Читать книгу Great Disasters and Horrors in the World's History - Allen Howard Godbey - Страница 8
CHAPTER IV.
TORNADOES AND CYCLONES.
Оглавление“O sad and mournful wind!
From what wild depths of human pain and sorrow
Could’st thou those tones of restless anguish borrow
As of a soul that dreams of no to-morrow,
O sad and mournful wind!
O, thou art fierce and wild!
Thy mighty chariot through the black skies lashing,
The cloud-shapes round the mountain-summits dashing,
The waves of ocean round the wrecked bark crashing—
O, thou art fierce and wild!”
EN find no difficulty in recognizing law and system in the phenomena that are of constant or frequent recurrence. That which is most difficult to explain, may pass without a serious thought so long as it manifests no stupendous or sudden power. The water may wear away the stone for centuries and its progress be unheeded by those who daily visit the pool.
So all observe and admire the beauty and order that prevails in the system of winds hitherto described. Their movements seem so simple and natural, that people take them as a matter of course. Rather, we should say, they may be depended upon with such certainty that the laws which they followed were unheeded for more than six thousand years. Relying on result, men gave themselves no concern about principle.
But in the sudden storm, the cyclone or tempest that comes sweeping the land with hardly any warning, flooding and destroying, men find mystery.
And who is not justly awed thereby? What other power so easily and frequently wrecks and ravages? Who may point out its course or stay its progress?
And indeed it would seem difficult at first to find any law or system that controls the motion of the storm. If a rain storm always came from the same direction; if unusually high winds always blew from the same quarters, just as the moderate breezes of spring and summer can always be expected from the same general direction, it would appear that there was much greater subordination to definite law. But what can be more perplexing than to have a storm blow violently from one quarter for a time, and after a brief calm to blow with equal violence the other way? Can such phenomena be explained by any principles hitherto discovered?
What is a storm? Strictly speaking, it is any marked or unusual disturbance of the normal atmospheric conditions. There may be excessive wind: there may be cessation of the customary winds. Two great classes are found: cyclonic, or low area storms, and anti-cyclonic, or high area storms. The former may be accompanied by heavy rainfall or snow; the latter is usually noted for absence of either. It is with the low area storm that we must deal at present.
This term is used to designate all storms which are marked by low barometer, and therefore it is clear that such are accompanied and partially occasioned by an unusual amount of moisture in the atmosphere. The resultant commotion is usually extensive, the storm centre traveling across the country; but occasionally the effects are perceptible only for a short distance, the storm centre either breaking up or ascending to the upper atmosphere.
By a cyclonic storm is signified a storm characterized by unusually low barometer, and a wind system blowing spirally inward, as in a genuine cyclone. They usually affect only the lower strata of the air. Quite frequently they are broken up by striking low mountain ranges, such as the Alleghany system: and often pass Mount Washington without making their presence felt at the signal station on its summit. To what extent they are influenced by or are due to the upper air currents is therefore unknown, though not a few of the attendant phenomena indicate that the latter are of no little importance.
Any one who has observed the waters at the junction of two streams, is familiar with the appearance of numerous tiny eddies or whirlpools formed at the point of junction. Such are perceptible also in every rapid stream, when the current, sheering sharply from a projecting point, is made in a measure to collide with itself. This is also the principle of the many tiny whirlwinds seen during the warm summer days: and such are also observable in winter, if there be snow enough to render their presence in the air clearly visible. Their results are most readily recognized in snow-drifts, where the wind meets some special obstruction. It does not often occur that a high fence is covered with a snow-drift: a great drift will be thrown up by it, but not against it: and the side next the fence will be curved inward, or concave. The wind strikes the fence and partially recoils, curving upward to pass over the fence. The drift is then built up between the wind and the current recoiling from the fence, and its inner curve shows the direction pursued by the rebounding current.
Now, when opposing air-currents meet each other on a large scale, the immense whirlwind that is produced is called a cyclone or tornado. It follows then, that if we would find any regularity or law in these unusual disturbances, we must know if there exists any permanent condition of atmospheric currents that is favorable to their generation.
That such a state exists, we have already learned. The great belts of calms that we have found between the trade-winds and the return trades and polar currents, are more appropriately called the zones of equinoctial storms. We have in them districts of general calms, with winds infringing upon either side. It is evident then that, as in the case of the fence, whose recoil-current curves the snow-drift, a whirling current of considerable magnitude may arise here at any time: hence, violent storms do arise in these regions more or less at all periods of the year. But we have seen that these zones of calms move slightly to the north or south with the course of the sun. It would then appear that at the equinoctial period, when they return from the mean position toward the extreme northern or southern limit, there would be opportunity for unusual disturbances, especially since the heavy rainfall of those periods would unusually affect the temperature of the atmosphere.
That is precisely what occurs. The equinoxes are marked by storms of unusual severity, and the influence of the sudden falls of rain is so great that some eminent men believe them to be nearly the sole factor in the formation of these storms. In one case they doubtless are. If a very heavy rain be decidedly local, there is low barometer at that place. Now, if on either side there be areas of high barometer, the opposing currents flowing toward the center of low area are sufficient to meet all the conditions necessary for a cyclonic storm. As the zone of calms is comparatively narrow, it is apparent that the diameter of the area of any storm, owing to the pressure exerted by the incoming currents of wind, must be still less. Hence, the cyclone center, at its time of formation, seldom exceeds one hundred miles in diameter. As it travels away from the compressing currents that formed it, it is clear that its centrifugal force must increase; hence, its area increases, and its violence correspondingly diminishes.
These facts refer to the unusually violent cyclonic storms, properly known as cyclones. But all low area storms are characterized by the upward spiral motion, though not strong enough in the case of ordinary summer rains and thunderstorms to be especially noticed. We shall see, by and by, how this spiral motion may result without the intervention of any strong opposing currents.
Why and how a cyclone travels, is a question that at once propounds itself. Its motion is in accordance with a fixed law, whose operation varies only as it may be affected by unusual peculiarities in the configuration of the surface over which it travels. The reason of the motion is not so easy to explain; neither is it easy to explain why heat expands objects: but its operation is none the less certain. And so the route pursued by any storm can be readily indicated in advance. It is not a matter of mere conjecture.
The motion of a cyclone or tornado is in accordance with the same law that governs the motion of planets around the sun. It can be illustrated in a very simple manner by the spinning of a top.
Spin a top on a perfectly smooth and level surface. It will be better if the peg of the top be blunt or round, so that there will be no tendency to settle steadily into some possible hole or depression.
Now, the instant any degree of steadiness is attained, the top begins to move in small curves. If it be spun on a marble slab smoothly coated with fine flour or sand, it can be made to record its motions, which may then be carefully studied. It will be found that the form of the curve is nearly the same with every start. It will describe a parabola, pause a moment, then describe a second, and so on.
The chief peculiarity of this separate curvilinear motion is that its direction is always in an opposite direction to that of the rotation of the top. If the top turn from left to right, it will move from right to left, and vice versa. The same tendency will manifest itself even if the peg of the top be placed in a slight depression or socket, so that the curve cannot be made. Then the upper portion of the top will incline to one side, and begin describing a curve: but, as before, in a direction contrary to the direction of rotation.
The common toy known as a gyroscope illustrates the last peculiarity also. It consists of a wheel within a metal frame, which has a peg like a top. If the wheel be made to revolve rapidly, the whole may be balanced on the peg: when the frame will begin to slowly revolve in the opposite direction: and if placed upon a smooth level surface, like the top it will tend to describe the same course.
Still other illustrations of this principle are even more familiar than the spinning of a top. Any one who has seen the game of soldiers in a bowling alley knows that in order to make the ball turn to the left as it moves forward, it must spin the other way; that is, with the hands of a watch. To travel or curve to the right, it must spin in the contrary direction. So in our “great national game,” base-ball, the pitcher curves the ball any way he pleases merely by following this law. It is not necessary to take into account, as many do, the return trades, as occasioning the travel of a whirling storm; and the fact is, that the cyclone frequently travels more rapidly than the ordinary wind moving in the same direction.
Now, the motion of the planets is similar: rotating in one direction, they travel in the other. So we find the general law is,
All revolving bodies, left free as to direction, travel in a curve in a direction opposite to that of their rotation. This curve is usually some form of conic section: an ellipse, parabola or hyperbola. The planets, and some comets, move in ellipses. Some comets travel parabolas or hyperbolas. And the parabola is the customary path of the cyclonic storm. As the cyclone in the northern hemisphere rotates from right to left, and in the southern from left to right, their paths must necessarily be in opposite directions, as may be seen by the accompanying diagram. So in either case, the direction of the path is always away from the equator.
As far as the United States are concerned, most non-cyclonic storms originate in the Saskatchewan country, or along the southeastern slope of the Rocky Mountains. By far the greater number pass over the St. Lawrence valley. A small number are developed in the Gulf, or in the Pacific: but these are much affected, often broken up, in crossing the Rocky or Appalachian systems. The usual course is somewhat north of east; but there are a few notable exceptions. The immense amount of vapor wafted up the Mississippi valley induces some low area storms to move southward from Manitoba into the upper Mississippi valley. In like manner, the excessive moisture along our north Pacific coast causes occasional storms to move southward from Alaska to Oregon.
But the course of a cyclonic storm, we have seen, must be different.
The accompanying diagram illustrates the fact that the wind blows from all directions toward the center of the storm. As the storm revolves, the wind would come apparently from the south for any one on the eastern edge of the cyclone of the northern hemisphere. Hence, in the case of a storm of large diameter, people in Richmond or Washington may often be surprised by an apparent northeast gale, which reaches them before it strikes New York or Boston. At the center of the storm is absolute calm. So if a cyclone pass centrally over any point in the northern hemisphere, a person at that place will find the wind blowing violently from the southeast: then after an interval of calm, it will blow with equal violence from the northwest. This will be the case if the path of the storm has already turned to the northeast, so that its northeast quarter may be called its front. If on the northwest course, however, the apparently alternate winds would be from the northeast and southwest. So one in the path of a southern cyclone would find the winds proceeding from the same quarters; for though it revolves in the opposite direction, its front or path is also in the opposite direction; so in either hemisphere, the southeast or the northeast wind will be the first felt by one directly in the track of the storm.
ROTATION OF STORMS.
Another result of the path of a cyclone is that the direction of its center from the stand-point of any observer is readily known. A glance at the diagram shows at once that if any one within the storm area of a cyclone of the northern hemisphere stands with his back to the wind, the storm center, where the barometer is lowest, is invariably on his left: but if he stand with his back to the wind of a southern cyclone, the storm center is always on his right. Hence, if a vessel be overtaken by a cyclone, the captain at once may know how to pass beyond its range, by shaping his course at right angles to that of the wind. Thus, if in a northern cyclone, he must sail to the right, supposing his back is to the wind: in the southern hemisphere, he would sail to the left.
As an example of the expansion of the storm area in its journey, may be mentioned the West India hurricane of 1839, which had, in the Antilles, a diameter of three hundred miles, which increased to five hundred at the Bermudas, and eight hundred on the parallel of 50° north latitude.
To draw again upon the illustration of the spinning top, it will be observed that the curvilinear motion is extremely slow in comparison with that of rotation, but increases as the rotation decreases. The same law applies to the movement of cyclones. The slowest motion forward is usually near the apex of the curve: and the progress on the ocean is much slower than on the land. Traveling over the latter, the irregularities of surface act in the case of the storm just as a rough surface does in the case of the top. The motion may be accelerated, but its regularity is lessened. So while at sea the parabolic path of the storm is almost absolutely perfect, but on reaching the land its motion is more rapid, and less regular, conforming somewhat to the configuration of the surface.
To illustrate, take the great cyclone of August 16th to 22nd, 1888. This started off Point Jupiter, Florida, with a rainfall of 2.2 inches in twelve hours, while the rotary velocity of the wind was sixty miles per hour. Its path across the Gulf of Mexico was a perfect semi-parabola, curving northward into western Louisiana; but rapid as was the rotary velocity, three and a half days were required for the journey across the gulf. Meanwhile, it was rapidly widening: for within a few hours of its reaching land, its eastern edge was assailing Mobile, Alabama, with a south wind of fifty-five miles an hour. Almost at the same time the western half was flooding Memphis and Vicksburg with an enormous rainfall—almost four inches in twelve hours, at Memphis. By the morning of August 21st, thirty-six hours after reaching land, it was central over middle Tennessee and Kentucky; heavy rains fell over the entire region. But by this time its eastern edge was in collision with the Appalachian chain; while a heavy local rain at the northern extremity of that chain created an additional diversion in a new area of low barometer. So it left the hitherto parabolic route, and shot away nearly at a tangent along the western Appalachian slope, passing from Tennessee to Newfoundland in thirty-six hours, thus moving nearly three times as rapidly as in the Gulf: while its violence, or rotary speed, was vastly lessened.
This storm was one of the most destructive of the recent cyclones that have swept our country, doing immense damage to crops, bridges, houses, herds—in short, everything that can be seriously damaged by wind or flood.
The damage in Louisiana alone was estimated at $500,000. But it was by no means the most destructive of the West India storms.
An examination of the areas of calms, which are the hot-beds of cyclones and hurricanes, shows that the region which produces the great cyclones of the United States lies in the Antilles and Caribbean Sea. In the Pacific the portion of the calm belt of the Tropic of Cancer causes the ravages of cyclones or hurricanes originating there to be felt chiefly in Japan and China. The storms of the Pacific arising in the equatorial calm belt, are most violent in the East Indies, and the southern peninsulas of Asia. As these regions are much warmer, and consequently the atmosphere may hold a much greater quantity of vapor, it follows that cyclones in that quarter much exceed in violence those of our own land.
Such are the general laws of these terrible disturbances of nature, as ascertained by years of careful observation. In the United States, our Signal Service, with well-equipped observatories at important localities, is able to make these principles of practical use: to detect the incipient storm and mark out its path, ere it strikes its fiercest blow.
It should be observed, ere leaving this topic, that a few would-be prophets have maintained that not only great storms, but also earthquakes, volcanic eruptions, tidal waves, etc., are due to planetary influences. Observing that the most violent hurricanes occur near the equinoctial period, they argue that the equinoxes of the planets ought to also disturb the earth. They ignore the fact that as to our own equinoctial disturbances, the change in the relative position of the earth and sun is sufficient to produce change in the location of heated air-currents and consequent storms. They seek to find in the Equinox Absolute, some strange mysterious magic, some inexplicable power or Deus ex machina, whose business it is to get up a disturbance here on earth at every possible opportunity, no matter in what planet he may be for the nonce located.
But it is difficult to rid any man of his hobby. In the question of the equinoxes of other planets, their recurrence is of sufficient frequency to allow the weather-crank full play for his imagination. Two of the major planets lie within the earth’s orbit, and their more rapid course about
WATER SPOUTS AT SEA.
the sun results in there being an equinox in one or the other of them about once in each month. So no matter in what month a great storm may occur, the enthusiast can point out that a neighboring planet is at or near an equinox.
A careful examination of the equinoxes of the inner planets for a period of fifteen years shows that the number occurring in the month of April was 22 per cent. above the average occurrence for any month: whence, it would appear that the disturbances at that period ought to be equally in excess. But as a matter of fact, storms on the earth are most numerous and violent at the time of the autumnal equinox—September and October—when no such departure from the average of equinoxes of the other planets appears.
If planetary influence were the cause of our storms, it would be reasonable to suppose that disturbances would be greatest when the planets are nearest to the earth: but the advocates of the theory do not seem to consider this a factor at all. Nor could the planetary equinox theory account for the fact that storms of peculiar character always originate in the same regions. For instance, why do cyclones always originate near the tropics and move away from the equator? If the planetary equinoxes produce violent earthquakes, why are they so partial to North America as never in our whole history to have given us a very serious shaking up? Why is it that of the hundreds of recorded tornadoes of the past century in the United States, only one has ever occurred west of Dodge City, Kansas? Clearly the adherents of the equinoctial theory will have to admit local terrestrial conditions that modify all their theories: and to make such an admission will be, in the end, to give up the fight.
The writer knew of a boy who wasted a pound and a half of bird-shot in trying to kill a small owl. The game was finally secured, and the young Nimrod discovered a hatful of feathers with the body of a robin—and of no earthly use.
Attacking the planetary theory is of little more use. But the theory is the resort of many would-be weather prophets, who needlessly alarm the ignorant with their gloomy forebodings.
In a country as large as our own, any sort of weather is a very safe prediction to make for any day in the century, as minor rains and cloudy seasons and small storms are merely local. Any sort of prediction would be nearly sure to hit some portion of the country; and one who is so disposed can easily win a cheap notoriety and gain scores of testimonials as to the correctness of his predictions. Every unusual catastrophe produces a brood of these gentry who are eager to make the trial. But those who endeavor to indicate the exact locality where any great disturbance is to take place, meet discomfiture with a uniformity that ought to be discouraging. The work of the Signal Service is as carefully done as may well be: yet its best men assert that an average of 90 per cent. of correctness in their prognostications is unusual, because of the extremely small areas covered by local disturbances. Rainy weather announced for western Missouri may be correct every time for Kansas City, but be 10 per cent. in error for Nevada, Missouri. When those whose time is devoted to the weather can not always be correct, it is useless to listen to charlatans.
A careful study of sun-spots with relation to storms has been made of late years. The fact is elicited that the spots seem to have a definite connection with electrical disturbances: but while there are numbers of coincidences between unusual sun-spots and great storms, the number of striking exceptions seems equally great. Hence, it can not be fairly inferred that there is any definite relation between them. And so far as electrical phenomena on the earth are connected with storms, they always appear as dependent upon rather than productive of atmospheric currents. Indeed, the most remarkable electrical disturbances occur at times when no atmospheric current is prevalent. The most beautiful electrical display, the aurora, appears when the air is abnormally still and unusually dry. The necessity of the latter condition accounts for the fact that it is usually observable only in cold weather and occurs with great frequency and in remarkable brilliancy in the polar regions. It results from electric currents passing through extremely rarefied and dry air, and may be produced on a small scale artificially.
Poe was right when he held that many things remain long secrets by reason of their very simplicity. Six thousand years steam hissed and fumed in men’s faces, and tilted the kettle lid, before they learned its expansive power. Six thousand years the lightning flamed and roared before man realized it could be made one of his most obedient servants. Six thousand years he cudgeled his wits to discover the secret of the wind: yet when he made a fire within his house, he closed the door to prevent unpleasant draughts of air. And so he continues, constantly endeavoring to find some strange mystery in the things that are dependent on simplest laws.
There was a time when men stood aghast at small-pox, cholera, yellow fever, and many similar calamities, and spoke with bated breath of the “mysterious visitations of providence,” the “scourge of God,” and so on. When the Turks once besieged a plague-stricken city, a comet appeared in the sky. The pious inhabitants prayed “O Lord, deliver us from the devil, the Turk and the comet,” and usually such people believed such plagues were the judgment of God on them for their sins. Modern science holds that about the only sin the Lord punishes in that way is the sin of filthy streets, or back-door cess-pools. When man has once learned the means of control and prevention, evils lose their mysterious witchery.
On the other hand, let the laws of any force in nature be every so well understood: yet, so long as they are beyond the control of man, they will retain for him an eerie uncanny fascination. The pigmy has harnessed the steam and chained the lightning: but when the storm clouds lower and the forests moan, the sea roars and the lightning glows, he stands in fear and awe before a power whose might he but vaguely comprehends. He may know of the winds, whence and whither bound; but when the Stygian darkness has passed on, leaving wreck and ruin, want and woe, desolation and despair, shattered homes and hopes, and bleeding hearts, this knowledge of law is, for the nonce, forgotten, and the hurricane is transformed, in his disordered vision, into a demon of wrath, or caprice; or he speaks, hesitatingly it may be, of the mysterious dispensation of an inscrutable providence. But in the mighty wind, as in the soughing breeze, there is only obedience to universal law. But when the Author of law displays his power, man’s instinct, however unwilling his reason, acknowledges a God.