Popular Science Monthly/Volume 37/August 1890/Thunder-Storms
By ROBERT H. SCOTT.
THUNDER-STORMS naturally attract universal attention when they occur, and it may perhaps be of interest to point out some particulars that have been ascertained about them.
The most obvious facts are that a heavy cloud passes over the observer, and that from it lightning appears, followed, after a greater or less interval, by thunder; and that usually heavy rain or hail falls from the cloud. The damage wrought by these occurrences, whether by lightning-strokes or by the hail, is so serious that, in countries especially liable to such visitations, hail insurance forms an important item in the farmer's calculations. In many countries such insurance is in the hands of the Government, and accordingly statistics as to the amount of losses are to be obtained; whereas where insurance is in the hands of private companies, information as to the expenditure of these companies is naturally not published.
As regards the liability of certain districts to suffer damage from thunder-storms, it has been maintained by several authorities that these visitations are steadily increasing in frequency. A most elaborate inquiry into the records of such occurrences was printed in the Journal of the Statistical Office of Berlin for 1886. From this it appears that the evidence indicated no general increase in the frequency of lightning-strokes, but, on the contrary, rather a decrease. Houses with soft or, in other words, thatched roofs are struck about seven or eight times more frequently than ordinary slated dwelling-houses. Houses in towns are much less frequently affected than those in the country.
The geological character of the soil has a very great influence on the risk. If this for a limestone soil be taken as one, that for a sandy soil is nine, and for swampy land twenty-two. As regards the different classes of trees, if the risk to a beech be taken as one, that to a conifer (fir or spruce) is fifteen, to an oak fifty-four, and to other deciduous trees forty. Another investigator accounts for the comparative immunity of the beech by the fact that its leaves are edged with short hairs, which allow the electricity collected in the leaves to escape quietly.
As to the actual origin of atmospheric electricity, authorities are not at all agreed, and the observations made on its phenomena (made at different stations) do not accord in a satisfactory manner. In fact, it appears as if the indications of the instruments are due to local causes, so that they* do not lend themselves to any useful generalizations. When a thunder-storm is actually raging in the neighborhood of a station, the indications of electrometers thereat are most erratic and violent, but it can not be said that any electrometer enables us to perceive the approach of a storm one whit earlier than we are able to do by careful watching of the clouds. As regards forecasting thunder-storms, this can be done in a general sort of way; but it is not practicable to predict which villages or parishes, or even counties, will be visited. When the daily weather charts are drawn, if we find that there is an unevenness in the isobaric lines—that is, if these are wavy, or bulge out irregularly—we know that thunder-storms are likely to burst somewhere or other over the country, but that is all we can say. At each station the barometer is unsteady—the mercury moving up and down in the tube—during the actual continuance of the storm; but this oscillation of the mercurial column has nothing to do with the irregularity in the isobaric lines above mentioned. Forecasting these storms is, therefore, always an uncertain and a thankless task, for local success is rarely attained.
Among the earliest symptoms of the approach of a thunderstorm is the appearance on the western horizon of a line of cumulus ("wool-pack") clouds, exhibiting a peculiar turreted structure. I say on the western horizon, for most of our changes of weather come from that quarter, and it has been proved that thunderstorms, like wind-storms, advance over the country, generally, from some westerly point. This bank of clouds moves on, and over it appear first streamers and then sheets of lighter upper cloud—cirrus, or "mare's-tail"—which spread over the sky with extreme rapidity. The heavy cloud mass comes up under this film, and it is a general observation that no electrical explosion or downfall of rain ever takes place from a cloud unless streamers of cirrus, emanating from its upper surface, are visible when the cloud is looked at sideways from a distance.
Thunder-storms are generally accompanied by falls of hail as well as rain, and these hailstones assume the form of lumps of ice—some even as large as hens' eggs, and weighing several ounces, having been known to fall. The stories of masses of hailstones, weighing many pounds, having been found after storms, are explained by the fact that the hailstones, after they have fallen, may have frozen to each other and formed a solid lump on the ground. Large hailstones are composed of alternate layers of clear crystalline and white porous ice, and many of them consist of an aggregate of smaller hailstones which have attached themselves to one stone as a nucleus, and then the mass so formed has received external coatings of ice. The compound structure of such stones becomes manifested when the mass gradually thaws. In some cases these stones are coated with crystals of ice in six-sided prisms and pyramids, as perfectly formed as the specimens of quartz or calc-spar crystals which are to be seen in mineral collections. It is very hard to believe that such beautifully formed crystals as these can be the product of any instantaneous process of formation.
It is these heavy blocks of ice which do the greatest amount of damage, as naturally a lump, weighing even an ounce, is a formidable missile when it falls from a height of even a thousand feet. When these falls are about to take place, observers have reported that a peculiar rattling sound is heard in the atmosphere, evidently from collisions between these stones striking one another in their fall. A very careful observer, who was overtaken by one of these falls in the Caucasus, near Tiflis, states that it occurred immediately after an ordinary hail-shower, and that he could see the successive showers marching over the country, and noticed that, between the last edge of the falling hail and the front edge of the falling ice-blocks, there was a distinct break, through which he could see the sun shining on the hills in the background. It was on this particular occasion that the best specimens of crystal-bespangled hailstones have been recorded and sketched, but others have been reported from Natal, and quite recently from Philadelphia, October 1, 1889. When such a visitation of ice-lumps takes place, the entire crops of the district affected by it are destroyed. Such a storm passed over Richmond in August, 1879, and in five minutes some ten thousand pounds' worth of damage was done, principally to conservatories. Naturally, Kew Gardens were among the principal sufferers.
It is a problem as yet unsolved to account for the suspension in the atmosphere of such objects as these hailstones, which frequently weigh much over an ounce. A recent theory, which seems to carry some probability with it, supposes that in the heart of every hail-cloud there is a whirlwind, or what is usually but erroneously termed a "tornado." It is well known that such disturbances exert a prodigious lifting power, raising heavy objects, such as carts, house-roofs, and even trees, and transporting them to considerable distances. The theory is, that when a drop of water in such a cloud is congealed it is carried round and round in the vortex and lifted up, more moisture being condensed and frozen upon it at each gyration, until at last it is thrown out and falls. This would account for the alternate layers of which I have already spoken, but will not account for the formation of crystals, a growth which usually requires a considerable time.
Thunder-storms have been scientifically studied in various countries, and the broad fact has been elicited that they travel over the earth's surface like wind-storms, but at a higher velocity. To give an idea of this, I may quote some statements made before the Royal Meteorological Society last June, in relation to the storm of the 2d of that month. This storm progressed from Wiltshire to Edinburgh, over a distance of four hundred miles, at a nearly uniform speed, the rate of travel being about fifty miles an hour. This is an unusually rapid rate of advance for a windstorm over these islands. I am not speaking of the velocity of the wind in the storm, but of the velocity of the storm system as a whole. In this storm many of the hailstones which were collected weighed over an ounce. Some at Docking, near King's Lynn, were said to be three inches in diameter, and to weigh three and a half ounces. One was weighed at Barden Mill, near Tunbridge Wells, and was said to turn the scale at half a pound. As regards the incessant character of the lightning in London, one observer at Highgate counted twelve hundred and forty-four displays during the two hours ending at 11.10 p. m., giving an average of over ten per minute. Another observer, at Westgate-on-Sea, gave a much higher figure for frequency; his attempt to count breaking down at the very high number of one hundred and thirty-one per minute.
Thunder-storms are much more frequent in low latitudes than in high. In some tropical countries they are said to occur regularly every afternoon. At Rio the story was that at certain seasons, in issuing invitations to afternoon parties, it was usual to specify whether guests were to assemble before or after the thunder-storm. In Abyssinia, D'Abbadie gives, as the average of four years, 410·6 as the annual number of these storms. Many of these, however, consisted of only one or two flashes of lightning. It was formerly believed that such storms never were noticed in the arctic regions, but this is not the case, for one was experienced at Bell Sound, Spitsbergen, in 78° north latitude, in August, 1873; and a succession of thunder-storms was reported for several days in July, 1870, on the west coast of Nova Zembla. At any rate, in such high latitudes they are very rare.
Thunder-storms are generally divided into two groups—heat thunder-storms and cyclonic thunder-storms. The former are the summer type, while the latter occur principally in autumn and winter. We may also say that the former are essentially continental, while the latter are characteristic of the ocean or island climate. In Iceland all the thunder-storms are of this latter type, and occur in winter. The same conditions show themselves on the British Atlantic coasts, where there is a decided maximum of frequency of such storms in winter, even in the latitude of the southwest of Ireland, at Valencia. These circumstances are accounted for by the fact that thunder-storms are always associated with great differences of temperature in adjacent masses of air. Such conditions are most likely to occur in hot climates, where the soil gets excessively heated in the daytime, while the air at some distance above it is cool. In cold climates they occur in winter, where a shift of wind from southwest to northwest is sometimes accompanied by a sudden fall of temperature of 15° or even 20°.
We of the British Islands owe our comparative immunity from thunder-storms to our damp climate. The fact is well known that it is comparatively difficult to perform any electrical experiments in these islands, and that all apparatus must be kept constantly in front of a fire in order to prevent moisture being deposited on it. Accordingly, we must suppose that the electrical disturbances which would give rise to explosions and severe storms in France or Germany may pacify themselves comparatively quietly in our atmosphere, and at most only give rise to phenomena of a very moderate character.
I must now say something about the actual lightning flash, which is neither more nor less than a violent electric spark. Three different forms of lightning are generally admitted to exist: (1) The actual flash, or what is commonly called "forked lightning." (2) "Sheet lightning," which usually is the illumination of the sky by a lightning flash which takes place below the horizon. (3) "Globular lightning."
1. As to the term "forked lightning" the representations of it given by artists, which resemble the so-called thunderbolts placed in the hand of Jupiter, are quite absurd. The flash, when photographed, exhibits itself as a line which is continually changing its course, and is described as "intensely crooked" by a very careful observer. It never proceeds for a time in a straight line, and then, turning at a sharp angle, going on farther in an equally straight line, as is represented in pictures. The forking of it is very marked, and this occurs by side flashes passing off from the main track, and eventually losing themselves, like the ramifications of tree-roots. Occasionally the lightning appears to start from a point from which several flashes diverge in different directions.
2. "Sheet Lightning."—Whenever a flash passes from cloud to cloud, or from cloud to earth, the light produced by it illuminates the sky in the neighborhood, and the more intense the flash, the more brilliant and extensive the illumination. At times sheet lightning has been proved to emanate from an ordinary storm distant more than a hundred miles from the point of observation. It is, however, maintained, and apparently with good reason, that occasionally lightning of the "sheet" type, such as what is called "summer lightning," takes place without any thunder; so that, in such cases, no actual thunder-storm is in progress.
3. "Globular Lightning."—This is a rare phenomenon, and one which no one has as yet been able to produce in the laboratory, whereas the phenomena of the two previous types are easily produced. The general description of the occurrence is that a luminous ball is seen, moving very slowly, not touching any object, and eventually breaking up with a violent explosion and the appearance of several flashes of ordinary lightning. It is reported that persons have gone out from a house into a street to follow such a ball and watch its movements, so that the occurrence must have lasted at least a number of seconds. Ordinary lightning, as is well known, is practically quite instantaneous. The size of the ball on different occasions has varied from that of an orange to that of a large glass lamp-globe, or even larger. Many physicists refuse to believe any accounts of this manifestation of the electrical discharge, but the reports of it are too numerous and circumstantial for us to consider them to be entirely baseless.
There is another way of classifying lightning flashes, and that is as to their color. The seven colors of the solar spectrum are well known, but the spectrum of the electric spark differs materially from the solar spectrum. It exhibits rays which extend far beyond the extreme violet of the solar spectrum. We see, therefore, that in the light of lightning a wide range of color is possible. If any of my readers have ever watched a storm carefully, they must have noted that some of the flashes were bluish, others reddish, etc. It is generally the blue tints which accompany the most destructive strokes.
Some attempts have been made to estimate the actual force exerted by a lightning flash. The late Mr. de la Rue constructed a magnificent electrical battery of many thousand cells. From experiments with this, the number of cells being raised to 15,000, and the "potential" of each being rather over one "volt," it was found that 9,700 "volts"—say 9,500 cells—were required to produce a discharge through one centimetre (·3937 inch). Starting from these data, the electro-motive force requisite to produce a flash of lightning one mile (63,360 inches) in length, at ordinary pressures, is 1,480,570,000 volts, practically given by a battery of fifteen hundred million cells.
A flash a mile in length is nothing very extraordinary, and it is therefore not to be wondered at that experiments to bring electricity down from the clouds are very dangerous, and have frequently had fatal results. Soon after Franklin, in the last century, had made his famous experiment with a kite, and proved that electricity existed in a thunder-cloud, natural philosophers generally began to imitate him. One of them in St. Petersburg, a Prof. Richmann, arranged an apparatus to collect this electricity. On the first occasion of a storm he went to his laboratory to observe the effects. A ball of fire was seen to leap from the apparatus to his head, and he fell lifeless.
A flash of lightning really consists of a discharge between two objects, say two clouds, or a cloud and the earth, oppositely electrified, the charges on which suddenly combine, with the manifestation of light and heat. Lightning conductors are contrivances by which the electricity of the earth is allowed to escape quietly into the atmosphere, where it meets with electricity of the opposite character from the clouds, and the two neutralize each other quietly, without any explosive discharge, or, in other words, without lightning. I need not go back to the first principles of electrical science and explain why it is that electricity passes most easily through metals, and escapes with greater freedom from sharp points than from rounded knobs. Assuming these elementary facts, I may say that on any object, such as a house or other building, the electricity tends to accumulate itself on all projecting portions of the roof, etc., and especially on the highest points of it. The ideal complete lightning-rod system would call for a sharp-pointed copper rod erected at each of these projecting pinnacles, and rising above it, and would then connect all these separate points by copper rods, and eventually carry down a stout copper rod to the earth. Care must be taken that due attention is paid to certain main precautions: (1) The point of the conductor must be kept sharp; (2) the section of the conducting rod must be sufficient to allow the electricity to pass along it; (3) the rod must be perfectly continuous; and, lastly (4), the rod must be efficiently connected with the ground.
1. The sharpness of the point is insured by gilding it or coating it with some metal which resists oxidation.
2. As to the section of the rod, a bar half an inch in diameter is sufficient for all ordinary buildings. Bars are not usually employed, as it is difficult to bend them over cornices, etc.; accordingly, either wire ropes or tapes are taken. The wire ropes are more liable to corrosion from wet getting in between the strands than are tapes, so that the latter are generally preferred.
3. The continuity of the metallic connection from the highest point of the rod to the ground can only be secured by having as few joints as may be, and by making those joints as true and firm as possible by soldering. The joints should be examined from time to time, for it is often found, on examination of old conductors, that while the copper wire or tape is quite sound along its straight reaches, at the bends or joints corrosion has set in. As a chain is no stronger than its weakest link, a corroded conductor, such as has been described, is perfectly useless.
4. The Earth Connection.—It is not easy in all cases to insure that this is satisfactory. Electricity will not pass at all so easily into dry earth as into wet earth, and merely plunging the end of the rope or tape into wet earth is not sufficient. The conductor from the building should be soldered at its end to a large sheet of copper, say at least two square yards in area, buried in damp soil, or else soldered to the water or gas mains, so as to insure that a large surface of metal is in contact with damp earth.
Supposing that the whole system of protection against damage from lightning has been properly planned, the work should be carefully tested after its completion, because injury to it often occurs at the very last, owing to accidental causes, or to the carelessness of workmen. Conductors should also be examined from time to time, throughout their whole length, to make sure that all the joints are sound. Care should also be taken that the earth in which the terminating plate is buried is kept thoroughly moist. If any of these particulars be neglected, the conductor will be practically useless, and will afford no protection to the structure.—Abridged from Longman's Magazine.