Popular Science Monthly/Volume 2/February 1873/The Law of Storms Developed
|THE LAW OF STORMS DEVELOPED.|
By PROFESSOR THOMPSON B. MAURY,
OF THE SIGNAL-OFFICE, WASHINGTON.
METEOROLOGISTS tell us that their science is as old as Aristotle. If we should judge by its progress up to the middle of the present century, its antiquity furnishes little to boast of; for, in the long lapse of centuries, it must have proved an incorrigibly dull scholar. Within the past few years, however, it has greatly improved, and, especially since it became identified with the popular and important systems of storm-warnings and weather-forecasts, it has been rapidly developed. This is peculiarly the case in America, and it is not wonderful, when we consider the comprehensive observations of our meteorological bureau, and the many beautiful phenomena which its publications disclose.
If Vasco Nunez, the discoverer of the great South Sea, was so awed by the grandeur and expanse of its waters, as seen with the naked eye, how much more may we be impressed as telegraphic meteorology enables us to discover, at a glance, the tossings and undulations of the aerial ocean over the larger part of the hemisphere!
It is to some of the deductions, that may be justly made from the extensive and synchronous observations of the modern weather-systems, as they bear upon those weather-problems, which, from time immemorial, have interested mankind, that we now ask attention.
Until the year 1821, "the law of storms," simple as it is, was unknown to the most profound meteorologists and expert seamen of the world. It was then first discovered and announced by Mr. William C. Redfield, of New York, and established by the labors of that great mind, against the constant perversions and opposition of the scientific empirics of his day. It can be easily comprehended in its great outlines, and as far as our present purposes require. It assumes nothing, supposes nothing; but, from thousands of actual and actually recorded observations, presents the phenomena of spiral currents of air seeking a common centre of depression, and, in the attempt to find that centre, acquiring a vorticose or rotatory motion. The direction of this rotation Mr. Redfield found to be uniformly, in our hemisphere, contrary to that of the hands of a watch, with its face turned upward; and, in the Southern Hemisphere, the rotation is with those hands, or with the sun in its diurnal round. It is easy to see that, if the atmospheric column, resting over any given area of the earth's surface, should, for any cause, be suddenly diminished, or its pressure and intensity be reduced, the gaseous fluid would rush in from all surrounding regions to restore the disturbed equilibrium; and, if the earth was not whirling around on its axis, every particle of the centre-seeking air would endeavor to move on the shortest, or the straight line. It is known, from the principles of mechanics, that this endeavor can never strictly be executed, because the axial rotation of the globe incessantly so acts as to throw every body, while in motion, in our hemisphere, to the right of the line on which it is moving, no matter whether that line be from east to west, north to south, or at any conceivable angle with the meridians or the equator. Obeying, in part, this tangential impulse, every particle of wind must take up a resultant motion. If it begins to blow toward the depressed centre of the storm as a north wind, it trends to the west, and is felt as a northeaster; if it begins as a south wind, it diverges as a southwester; if as an east wind, it becomes a southeaster; and, if as a west wind, it soon changes into the boreal northwest wind.
It has often been asked whether the storms of our latitudes attain the immense size formerly attributed to them; and many eminent writers have denied the possibility of their reaching a diameter of more than two or three hundred miles. Mr. J. K. Laughton, in his recently-published "Physical Geography," would have us believe that cyclones "do not attain the enormous magnitudes which have been assigned them." But this opinion rests merely upon conjecture, not yet upon a correct physical theory.
It is a well-known fact that the monsoons generated on the central plateau north of the Himalaya Mountains, and the whole system of Asiatic wet monsoons, may be regarded as an immense and prolonged cyclone; and extend their "backing" influence into the Indian Ocean, and reach far to the south, through more than forty degrees of latitude (a radius of 2,500 geographical miles), and from the 69th to the 140th meridian of east longitude, far out into the Pacific, beyond the Bonin and Ladrone Islands, southeast of Japan. The whole system of wet monsoons may also be justly regarded as a grand cyclone, whose centre is stationary over the heated plains of Central Asia, whose intro-moving winds, bearing the evaporations of the Asiatic seas and oceans, feed it with meteoric fuel for six months in the year, and whose periphery may be regarded as embracing nearly one-third of the entire Eastern Hemisphere. Analogy, therefore, warrants the idea of a great cyclone. But, apart from all this, actual observations in different parts of the globe prove the frequency of storms of enormous magnitude. Thus, in the celebrated Gulf-Stream storm of 1839, as Sir David Brewster long ago pointed out, several stanch merchant-men were foundering off the coast of Georgia, near Savannah, in the very heart of the gale, at the same hour that the winds in its north-west quadrant were taking the roofs off houses in New York and Boston, more than 800 miles distant—clearly revealing a cyclone whose formation was symmetrical, and whose diameter must have been nearly 1,300 miles. But, not to go back to old data, the West-Indian storm of the 18th of August, 1871, before its centre had moved north of Florida, had begun to draw upon the regions of high barometer in the northern States, had exerted its influence as far north as New London, Connecticut, and gave us the northeasterly cyclonic winds in the northwest quadrant of the whirl, on the entire Atlantic coast. The more furious cyclone of the 24th of August, discovered to be then south-east of Florida, and telegraphically foreannounced as likely to endanger the coasts of the Southern States in less than forty-eight hours, appeared on the 26th in full force in Northern Florida, but not until some eight or ten hours after it had set the atmosphere all around it (as far north as Boston) in cyclonic motion, and had caused the storm-cloud to spread itself over the entire region of the United States on the eastern slopes of the Alleghanies, and as far westward as Knoxville, Tennessee. It is no uncommon thing, as Redfield, Espy, Henry, Loomis, and others, long ago showed, for an area of depression on the upper lakes to make itself simultaneously felt as far south as the Gulf of Mexico, and as far east as New England.
If it fell within the scope of the present design in this paper to consider the final cause of storms, it would be easy to show that, unless the law of storms ordained a large area, and a far extended path for the meteor, in some degree commensurate with the area of our immense continent, the meteor could not fulfil its office in the terrestrial economy—an office which, apparently, imposes upon it the task of gathering to its centre, through the agency of its intro-moving winds, the idle and inappreciable moisture scattered over the surface of the earth, condensing it into rain and snow, and diffusing it, in these forms, over immense districts of country.
It is of incalculable importance to observe, and carefully digest the fact, that, when a storm-centre or area of low barometer is once formed, it is the nucleus for a vast aggregation and marshalling of meteoric forces. No matter how small at first, under favorable atmospheric conditions, the courant ascendant is formed, condensation aloft sets in, and the precipitation only serves to add "fuel to the flames" of the cyclonic engine. This process widens in geographical area, and, after a few hours have elapsed, the storm may so develop as to cover a continent with its portentous canopy of cloud, while simultaneously strewing an ocean with wrecks, and throwing out, in the upper sky, more than a thousand miles in its front, the fine filaments of the premonitory cirrus and cirronus.
In close connection with the size and magnitude of cyclones must be considered the distance over which they pass from their initial point. Much has been said on this part of our subject, and not a few writers have accepted the doctrine of Admiral Fitzroy that they progress over but comparatively short distances. For such a view, however, it is impossible to find, either in the nature or physical office of the cyclone, any support whatever. The storm once engendered, no matter in what part of the world, may be stationary or progressive. There are well-authenticated instances of almost stationary cyclones and almost stationary typhoons, of which latter will be remembered the famous gale of the ship Charles Heddle—an Indiaman, carried round and round the storm-centre for five days—which progressed not more than ninety miles a day. Indeed, we may, as has been said, regard every wet-monsoon region as a stationary and semi-perennial cyclone. Such a meteor has been shown to resemble an eddy moving in the current of a rapid river. The latter may be large or small, while it does not determine, but is determined by, the course of the on-flowing stream. It is true the centre of an eddy or water-hollow may soon be filled up and the whirl disappear; but it is because the depression is not maintained. If the depression could be maintained, it is easy to see that the eddy would continue, and pursue its way, as long as the current in which it is embodied continues to flow: it might be through the length of an Amazon or a Mississippi River. In the case of a cyclonic eddy or whirl, we know the atmospheric depression is maintained as long as the centre moves in a region sufficiently supplied with aqueous vapor to feed it. It is a physical impossibility, as has been often shown, that any storm, however vast or however violent, can prolong its advance or sustain its fury over a dry and desiccated surface. The most extended typhoons of the East, upon entering the dry and rainless continental regions, dwindle into the well-known and diminutive dust-whirlwind, such as Sir S. W. Baker describes as witnessed in Nubia, and as here illustrated. The Sahara is a more formidable barrier to the
passage of a storm than the majestic mountain-wall of the Alps, and the simoom is, notwithstanding the stories of travellers and the legend of swallowing up of the army of Cambyses on the African desert, a wasted and worn-out cyclone. In his "Desert World," Mangin, compiling the more accurate observations of the phenomenon says: "It never prevails over any considerable area, and beyond its limits the atmosphere remains serene and calm; the phenomenon is of brief duration, the atmospheric equilibrium is speedily restored; the heavens recover their serenity; the atmosphere grows clear, and the sand-columns, falling in upon themselves, form a number of little hills or cones, apparently constructed with great care, like those mimic edifices of sand made by children in their pastimes." The same writer also mentions a severe simoom which was "over in a couple of hours."
Embedded in the great aërial currents, however, and supplied with abundance of moisture, there is nothing to arrest either the rotatory or progressive movements of the storm. Like the drift-bottles cast upon the current of the ocean, and found after months to have been carried thousands of miles, from the equatorial to the polar parallels, there is every reason to suppose the tropic-cradled gale, and the minor storms also, are borne in the great atmospheric currents through quite as great distances. There is an authentic and well-attested account of a Japanese junk, lost or deserted off Osaka, drifting through the immense arc of the Kuro Siwo's recurvation, and encountered (in latitude 37°, by the brig Forrester, March 24, 1815) off the coast of California. That tiny craft must have followed in the bands of westerly winds and warm waters for seventeen months. Why, upon theoretical grounds, should we reject the hypothesis which represents the movement of storm-areas as prolonged for many thousands of leagues, or indeed that which represents them perpetually in motion around given centres of cyclonic or anti-cyclonic areas, keeping pace with the great winds in their eternal circuit?
As a striking corroboration of all this we find—what might have been assumed on theoretical grounds—that the logs and special observations of the Cunard steamships show that a vessel bound from Liverpool westward encounters frequent advancing areas of low pressure, indicating a number of rapidly-succeeding barometric hollows or depressions, "each with its own cyclonic wind-system, moving across the Atlantic as eddies chasing each other down a river-current."
The word cyclone has frequently, but incorrectly, been used as significant of an enormous or very violent meteor, as if its application was to be confined to the devastating hurricane of the West Indies or the terrific typhoon of the China seas. It simply means a storm which acts in a circular direction, and whose winds converge, by radials or sinuous spirals, toward a centre, moving in our hemisphere in the opposite direction to that of the hands of a clock, and in the Southern Hemisphere in a contrary direction. Taking this as the definition of a cyclone, it seems clear, from observation alone, that all storms are to be regarded as cyclonic. Volumes have been written to prove that this is not the case. But we have only to examine a few series of weather-maps from week to week to see that, wherever you have an area of low barometer, into its central hollow the exterior atmosphere from all sides will pour, and that in so doing a rotatory spiral or vorticose storm is generated. The tornado, the simooms, the dust-whirlwind, the fire-storm, even the slow and sluggish storm which moves on our Western plains as the laboring wheel of the steamship buried in a heavy sea, all attest that a body cannot move on the earth's surface in a straight line. It is not more true with us that the Gulf Stream turns to the eastward, the Polar Stream to the westward, and the equatorial currents to the northward, than that every air-current, in obedience to the same law, should turn to the right of the line along which from any cause it is called to move. The meteorist has therefore only to ascertain by observation where the barometer is lowest, to know at once the direction of the winds from the circumjacent districts, far and near, or at least to test the mathematical law by a grand experiment.
The tangential and centripetal forces, acting at the same time on any particle of air in the storm, may be equal or very unequal, and the cyclonic character of the gale may be well marked or partly concealed. In the tornado, with a diameter of only a few hundred feet, the tangential force may not be appreciable to an observer, but it is present, and intensely assists in communicating vorticose motion to the storm, whose roar is heard with awe by the stoutest heart, as it crashes through the forest and even ploughs up the soil of the earth. If the cyclonic or spiral feature should fail to manifest itself in any storm, we ought to look for such failure in the tornado. It is true that no barometric readings have ever been taken in the narrow heart of a tornado, but abundant evidence exists of the fearful rarefaction in the centre. While the meteor, once set in motion, may move forward with great velocity and destructiveness, the danger is clearly due to the intro-rushing and gyratory winds. There is not an instance, it is believed, recorded in which a tornado moved as much as 100 miles an hour; probably one-half that velocity would be too high an estimate for its usual and ordinary motion. But the wind, moving straightforward at the rate of 60 or 80 miles an hour, never worked any thing like the disaster of a tornado. In the West-Indian hurricane, blowing at the rate of 100 miles an hour, houses have been blown down, shipsstranded; but all this is mere child's-play compared to the suction and whirl of the tornado. The conclusion forced upon us is, that the ravages of the latter are due, not to the weight of the atmosphere, moving as a river-torrent in a straight line, nor to the rush of air behind the travelling vacuum, but to the torsive, racking motion—imparted to every object in its path—due to its gyration. To prove that this gyration is always from right to left, or against the hands of a watch, is, of course, practically impossible; but such a direction has often been observed in tornadoes.
It may, therefore, be safely concluded that, for all processes of meteorologic calculation, the disturbance, if not such at first, will soon become cyclonic. All daily weather-charts demonstrate this, not by a laboratory or lecture-room experiment, but on an infinitely wider and grander scale, and in a manner far more conclusive than any merely manual experiment could possibly make to appear. As one has happily said, "Nature makes no distinction between small and great; the drop of mist that lights gently down on a delicate flower, and the avalanche that sweeps away a village, fall in obedience to one universal law."
It has been asserted lately that the Gulf Stream has no influence upon storms; that they have no tendency to run toward it or to run upon it; and that what geographers and seamen have always said about the Gulf Stream as a "weather-breeder" and "storm-king" is absurd. I think it can be demonstrated that this well-known popular belief is not absurd.
It is an observation, as old as Aristotle, that the storms of the middle latitudes in the Northern Hemisphere advance from west to east. This is obviously partly due to the fact that the winds on their eastern sides are southerly, that they come from the equatorial regions, and hence are highly charged with aqueous vapor. This vapor is absolutely essential to the sustenance of the storm. Moreover, the law of storms requires that the southerly winds should enter the storm-vortex on the eastern side, and as this is the side on which the greatest quantity of vapor is found, and the side of greatest condensation, of the greatest evolution of latent heat, hence of the greatest aërial rarefaction and barometric fall, to this side the heavier air from the west will push as into a great hollow. Thus do we actually find that all storms, formed west of the Gulf Stream, are actually propagated toward it. It may be argued from the above facts that the anti-trade winds are thus maintained by storms incessantly making the circuit of the globe within the temperate zone. But in reality, instead of being the effect of storm-influence, the anti-trades are originated by independent solar agency, as are the trades, and they are potential and causal in producing the eastward progression of all cyclones. It must be conceded that the pressure of vast aërial currents does serve to force the meteor along with them as the river-eddy is carried down stream with the water-current; otherwise it is impossible to explain the westward progression of tropical hurricanes. While yet in the band of easterly trade-winds the storm will invariably work its way or be propagated toward the most humid region, unless mechanically borne in another direction by the great atmospheric current in which it is often embedded as an eddy in a river. The cyclone-tracks over all the oceans lie in the central bands of the great ocean-currents of high temperature and great evaporation, and the band of cyclonic violence is often beautifully coterminous with the sharply-marked edge of the Gulf Stream. Thus, in the Pacific, the Loochoo Islands lie just in the path of the Kuro Siwo, the great Pacific Gulf Stream of the Japanese, and are visited by the most fearful typhoons; but the Bonin Islands, in the same parallel, but on the extreme margin of the Kuro Siwo, have very mild and moderate storms. "If a storm commences anywhere in the vicinity of the Gulf Stream, it naturally tends toward that stream, because," as Loomis says, "here is the greatest amount of vapor to be precipitated, and, when a storm has once encountered the Gulf Stream, it continues to follow that stream in its progress eastward." Vessels and Japanese junks, dismasted in gales off the Asiatic coast, have been drifted for many days in the current of the Kuro Siwo, to the coast of California, just as West-India beans, cocoa-nuts, and vegetables, have been drifted to Iceland, Greenland, and Spitzbergen, on the extension of the Gulf Stream. According to all meteorological observations of the tracks of storms, we are warranted in believing that cyclones and hurricanes do, as a matter of fact and of atmospheric law, run on the hot currents of the sea as naturally as the water-course clings to its bed. Practical seamen, though unable to explain the fact, are always on the lookout for these furious gales when sailing on the axial lines of the Gulf Stream, on the hot Mozambique current (the Gulf Stream of the Indian Ocean), and on the dark superheated waters of the Kuro Siwo of the Pacific.
So dangerous and disastrous are the storms which course along the Gulf Stream that sailors avoid it, and the American Sailing Directions and those of the British Admiralty advise all vessels, sailing from the West Indies to New York or Liverpool, to beware of taking advantage of its current, although it would help them along from three to four miles an hour. Close observation has traced these storms continuously from the Florida coast to New York, through Redfield's labors, and thence to England, through the record of the Cunard steamships, and thousands of detached observations.
We have now reached a point where we can properly and intelligently consider a question that has always baffled meteorologists—the origin of cyclones. The diagnosis of the phenomenon necessarily precedes its explanation. This subject has engrossed many minds, and various have been the ingenious devices for unravelling its mystery. Mr. Redfield—the father of storm physics—in his modesty and diffidence, so distrusted himself and in his day so keenly felt the need of a more enlarged induction of facts, that he has scarcely left us his opinion. The theories of other writers have all long since been abandoned by themselves or suffered to drop from the notice of the scientific world as evidently incapable of explaining the phenomena of cyclones. This has been the fate of them all, unless possibly we except the theory advanced by the great meteorologist, M. Dové, of Berlin. Briefly stated, the latter hypothesis is this (at least in its application to West-India hurricanes), viz., that "they owe their origin to the intrusion of the upper counter trade-wind into the lower trade-wind current" (Dové's "Law of Storms," p. 264).
Without pausing here to examine this theory upon its merits and upon the facts, we hasten to mention a different hypothesis advanced, nearly two years ago, as a substitute for that of M. Dove, and as affording an entirely original and satisfactory explanation of the origin of cyclones.
The hypothesis was likewise based upon the agency of the trade-winds, but in a manner wholly different from that elaborated by the German meteorologist. In the original paper in which my views were published, the following statement was made: "It can be demonstrated that the origin of cyclones is found in the tendency of the southeast trade-winds to invade the territory of the southeast trades, by sweeping over the equator into our hemisphere."
The hypothesis advanced, in lieu of another seemingly less satisfactory, claimed to rest upon observations conducted in the very region most notorious for the generation of cyclones.
To test this, we need only to examine the Atlantic trade-winds.
Theoretically, physical geography has generally represented the motions of the atmosphere somewhat as is represented in the accompanying diagram of the winds, as projected by Prof. William Ferrel, of Cambridge. The elaborate pages of Prof. Coffin, in his invaluable volume on the "Winds of the Northern Hemisphere," as deduced from myriads of observations, show that the graphic illustration furnished by the following diagram is approximately correct.
The region of the trade-winds, it will be seen, more than covers the torrid zone of the earth, and all seasons of the year overlaps both the northern and southern tropics. While this is theoretically true, and is usually put forth as fact, it must be accompanied with one or two important qualifications and additions.Let us see what these are: The well-known oscillation or swinging of the belts of winds to and fro on the meridians, which is kept up in never-ceasing response to the apparent annual motion of the sun as he crosses and recrosses the equator, must ever underlie the conception we form of the trade-winds and be perpetually present to the mind's eye. This oscillation has never yet received the popular attention it needs. The sun traverses (apparently) an arc of 23½° on either side of the line; and we might, a priori, suppose that the thermal or meteorological equator, the thermal or meteorological Tropics of Cancer and Capricorn, and all those phenomena which lie between them and beyond them, move over an arc of as many degrees as they traverse. Such an inference, however, is not borne out by observation, and we propose to confine ourselves strictly to what may be proved by observation. It is clear that the trade-wind belt does traverse or vibrate over a wider zone than any physicist has yet assigned to it, which is not more than ten degrees of latitude north and south respectively of the Tropic of Cancer and that of Capricorn. These winds, when first experienced by Spanish sailors, gave, to that portion of the
the atmospheric movements.
Atlantic over which they blew, the name el Golfo de las Damas (the Ladies' Sea) because they rendered navigation so easy that a girl might take the helm. But, "gentle" as they are, they have a wide sweep, and, in the summer of the Northern Hemisphere, extend far beyond the Tropic of Cancer. They have often been distinctly felt at Madeira and the Azores (near the 40th parallel) in summer, and it is highly reason-able to suppose that they then fully reach the latitude of 40° N. The equatorial side of the northeast trade-wind belt, of course, vibrates with the sun. In summer it stretches along between the 10th and 12th parallels of north latitude, verging in August on the 13th parallel, and, according to one writer, occasionally the northeast trades at that season do not extend south of the 15th parallel of north latitude. Dampier, "the prince of navigation," as the English call him, gives the direction of the wind in the summer months, between the equator and 12° north, as south-southeast, south-southwest, and south-west.
The equatorial side of the northeast trade-wind belt in winter approaches very nearly to the equator, and may be located in January at least as far south as the latitude of 2° north.
The freshest trade-winds in the North Atlantic are generally found between the parallels of 10° and 25°, and by long-protracted experiment in seamanship they have been found to have an average propelling power, when the wind is taken just abaft the beam, of about six knots an hour. But, of course, the northern boundary of the south-east trade-wind likewise varies and vibrates with the seasons. So, also, and under the same condition, does the southern boundary of this trade vary and vibrate with the seasons. Its normal and mean position is a little south of the parallel of 25° south, but in the winter of our hemisphere it is pushed much farther south, and in the vicinity of 35 south latitude. The charts of Captain Wilkes give easterly winds for the east coast of Australia, and also for the south coast of Africa. Sir John Herschel, speaking from knowledge gained by his long residence at the Cape of Good Hope, tells us that there "the southeasterly wind which sweeps over the Southern Ocean, infringing upon the long range of rocks which terminates in the Table Mountain, is thrown up by them, makes a clean sweep over the flat table-land which forms the summit of that mountain (about 3,850 feet high), and thence plunges down with the violence of a cataract" ("Meteorology," p. 96).
From these high southern latitudes, we must conceive the motion of the southeast trades, extending northward in summer to the neighborhood of the parallel of 10°.
From the Cape of Good Hope, in a straight line toward the projecting eastern coasts of Brazil, mariners have found a peculiar streak of southeasterly winds. Between the island of Tristan da Cunha and the Cape, and northward and westward to the island of Fernando Noronha, this streak of powerful winds, with which nothing in the trade-wind region of the North Atlantic can compare, has its atmospheric current as sharply marked as the dark blue and rapid current of the Gulf Stream in the Narrows of Bernini. It is, doubtless, the region or band of most intensely acting southeast trades, and is probably due to the peculiar configuration of the shores of the South Atlantic, and to the wall of the South American Andes. It is a well-known fact that the volcanic cone of Teneriffe, which lies in the zone of northeast trades, intercepts the wind and gives it a lateral deflection; so that, while the tirades are blowing strongly on the northeast side of the island, on the opposite side there is a distinctly-marked and carefully-measured calm shadow. Now, the chain of the Andes endeavors to exert on the southeastern trades just such an influence as is exerted by the Canary Islands on the northeast trades. This influence, in the former case, suffices to throw off from the Continent of South America a large body of the southeast trades, and to deflect it to the eastward, giving it the character of a south-southwest wind, and, at the same time, by forcing a greater or more concentrated body of air into the regions northeast of Brazil, imparting an increased velocity and violence to the air-current. It is, therefore, in the air-current that the homeward-hound vessel from the Cape of Good Hope aims to steer, because she is sure of being wafted happily and swiftly to her destination.
It has long ago been demonstrated by meteorologic observations, taken both at sea and on land, that there is very much less atmosphere in the Southern Hemisphere than in the northern, and for a long time physicists were at a loss to account for the difference. It has been, however, very satisfactorily explained by the eminent American mathematician, Ferrel, in his work on the "Motions of Fluids and Solids, relative to the Earth's Surface," where he proves at length, and states in detail (p. 39): "As there is much more land, with higher mountain-ranges, in the Northern Hemisphere than in the southern, the resistances are greater, and consequently the eastward motion of the air, upon which the deflecting force depends, is much less; and the consequence is, that the more rapid motions of the Southern Hemisphere cause a greater depression there, and a greater part of the atmosphere to be thrown into the Northern Hemisphere" It is, doubtless, to this tendency of the Southern Hemisphere to throw off much of its atmosphere north of the equator that we may attribute in part the superior force and power of the southeast trades, and their well-known ability to battle with the northeast trades, and drive them from their own territory, at least all summer, and even in winter, as far back across the line as 3° or 4° north latitude. Mr. Ferrel, speaking of the principle just enunciated, well says: "This also accounts for the mean position of the equatorial calm-belt being, in general, a little north of the equator. But, in the Pacific Ocean, where there is nearly as much water north of the equator as south (and the resistances are usually equal), its position nearly coincides with the equator." In other words, just as a bucket full of water set to revolving on a perpendicular axis would show a depression in the centre, and the fluid be thrown from all sides of its rim, the Southern Hemisphere throws its water and its atmosphere into the Northern Hemisphere, all along the equator.
It is, therefore, a mathematical and mechanical certainty that there is an invasion of the northeast trade-wind belt from the southeast trades, and observation powerfully bears out the deduction of the mathematician. Auste states, in his cautiously-written "Physical Geography:" "The southern trade-wind region is much larger than the northern in the Atlantic Ocean. In this sea, the southeast trades are fresher, and blow stronger, than the others, and often reach to the 10th or 15th parallel of north latitude; whereas, the northern trade-wind seldom gets south of the equator, and usually ranges from 9° to 29° north latitude" (p. 253). It is easy to see how easily it happens that a very small atmospheric eddy found in the tropical Atlantic by the conflictory northeast and overleaping southeast trade-winds may soon become a hurricane of wide extent and of tremendous energy. All that is necessary, as we have before seen, is that an initial impulse of gyration be given to a body of air. The moment that this takes place by mechanical influence, and, centrifugal force creates the smallest eddy or vortex, the surrounding air, already highly charged with moisture, begins the process of convergence and ascensional motion, followed rapidly by condensation aloft, small, centrical, and upright.
The storm-cylinder—the nucleus of the hurricane—originally very small, is instantly enlarged and expanded by the evolution of latent heat stored away in the vesicles of aqueous vapor. For some hours, as all observations show to be actually the case, the incipient cyclone scarcely moves, while gathering in its energies and laying tributes upon all contiguous regions. The process continues with momentarily increasing intensity, and, before the sun has made his daily circuit, the meteor is formed.
If it be asked along what parallels of latitude in our hemisphere this formation takes place, the intelligent reader will at once answer, Near the terrestrial circle of trade-wind interference. This, we have already seen, is in summer, from the 10th to the 12th parallels of north latitude.
This slender zone of debatable ground is the battle-ground of the two opposing bands of the trades. There is really no need of observations to tell us as much. But millions of observations attest the fact. Every seaman knows it. Every meteorological writer tells the same story. You have only to examine physical charts from the time of Columbus and Magellan to this, to see the absolute unanimity of testimony, and to discover that the hypothesis now advanced, and the known facts of the case, are in perfect and minute accord.
If it be asked whether the origin of the West-Indian gales is solely due to mechanical interference, the proper reply, it would clearly appear, should be in the negative. As the southeast trade-wind comes laden with the vapor of the southern or water hemisphere, which Dove well called "the boiler" of the globe, it is met by the cold northeast trade from the northern, or land hemisphere. There must be a great difference in their temperatures, and consequently extensive condensation, which, by the reasoning of Mr. Clement Ley, would, of itself, explain the formation of the storm. That condensation greatly assists in producing and intensifying it, cannot be doubted. In the high latitudes, where the polar air-current is sometimes forced by barometric pressure into the southerly or equatorial current moving over the warm waters of the ocean, and thus heavily vapor-laden, the consequence is illustrated by such terrific and sudden tempests as that of the Royal Charter, distinctly proved by Admiral Fitzroy to have been generated between the opposite polar and equatorial currents off the coast of Wales.
But that the origin of great depression systems is solely due to
weather-chart of great britain, before royal charter storm.
Full-feathered arrows show Polar current; half-feathered arrows show Equatorial current; dark-colored surface not reported by vessels or land-observers.
- See Redfield's Report.