Hurricane (Span, huracari), a word of undetermined origin, signifying a violent storm of wind and rain, generally accompanied with intense displays of lightning and thunder. Although this term was originally special in its application, it is now frequently used to designate not a peculiar class of storms, but in general the strength of the most violent winds known to mariners; thus we may have storms in any part of the world whose severest winds may attain to the force either of a gale, a storm, or a hurricane, according to the circumstances that attend their development. The hurricanes of the Pacific ocean, the China sea, and the northern portions of the Indian ocean are called typhoons, and are from a scientific as well as a practical point of view to be classed in the same category with the hurricanes proper; but in what follows we shall give only such facts and theoretical views as belong specially to the hurricanes of the Atlantic and southern Indian oceans. The general subject of storms in their various aspects will be treated under that title. - To a person occupying a stationary position toward which a hurricane is approaching, it is said that the storm is frequently heralded a day beforehand by a peculiar haziness of the atmosphere, a cessation of the regular trade winds, a lassitude perhaps induced by the hy-grometric condition of the air, and an ominous stillness.
Then follow a steady slow fall of the barometer, light breezes increasing to high winds from some new quarter of the compass, generally in the West Indies between S. E. and N. E., and the obscuration of the entire heavens by a uniform sheet of cloud of increasing density. When the storm has, in the course of from 4 to 24 hours, finally arrived at its greatest severity, the fury of the wind and the confusion of the scene become indescribable; in the midst of a drenching rain and a steady wind that, fills the air with a deafening roar, there occur prolonged gusts whose violence equals or excels the force of the strongest waves; in such gusts the largest trees are uprooted, or have their trunks snapped in two, and few if any of the most massive buildings stand uninjured. In the midst of the confusion incident to the general destruction of property and life, there occurs a mysterious calm, while a break in the clouds and the diminished rainfall seem to denote the end of the storm. But in the course of from five minutes to five hours the wind bursts with additional force from a direction opposite to that which had before prevailed; whatever had escaped the destructive force of the first half of the hurricane is likely to yield to its subsequent fury, and the shipping which before perhaps had been blown out to sea, is now driven back upon the shore.
If now, instead of watching the storm from a fixed standpoint, we take a general survey of the ocean over which it rages, we shall observe that the interval of calm in the midst of the storm, as observed at the fixed station, corresponds to a central spot in a large region of violent winds and heavy rain; these winds are found to blow in spiral lines toward and around the central region of calms, increasing in force as they approach that centre. It will also be seen that the whole system of winds moves bodily over the surface of the earth. It is thus easily understood why the stations over which the centre of the hurricane passes should experience, after the central lull, a wind from the opposite quarter to that which prevailed immediately before. - In the "Philosophical Transactions" for 1698 Langford represents the hurricanes of the West Indies as whirlwinds advancing in a direction opposite to that of the trade wind. Dampier (1701) says the West Indian hurricanes and the Chinese typhoons are of the same nature. In 1801 Capper published a work on winds and monsoons, in which he advanced the opinion that the hurricanes at Pondicherry (1760) and Madras (1773) were of the nature of whirlwinds whose diameter would not exceed 120 miles.
In 1820 and 1826 Brande broached the theory that the currents of air in great storms flow from all directions toward a central point. Dove (1828), in controverting the views of Brande, explained the observed directions of the winds on the assumption of general rotary currents or whirlwinds. In 1831 Mitchell expressed the opinion that the phenomena of storms are the result of a vortex or gyratory motion. The scanty observations accessible to the authors previously mentioned were supplemented in 1831 by Mr. Redfield of New York, who then published the first of a series of remarkable papers on the phenomena of storms, in all which he maintained that hurricanes were progressive vorticose whirlwinds. His views were for a long time controverted in America by Espy and Hare. Sir William Reid published his first papers on hurricanes in 1838, and subsequently other works, in which he developed views similar to those of Mr. Redfield. Of the authors previously mentioned, some laid a special stress on the tangential, and others on the centripetal movements of the winds; at present, however, following the studies of Redfield (1839-'56), Espy (1840-'57), Thorn (1845), Piddington (1839-'54), Reid (1838-'50), Ferrel (1858), Mel-drum (1851-'73), Mohn (1870), Reye (1872), and many others, it is generally acknowledged that the combination of both these movements with an upward one is an essential feature of every hurricane, so that the movement of the surface wind is more correctly described as an ascending spiral.
Concerning the direction of this movement, Dove, and independently of him Redfield, concluded that in the storms of Europe and the American coast the winds move in a circuit about the storm centre, contrary to the direction of the motion of the hands of a watch when the latter is laid on the ground with its face upward. Furthermore, Dove made the important remark that in the hurricanes of the southern hemisphere the air revolves in an opposite direction; this generalization, announced by him, apparently with some limitations, was by the labors of Reid (1838) converted into an accepted law. The law of the rotation of winds around the storm centre is considered to be of the highest importance in its practical bearings on the interests of navigation, and may be stated in other words as follows: If in the northern (or southern) hemisphere you stand with the centre of the hurricane on your left (or right) hand, the wind will be on your back. The determining cause of this law of rotation, and of the distinction between the hurricanes of the northern and southern hemispheres, was imperfectly understood by early writers, as Taylor and Herschel, but was rigidly demonstrated in a remarkable mathematical memoir by Ferrel in 1858, who showed that the rotation of the earth on its axis affects the direction not merely of north and south winds, but of every wind, in such a manner that in the northern hemisphere winds tend as they move forward to deflect to the right hand, but in the southern hemisphere to the left hand.
This tendency, which is known either as Poisson's or as Ferrel's law, is in large storms sufficient to determine the direction of rotation, while in storms of comparatively small dimensions accidental circumstances may conspire to annul or even reverse the direction of rotation. Thus we are provided with the means of harmonizing, at least in great part, the views of Hare, Espy, and others, with those of Redfield and Reid. - There are unfortunately but few actual measurements of the velocity of the stronger winds that occur within the limits of a hurricane. In general it appears that the velocity increases as we proceed from the outer limits toward the centre of the storm, but suddenly diminishes to feeble irregular winds and calms within the central space. From the observed destructive force of some gusts it has also been contended that a velocity of 10 m. per minute must have been momentarily attained, but such computations are not very satisfactory. The highest hurricane winds that have ever been actually observed have on the British coast attained a velocity of 130 m. per hour; in the comparatively small hurricane of August, 1871, the observers in Florida of the United States army signal corps recorded a velocity of 85 m. per hour; all these winds of course were interspersed with gusts of great violence.
The diameter of the region of calms varies from 30 m. to a much smaller size, and probably even to nothing. It would seem that in some hurricanes, as frequently in the smaller tornadoes on land, the so-called axis of the storm rises temporarily above the surface of the earth. The central space in general, according to Redfield, increases in diameter as the storm moves away from the equator northward or southward. - A heavy rainfall extending far beyond the region of most violent winds attends all hurricanes. The quantity of water that falls during the prevalence of these storms forms a large percentage of the total annual rainfall over the hurricane regions, and in this respect they perform an important service to mankind. At Mauritius in the Indian ocean a single storm has been known to be attended by a rainfall of more than 10 inches. The area of cloud and rain is especially extended on the N. and E. quadrant of the storms of the North Atlantic; it is sometimes much contracted, though rarely wanting, on the west side of the hurricanes of both the northern and southern hemispheres.
The movements of the clouds have been carefully observed, especially by Redfield (1832-'42) and Ley (1866-'70), and the result is well expressed by Reye (1872): "While on the earth's surface the storm wind in spiral curves gradually flows inward, it forces the flying storm clouds in spiral curves outward, and removes them away from the axis of the cyclone." This generalization was fully explained from a theoretical mechanical point of view by Ferrel, and was shown by him to be a consequence of the rising or upward movement of the masses of air that are drawn into the whirlwind. The clouds then must move in spirals opposed to the movements of the lower winds. Redfield estimates the angle between the winds below and the clouds above to be about 22.5°. - The barometric disturbance is one of the most remarkable features of a hurricane. The nearer one approaches the centre, the lower is the barometric pressure, and at the centre the depression is frequently two or three inches. The first notice of an approaching hurricane, when it is yet 100 to 400 m. distant, is usually given by the steady fall of the barometer; as we approach the centre the fall is more rapid.
The law by which the pressure diminishes, as well as the variations from it, may be illustrated by two examples, the first showing a very regular depression, the second giving a great and rapidly increasing rate of fall. The first example is Redfield's Cuba hurricane of Oct. 4-7, 1844, for which we have the following pressures: at the centre, 27.7 in.; at 100 m. distance, 28.0 in.; at 200 m. 29.0 in.; at 300 m., 29.5 in.; at 400 m., 29.8in. The second example is from Buchan (1871), and relates to the Bahama hurricane of October, 1866. On the evening of the 1st of October we have the following pressures: at the centre, 27.7 in.; at 15 m. distance, or the radius of the central column, 27.8in.; at 300 m., 29.7in.; at 500 m., 29.8in.; and at 800 m., 30.0 in. The ratio at which at a fixed station the barometer falls on the approach of a hurricane differs from the preceding by reason of the progressive motion of the storm toward or from the station; on .board a vessel, the barometric fall is further complicated by the movement of the observer. The best idea of the barometric disturbance is given by a chart of synchronous observations on which isobaro-metric lines are drawn.
These isobars will be found to be crowded together on one side (generally the advancing half) of the storm more than on the other, and to enclose a small oval or circular region of lowest pressure, almost if not quite identical with that of the area of calms, though sometimes apparently in advance of it. In a general way it may be stated that the velocity of the wind increases with the crowding of the isobarometric lines. The exact relation between the two is quite complicated, and may be deduced from the formulas of the above mentioned treatise by Ferrel, combined with the considerations introduced by Peslin in 1867 and Reye in 1872. It is evident that the law above given for the rotation of the wind may be converted into a rule for finding the centre of calms, which will also hold good for finding the centre of lowest barometer; this latter is generally spoken of as the storm centre or axis. Buys-Ballot has expressed this generalization in the form known as Buys-Ballot's rule, viz.: in the northern hemisphere stand with your back to the wind, and the lowest pressure will be on your left hand and somewhat in front thereof; a rule that applies especially to, and was apparently suggested by, the behavior of the winds of hurricanes and similar storms. - The dimensions of hurricanes generally increase from day to day until the dissipation of the entire storm, while the intensity of the winds is believed on the average to diminish somewhat; this will however depend upon the atmospheric conditions favoring the development or the decadence of the disturbance.
Given a proper supply of warm moist air, and it can be shown that the central depression with the attendant wind and rain must steadily increase up to a certain limit. These favorable circumstances are generally found combined in a remarkable degree in the region of the Gulf stream, the Kuro Siwo, and similar ocean currents; accordingly, on reaching these the area of cloud and rain expands, as also do the diameters of the isobaric curves. The dimensions of the central depressions vary quite irregularly, but appear on the average to increase as the storm continues; while the actual height of the barometer at the centre changes much less, but is believed to diminish gradually so long as the intensity of the wind increases. If a curve, enclosing a region in which the winds attain the force ordinarily described as a moderate gale, be assumed as the limit of the storm, it will be found that in the earliest stages of the hurricane it has a diameter of from 50 to 200 m., which increases in the course of 5 or 10 days to from 400 to 1,200 m.; thus a disturbance that may have been originally designated as small or local, increases so as to involve half the surface of the North Atlantic ocean. - The track of the centre of the hurricane is a fair indication of the progress of the storm over the earth, and much labor has been bestowed upon such collations of logs of vessels as would elucidate this important branch of the subject.
But notwithstanding the labor expended, there have as yet been very few hurricanes traced back to what appears to be very near their origin, and in not a single instance has unmistakable evidence of their origin been adduced. The general position of hurricane tracks in the earlier parts of their course therefore remains obscure, although the immense accumulation of material by the labors of the various national government weather bureaus is rapidly dissolving our ignorance on this point. So far as the known hurricane tracks are concerned, it may be stated that in the North Atlantic ocean each uniformly appears to be a segment of a parabola having its axis coincident with the parallels of 25° to 35° N. latitude, and the longitudes of whose apices fall between the meridians 40° and 100° west of Greenwich, but mostly between 65° and 85°. At the southern extremity of the parabolic track, the branch passes either to the north of or over the Windward islands, while the northern branch passes to the south of or over Newfoundland. In a few cases the first portion of the track has been traced southeastward nearly to the coast of Senegambia, and the latter portion of the track northeastward to the ocean between Iceland and Scotland; some tracks that curve northeastward before reaching Ion. 40° may even strike England or France. The hurricanes of the southern hemisphere describe similar parabolic tracks, which lie at a corresponding distance south of the equatorial belt of calms, and are symmetrically disposed with reference thereto.
Very few have been traced in the South Atlantic ocean, but in the southern Indian ocean the majority of the hurricanes pass from Sumatra and Java southwestward to within 500 m. of Madagascar, then southward and southeastward. In general, Mohn (1870) and Reye (1872) state that all cyclones (of which hurricanes are the grandest examples) move in the direction in which for the longest time the warmest and moistest air has been rising, and producing the heaviest cloud and rainfall. If we combine with this law the tendency of the whirlwind as a whole to move away from the equator, as proved by Ferrel, it seems to the writer that we have a very close approximation to the full statement of the reason for the parabolic form of their orbits. - The rate of progression of the West Indian storm centres varies from 50 m. per hour in a few cases to 10 or 15 as the other extreme; that of the storms of the southern Indian ocean varies from 1 to 20 m. The rate in general in the North Atlantic increases with the growth and northward movement of the hurricane, and, though sometimes quite variable, is not so much so as in the case of the similar storms of the Indian ocean.
The rate of progress must be carefully distinguished from the velocity of the wind, as the latter has no known relation to and far exceeds the former. - The waves and swells produced by the hurricane winds are a most important feature; these waves are the largest and most formidable known to the mariner. They form with greatest regularity at points directly in advance of the approaching storm centre; at other points they form a confused mass of crossed sea; in the neighborhood of the land the confusion is increased by the waves reflected from the shores. Such is the equality of the contest of opposing waves, that near the central region these sometimes lose their progressive movement and become stationary pyramidal waves, simply rising and falling. The smaller waves that are propagated in all directions from the region of severest winds, degenerate into long gentle swells that outrun the storm in its progress, and announce its presence several hours or a day in advance of its arrival. Besides these waves, it is believed that the extended region of low barometer allows the formation of a peculiar "cyclone wave," which is similar to the tidal wave of mid-ocean. The cyclone wave is coextensive with the area of low barometer; it is highest at the central lowest pressure, where its elevation above the ordinary sea level should be a foot or more for each inch of barometric depression. - From the earliest times the months from July to October have been known in the West Indies as the "hurricane season." A table published by Poey in 1855 gives the distribution by months of 355 hurricanes recorded on the Atlantic between 1493 and 1855. According to this work, there are recorded in this period in all in January 5, February 7, March 11, April 6, May 5, June 10, July 42, August 96, September 80, October 69, November 17, December 7; but the annual period is probably not very correctly shown by this list, because of the imperfections of the earlier records.
More recently Poey has revised his list and added many later hurricanes, and has published in the Paris Comptes Rendus for Nov. 24, 1873, and Jan. 5, 1874, the results of a comparison between hurricanes and the frequency of solar spots. His results seem to remarkably confirm those of Meldrum, who had previously studied the hurricanes of the Indian ocean from the same point of view. Poey states that in the majority of cases the years of the greatest number of hurricanes are also the years of the greatest sun-spot frequency. The extensive researches of Koppen (1873) have shown that the amount of heat received from the sun varies annually with the sun spots, whence we infer that the variations in solar heat produce a similar variation in the terrestrial evaporation, and an increased tendency to the formation of hurricanes. The actual number of hurricanes visiting any limited region is of course very small. Since the year 1700 the centres of about 25 have been known to pass quite near the coast of Georgia and South Carolina, which is by far the most frequently visited portion of the United States. Nearly all those of the Indian ocean pass near to the islands of Mauritius, Rodriguez, etc. - Concerning the origin and cause of the hurricanes of the Atlantic ocean comparatively little is positively known, but it seems by analogy that they may originate wherever the lower stratum of warm moist air is rapidly elevated above the sea level, whether (1) by being pushed up over an elevated plateau or mountain chain, or (2) by the under-running of a layer of cold dry air, or (3) by the conflict of two opposed and nearly balanced currents of warm moist air.
In numerous instances one or the other of these cases seems to have occurred; and as these, combined with (4) the radiation of heat into space, are the prevailing causes that determine the origin and growth of storms in general, there seems no reason in the case of hurricanes to appeal to more forced theories. The immense mechanical power stored up in the heat and vapor of moist air has been abundantly demonstrated by Espy, Peslin, and Reye. Whenever, by the action of either of the four causes just mentioned, the process of condensation of vapor into cloud, rain, or snow begins, there at once occurs an influx of air from all sides, and from below as well as from above, to fill up the partial vacuum thus created; this influx toward a central region is immediately followed, as shown by Ferrel, by the formation of a whirl whose subsequent development is entirely dependent on the supply of moist air. The hurricanes of the southern Indian ocean are thus generated in the region of calms between the N. W. monsoons and the S. E. trade winds of that ocean. Similarly hurricanes have been known to originate in the neighborhood of Florida when a cold north wind has swept under the warm moist air of the gulf and ocean.
Another class originates in a similar manner in the western portion of the gulf of Mexico after a Texas norther has prevailed for a few days. A few begin in the interior of Texas when a high barometric pressure on the gulf, or a low pressure in the western territories, forces or draws the air of the gulf up over the plains of Texas. But by far the larger class of the Atlantic hurricanes, including those of greatest extent and violence, appear to originate between the Windward islands and the African coast, and generally quite near to the latter; apparently these begin with heavy rains in the region of calms, such as are accompanied on the African mainland by the peculiar harmattan and tornadoes of that coast, which may be, so far as we know, either the consequence or the determining cause of the heavy rains. The storms that originate here may either move as far west as the American coast before recurving toward Iceland and Norway, or may describe a much shorter route, and finally arrive at Great Britain, or possibly at Portugal. - Rules for the Avoidance of Hurricanes at Sea. The researches of Redfield first led to the suggestion of certain rules for the direction of navigators.
The erroneous theories of the purely circular and of the radial movement of the hurricane winds early led their respective advocates to the suggestion of rules for avoiding the dangers of these storms, which later and more correct views as to the spiral or vorticose movement have somewhat modified. It may in general be said that a vessel's safety can only be assured by the possession of a reliable barometer, either aneroid or mercurial; and having this, the navigator should proceed thus: First, as soon as the ocean swell, the falling barometer, the clouds, and the rain announce that a hurricane exists, though it may be 500 m. from him, he should at once lay to long enough to ascertain how rapidly the barometer is falling and the wind increasing, and in which direction the course of the wind is changing. If the wind increases without materially changing its direction, the storm centre is advancing directly toward him; if, however, the wind veers or backs, the direction in which the centre is at any moment may be approximately determined by the rule above given, viz.: "in the northern or southern hemisphere, stand with your back to the wind, and the centre will be on your left or right hand, and in front." The mariner may then by due consideration of his own desired course, and the customary track of hurricanes in that part of the ocean, so alter his course as to avoid the storm centre on the one hand and a lee shore on the other, and may indeed, if there be plenty of sea room, take advantage of the strong wind to hasten his own course.
Further details on this subject are given in all works on navigation. It is very rare that a navigator cannot by cautious manoeuvring thus avoid the dangerous portions of a hurricane; on the other hand, it is said that many ocean steamers, relying upon the power of their engines, the strength of their build, and their great speed, deliberately plough through the heart of the severest storms rather than incur a possible delay of a few hours in order to avoid them. The hurricane of August, 1873, which destroyed over 1,000 vessels on our Atlantic coast, and those of October, 1873, and February, 1874, afforded numerous instances of such bravado.