THUNDEBSTORM. All the more ordinary pheno-mena of thunderstorms had, about 1750, been conclusively traced to electrical charges and discharges (ELECTRICITY, vol. viii. p. 6), so that they could easily be reproduced on a small scale in the laboratory. To the article cited we therefore refer for their explanation. Some of the laws of relative frequency of thunderstorms, in different places at the same season or in the same place at different seasons, will be found in METEOROLOGY (vol. xvi. p. 128). A discussion of the cause of thunder, and of the circum-stances which give rise to a crash, a roll, or a peal of thunder is given under ACOUSTICS (vol. i. p. 107). In what follows, therefore, the rarer phenomena of thunder-storms, and the possible sources of the atmospheric elec-tricity, will be the chief points treated.
There can be little doubt that atmospheric electricity, at least in the great developments which characterize a thunderstorm, is due in some way to water. Before a great thunderstorm the lower air is usually at an abnor-mally high temperature, and fully saturated with water vapour, so that it is in a thoroughly unstable condition. Immense cloud masses, often miles in vertical thickness, which produce almost midnight darkness by day in the region of the storm, and which appear, when seen from a distance, as if boiling upwards, are always a notable feature of great thunderstorms. These are usually accom-panied by torrents of rain, or by violent hail-showers. And it is commonly observed that each flash of lightning is followed, after a brief interval, by a sudden but temporary increase in the rate of rainfall. At what stage of its transformations the electrification is developed by water-substance is, as yet, only guessed at,—though it seems most reasonable to conclude that it is anterior to the formation of cloud, i.e., to the condensation of vapour. And, though the idea was at one time very generally held and still has many upholders, it seems unlikely to be the direct result of evaporation. For, were it due directly either to evaporation or to condensation, it is almost im-possible to doubt that proof would long since have been furnished by careful experiment, even if made on a scale so limited as that afforded by our laboratories. No trace of electrical effect has been found to attend the precipita-tion of moisture; and the electrical effects, sometimes con-siderable, which have been found associated with evapora-tion have always been accompanied by relatively violent physical and mechanical actions which are not observed in conjunction with atmospheric electricity. It has been suggested by some authorities that the electricity of a thunderstorm is developed during the formation of hail, by others that it is due to the molecular actions which accompany the diminution of total surface when two or more drops of water coalesce into a single one. It has been ascribed to the friction of moist against dry air, and to the dust-particles which appear to be necessary for the condensation of vapour. Again, it has been suggested that it may be a mere phenomenon of contact electricity, due to the impact of uncondensed vapour particles on particles of air. It is almost unnecessary to observe that, whatever hypothesis we adopt, some explanation must be given of two important points :—(1) What becomes of the electricity equal and opposite to that in each drop, which must be produced simultaneously with it ? (2) By what means is the attraction between the drops and the recipient of the opposite charge of electricity overcome so that the drops may be enabled to part with their charge 1 It is to be presumed that gravity satisfies the second of these questions. As to the first, it seems to necessitate the presence of something besides water, in order that the electric separation may be commenced, and thus appears to be fatal to the capillary theory indicated above. Whatever be the true source of the charge, it is easy to see, by known properties of electricity, that even an exceedingly small charge on each vapour particle would lead to a very high potential as soon as a visible drop is formed, and that as a drop increases in size its potential is proportional to its surface. That drops of rain are often individually electrified to a very high potential is proved by the frequent occurrence of " luminous rain," when the ground is feebly lit up by the multitude of tiny sparks given out by the drops as they come near it. The flakes of falling snow, also, are often strongly electrified, so that smart sparks have been drawn from an umbrella on which the snow was falling. But the law of electric repulsion shows us at once that, as soon as the drops in a cloud are sufficiently electrified, at least the greater part of their charge must pass to the boundary of the cloud. When this occurs, the nature of the further behaviour of the charge presents no difficulty. The reason for our singu-larly complete ignorance of the source of atmospheric electricity seems to lie in the fact that it can only be discovered by means of experiments made on a scale very much larger than is attainable with the ordinary resources of a laboratory. The difficulties will probably be easily overcome by the first nation which will go to the expense of providing the necessary means.
Numberless other explanations of the origin of thunderstorms have been suggested; but the more reasonable of these do little more than shift the difficulty, for they begin by assuming (without any hint as to its source) an electrification of the earth as a whole, or of the lower (sometimes the upper) layers of the atmosphere. Induction, convection, &c, are then supposed to effect the rest.
Another and much less reasonable class of explanations depends upon magneto-electricity. Some of these introduce the so-called " unipolar " induction supposed to be due to the rotation of the earth, which behaves like a gigantic magnet. Of this nature is the suggestion of Edlund, which was recently crowned by the Academy of Sciences of Paris. That rapid variations in the earth's magnetic elements, such as often occur on a large scale, as in a " magnetic storm," have at least a share in the production of the aurora is a perfectly reasonable and even plausible hypothesis, long ago brought forward by Balfour Stewart. But we have yet to seek the source of these variations.
The brightness of a flash of lightning is usually much underrated. It is true that it rarely gives even at night an illumination greater than that due to moonlight. But it must be remembered that Swan has proved that the impression of a flash on the eye depends upon the duration, being nearly proportional to it, and steadily increasing for about a tenth of a second. Now the duration of a light-ning-flash is (roughly speaking) only about one millionth of a second. This is proved by the fact that the most rapidly rotating bodies appear to be absolutely steady when illu-minated by it. Hence, if it could be made to last for a tenth of a second, it would give near objects an illumina-tion one hundred thousand times more brilliant than that of moonlight. It must be remembered that the flash is not a mere line, but a column, of intensely heated air, driven outwards from the track of the discharge at a rate initially far greater than that of sound.
What is called "summer lightning" or "wild-fire" is sometimes a rather puzzling phenomenon. In the majority of cases it is merely the effect of a distant thunderstorm. It is also often due to a thunderstorm in the higher strata of the atmosphere overhead,—the reason why we hear no thunder being not so much the distance from the spectator as the fact that sounds generated in rarer air lose rapidly in intensity as they are propagated into denser air. But, besides these more common forms of the phenomenon, there is certainly a form of sheet lightning which occurs, without either sound or cloud, often close to the spectator. The cause of this is not at all obvious.
But the most mysterious phenomenon is what goes by the name of " globe lightning " or " fire-ball," a phenomenon lasting sometimes for several seconds, and therefore of a totally different character from that of any other form of lightning. The fire-ball is almost incomparably less brilliant than forked lightning, because, though it lasts long enough to give the full impression of its brightness, it is rarely brighter than iron in the state which we call " red-hot." It is always spherical, often more than a foot in diameter, and appears to fall from a thunder-cloud by its own gravity, sometimes rebounding after striking the ground. It usually bursts with a bright flash and a loud explosion, occasionally discharging flashes of lightning. No experimenter has yet succeeded in producing artificially anything resembling these natural and intensely charged Leyden jars.
The term " thunderbolt," which is nowadays rarely used except by poets (and by the penny-a-liners), preserves the old notion that something solid and intensely hot passed along the track of a lightning flash and buried itself in the ground. Two distinct classes of phenomena probably gave rise to this notion. When lightning strikes the ground it often bores a hole of considerable depth, which is found to be lined in its interior with vitrified sand. This presents no difficulty. But AEROLITES (q.v.) are often found, in the holes which they have made, still intensely hot, in conse-quence of their rapid passage through the air. A hasty generalization seems to have connected these two entirely independent phenomena, and thus given rise to the notion of the thunderbolt. The ancient notion that a lightning flash could occur in a clear sky is probably to be accounted for by the occasional appearance of these ultramundane visitors.
The sulphurous smell of lightning, which is vividly de-scribed in the Odyssey, is now known to be due to the formation of OZONE (q.v.).
For the precautions necessary to prevent danger from a thunderstorm, see LIGHTNING CONDUCTOR.
A whole volume of Arago's collected works is devoted to thunder-storms, and many important observations are to be found in the writ-ings of M, D'Abbadie and other scientific travellers. (P. G. T.)