Popular Science Monthly/Volume 7/July 1875/Thunder-Showers
THE thunder-shower of Southeastern Vermont generally comes from the southwest. To understand why it should take this course instead of any other, we must examine the topographical character of the country.
The chain of Green Mountains extends throughout the State from south to north, inclining some degrees to the east of north. It presents a barrier to the prevailing general current of southwest wind, and in summer condenses the vapor which that wind bears, thus forming piles of cumulus cloud over the higher summits, or most wooded districts. The deeper ravines, or river-beds, on the eastern slopes of the mountains, run to the southeast, and open out on the wider valley of the Connecticut River.
In order to convey a more definite idea of our theory, we will choose a certain locality which may serve the purpose of a diagram to our demonstration; and this locality shall be the region of West River, This river takes its rise among the forests near the summit of the Green Mountains, at a height of some 2,000 feet above the level of the sea, and, flowing southeasterly forty or fifty miles, empties into the Connecticut River about ten miles from the southern boundary of the State.
During a hot summer day the sides of the deep valley of this river reek with intense heat, and cause a flow of moist air upward toward the summits of the mountain-ridge, from the valley of the Connecticut, and also from the sea. This moist air, meeting with the general current from the southwest, piles up an immense mass of cumulus cloud, of many square miles in extent. So long as the intense heat prevails, this cloud increases in size; grows black and blacker with its dense vapor, and casts a gloomy, lurid glare over the face of Nature, darker than that of any eclipse. The vapor, pushed up by the ascending currents of heated air, attains to a great height above the sea, where the temperature is very low. But finally, at that hour of the afternoon when the heat begins to decline, the accumulated vapors, no longer augmented or sustained by heated air from the valleys below, fall in rain.
The effect of large cold drops of water, or perhaps of ice, making altogether millions of tons in weight, falling from a great height into a deep, narrow valley, is, not only to beat down the air into that valley, but to chill the air there; and the cold air, seeking the lowest level, tends to rush down the valley, at first near the surface of the earth, but growing deeper and deeper, until the cloud itself is borne away on the swift-rushing air-freshet of its own making.
The land beginning to cool with the declining sun and the cooling rain, causes the southerly breeze to slacken and die away, and the storm-cloud rushes on unobstructedly down the West River and the Connecticut, deluging and fertilizing the fields along its course, while its quick lightning and oft-repeated claps of thunder flash and resound among the reverberating hills.
The cloud passes on, and often the sinking sun comes out from behind it; the late hushed and frightened birds gush forth with new song; myriad drops hang glittering on the spray; the green is flushed with a brighter, fresher hue, and the glowing rainbow smiles serenely from the dark, retiring, and still grumbling storm.
This storm is followed the next day by delightfully clear weather, with a cool, exhilarating breeze from the northwest; though this is not always the case, the cloud sometimes overspreading the sky, losing its motion, and leaving the air damp and murky.
The thunder-shower, as we have thus described it, though limited to a small district of country, may be regarded as the type of all similar showers that occur in mountainous regions everywhere. Numerous modifications, however, of local origin will occur, due to various causes; and it would be a highly-interesting and valuable study to ascertain these causes for every particular case.
Yet as to whether the moving force of the thunder-gust is limited wholly to the causes here given, may well admit of a question. It is not improbable that a cloud, from its great height, may penetrate a high upper current from the northwest, and that both this upper and the lower current may contribute to its rapid motion of translation. It is well that these thunder-showers are movable, instead of being stationary, as they often are at sea, for otherwise summer rains would not be evenly distributed over the face of the country; and the hind in some places would be subject to exceeding moisture, while in other places it would suffer from the drought.
A few days after the above was written, a violent thunder-gust closed a warm afternoon. It was on the 1st of August, 1873. The day had been hot and peculiarly oppressive, as is usually the case before a violent storm. Between four and six o'clock p. m., a thunder-shower came down the valley of West River, and corresponded in its general features with the description given above; but it exhibited in addition other features which were entirely peculiar. The lightning struck in five notably different places in the village of Brattleboro, which partly borders the valley of West River near where it disembogues into the Connecticut River, and these places, instead of being elevated points, were, in all cases except one, among some of the lower ones. And they were nearly all in the same straight line, about a half or three-fourths of a mile in length, and at a short distance from the Connecticut.
The strokes that fell upon these points followed each other in pretty rapid succession, and were accompanied by thunder that had a sound as if partly suppressed. It was neither loud nor jarring, as thunder sometimes is. The rain fell in floods, and was very copious. Its abundance, rendering the air seemingly nearly half water, doubtless occasioned the subdued sound of the thunder, and perhaps greatly reduced the force of the shocks; for in no case was any considerable damage done. An upper corner of a two-story house was shattered, two other buildings were slightly injured, and several trees were marked by narrow channels down their trunks or branches. Together with the first house struck, one of two fir-trees standing near was grooved at the same time, and some of the splinters were found in the chamber nearest the shattered corner, although the window-blinds were closed and fastened. These splinters must have been driven up between the slats of the blinds, which would seem to show that the stroke was upward instead of downward. A window-curtain near the corner was torn to shreds. In the lower room nearest the corner there were no effects of the shock observed except upon a gilt cornice which was marked at intervals by black perpendicular bars, the gilding there having been burned or melted. The intervals between these bars were in some cases very narrow, and at others very wide. Two persons sitting in this room perceived no effect from the shock.
At one moment during the storm the wind came from the north or northeast. This wind was probably highly charged with electricity, which, being added to the electricity of the northwest current, produced such an excess of the fluid as to result in the rapid and numerous discharges which took place. The most of these discharges apparently occurred along the line where the two currents of air met. The more easterly current of air may have come over the shoulder of the mountain on the opposite bank of the Connecticut River, or it may have come down the valley of that river, and met the current coming down the West River in the village of Brattleboro.
With respect to the direction in which the lightning struck whether up or down, it is not improbable that in every stroke of electricity there are two opposing currents, one up and the other down. The splinters which adhere to the first tree struck show this, some remaining attached by the upper end, and others by the lower.
Beginning in the north, the first in order of the objects struck were a house and a tapering fir-tree near by, within about ten or fifteen feet, and towering considerably above the house. The house had no conductor. A hundred paces from there, in a southeasterly direction, a locust-tree was struck. It stood in a grove of locust, maple, poplar, butternut, fir, and other trees, within about thirty paces from a conductor upon a neighboring house, and not far from a tall Lombardy poplar. A hundred paces farther on, and at a lower level, one of the higher branches of a lofty elm was struck. At the distance of another hundred yards, in the same general direction, stands the Congregational church, and near it the Baptist church, both about 130 feet in height, and with conductors apparently in good condition. These churches were unharmed. About 400 paces from there, and at a still lower level, stands the fourth point struck, which is a three-story grist-mill; and, finally, some 300 yards or more farther on, and more to the westward, on a comparatively high point of land stands the dwelling-house, the fifth and last point known to have been struck—the last, we mean, in following the direction, and not in the order of time. The effect of the strokes at the two extreme points was severer than any of the others.
Reports from other quarters of the country show that the electrical condition of the atmosphere of New England on the 1st of August was considerably disturbed, thunder-showers occurring at many different places. When this is the case, it is reasonable to suppose that two showers, following down two neighboring valleys, may come together, and thus double the amount of electricity that might be possessed by one alone.
The question here occurs, "Is there any common origin between these thunder-showers and the northern lights?" Are they not each but a different means of restoring a disturbed electrical equilibrium? If this is the case, we might infer that, when thunder-showers are numerous and violent, the displays of the northern lights will be less frequent and less active, and vice versa; though there may be cases in which both become more than ordinarily active.
One of the discharges of electricity which we happened to observe during the shower was perhaps that one which fell upon the grist-mill. Amid the floods of descending rain, it looked like falling sparks of lire. It was about 600 or 700 yards from where we sat, and the sound of the thunder was more like that of the rocket than that which usually accompanies electrical discharges. In fact, the cloud probably passed through the village rather than over it; and the discharges were necessarily short, close, and without prolonged reverberations, such as may occur when the stroke is high, partly in dry air, and several miles in length, the sound from which must reach the ear at different intervals of time, thus producing a continuous rolling noise.
We are here reminded of another thunder-shower, of peculiar features, which occurred in Brattleboro in November, 1860. It was on the day of the first election of Mr. Lincoln to the presidency. A pinetree was rent into fragments by it, and a passer-by, a voter, on seeing the extraordinary havoc that had been made, the white, shining splinters lying scattered over the ground, in all sizes, from the smallest sliver up to strips long enough for rails, exclaimed in great excitement: "The thing is all up now; for the old 'Rail-Splitter' is around at his work!" Even thunder-showers are wrested by some men into a political significance!
The circumstances of the case, however, would appear to have been these: A dense cloud, borne upon a low southwest surface-current of wind, was passing across the deep valley of the West River, half a mile or so from its mouth, when it was probably struck by a cool, dry mountain-breeze flowing down the valley. This breeze imparted new electricity to the cloud, which, being thus overcharged, gave out its surplus in a sudden shock, which took effect upon, a group of pines. Every drop of water of which the cloud was composed we may regard as a small Leyden jar, as it were, the united force of which proved sufficient to rend in pieces one of the pines in an instant of time. The tree was some seventy feet in height, two feet in diameter, and stood, not on the heights immediately under the cloud, but low down, within a few paces of the river. It was broken square off twenty or thirty feet from the top; and this top fell straight down and stood leaning against the shattered stump, showing that the trunk had been rent asunder so suddenly as to occasion no obstruction to its fall! There were but two discharges of electricity from this cloud; and soon afterward the weather cleared up from the northwest.