J), called a stream­er, between the two plates. It is this streamer that pro­vides the channel for a spark or for lightning. The next

drawing illustrates the process by which streamers form branches. A streamer (M) attracts two avalanches (K and L). The avalanches are then reversed to form a

bl'anehed streamer. The remaining example shows how a streamer may begin before an avalanche has reached positive plate. Streamer then works towards both plates.

vances at the rate of 20 million centi­meters per second. The ionized path that is formed as the end result of the process is called a streamer, and it is this stream­er that provides a channel for the spark or actual lightning stroke.

In a thunderstorm, such a streamer bridges the gap between the charged cloud and the ground. The bridge is a conducting filament of ions with elec­trons streaming over it, and it acts as a kind of tear in the electrical field, ac­centuating the electrical stress at its ends. The instant the bridge is com­pleted, it releases a cataclysmic burst of electrons from the negative terminal-in this case the ground. The burst of elec­trons sends a potential wave up the streamer channel to the cloud. This lit­erally tears electrons out of most of the

molecules in the centimeter-wide chan­nel. The cloud's charge and energy are then drained away down the conducting channel for some 10 microseconds, mak­ing the channel brilliantly luminous. The speed of the potential wave, called the return stroke, is from one to ten billion centimeters per second-one thirtieth to one third of the speed of light. This brilliant flash constitutes the phenome­non we call lightning.

Once the spark channel has been es­tablished, there may be repeated strokes from the cloud down the same channel. The discharge of the section of the cloud from which the stroke comes changes its potential with respect to other sections of the cloud, and strokes within the cloud then recharge this section, causing new strokes to the ground. As many as 40

successive strokes down a single channel have been observed-the legend that lightning never strikes twice in the same place is wrong in more ways than one. The repeated strokes follow one another very rapidly, at intervals ranging from tenths to hundred-thousandths of a sec­

ond. The process that has been described

is that for a stroke from a positive cloud to negative ground. For a stroke from a negative cloud the mechanism is similar, except that the streamer is built up by steps.

The foregOing description represents the fundamentals of the process, but in an actual lightning storm the sequence is a bit more complex because of the length of the strokes. \Vhen Schonland photo­graphed the lightning discharge with his

25

© 1949 SCIENTIFIC AMERICAN, INC

A V ALANCHE of electrons, depict­ed in drawings on pages 24 and 25, is photographed in a cloud chamber.

GRO\VTH of avalanche is shown in third photograph. Electrons ionize atoms so droplets condense on them.

26

ADVANCE of avalanche within the chamber is apparent in a photograph made tiny fraction of a second later.

STREAMER develops from another avalanche. These experiments were done in the lahoratory of H. Raether.

revolving-lens camera, the pictures showed that the stroke advanced in a series of jumps. The mechanism for this process is deduced to be as follows: When a streamer is initiated from the cloud it starts towards the ground as a "pilot" streamer. After some 30 to 90 microseconds, during which the tip of the streamer has advanced 10 to 30 feet, the ionization in the streamer's upper or cloud end decays as the result of the recombination of ions. This builds up a high resistance, and in consequence a high potential, at the upper part of the channel. When the potential reaches a critical limit, the stress is great enough to re-ionize the part of the channel on the earth side of the region of stress. A rapid pulse of ionization then sweeps down the channel, increasing in speed and intensity as it approaches the tip of the streamer. \\Then it reaches the tip, the latter is given a boost in energy. It lights up brilliantly, and often produces branches at this point. The pilot streamer then advances for another 40 to 90 microseconds, whereupon decay again sets in at the cloud end and the process is repeated. Thus the pilot streamer forges ahead, the mechanism being known as the "stepped leader" process.

AS the streamer approaches the ground, 1'\. the field distortion, particularly near conducting elements in the ground, in­creases. The streamer speeds up and heads in at a nearly vertical angle to the ground. If the stroke is from a negative cloud, in the last microsecond before the streamer is completed a positive streamer may advance from the ground to meet the pilot streamer, usually 15 to 40 feet above the ground. In any case, as soon as the pilot streamer makes contact with the ground or with a positive ground streamer, an enormously steep potential wave sweeps up the channel to the cloud. This return stroke, which ionizes from one to 30 per cent of the gas mole­cules in the channel, is the lightning flash that we see. After this stroke dis­charges the section of the cloud to which the channel leads, the cloud is recharged as we have already indicated, and it then yields a new discharge towards the ground. Because the channel is now fully ionized, the discharge proceeds not by steps but directly to the ground. This wave, called a "dart leader," accounts for the repeated strokes of lightning. When the dart leader reaches the ground it calls forth a new bright return stroke. Dart leaders and return strokes repeat themselves in rapid sequence until the cloud element is drained of its high potential.

-

Leonard B. Loeb is pro­fessor of physics at the University of California.

© 1949 SCIENTIFIC AMERICAN, INC

COMPLEX STRUCTURE of lightning is illustrated by a pbotograph of a single holt striking the Empire State Building. This photograph was made by researchers of

the General Electric Company with the revolving-lens Boys camera. At the top the stroke is dissected into one long discharge (at right) and six subsequent ones.

27

© 1949 SCIENTIFIC AMERICAN, INC