The Hurricane Guide / Being an Attempt to Connect the Rotary Gale or Revolving Storm with Atmospheric Waves.

Should a gale be observed commencing without its having been preceded by an unusual elevation of the mercurial column, and consequently no additional observation have been made; when the force of the wind is noted in the usual observations at or above 5, then the three-hourly series should be resorted to, and the same care taken in noting the direction, changes, and force of the wind as pointed out in the preceding paragraph.

The foregoing remarks relate especially to the central and western portions of the North Atlantic; they will however equally apply to the remaining localities of storms. Under any circumstances, and in any locality, a high barometer not less than a low one should demand particular attention, and if possible, hourly readings taken some time before and after the passage of the maximum: this will be referred to more particularly under the next head.

Preceding and Succeeding Accumulations of Pressure.—Mr. Redfield has shown in his Memoir of the Cuba Hurricane of October, 1844, that two associated storms were immediately preceded by a barometric wave, or accumulation of pressure, the barometer rising above the usual or annual mean. We have just referred to the importance of hourly observations on occasions of the readings being high as capable of illustrating the marginal phænomena of storms, and in connexion with these accumulations of pressure in advance of storms we would reiterate the suggestion. These strips of accumulated pressure are doubtless crests of atmospheric waves rolling forwards. In some cases a ship in its progress may cut them transversely in a direction at right angles to their length, in others very obliquely; but in all cases, whatever section may be given by the curve representing the observations, too much attention cannot be bestowed on the barometer, the wet and dry bulb thermometer, the direction and force of the wind, the state of the sky, and the appearance of the ocean during the ship's passage through such an accumulation of pressure. When the barometer attains its mean altitude, and is rapidly rising above it in any locality, then hourly observations of the instruments and phænomena above noticed should be commenced and continued until after the mercury had attained its highest point and had sunk again to its mean state. In such observations particular attention should be paid to the direction and force of the wind preceding the barometric maximum—and the same phænomena succeeding it, and particular notice should be taken of the time when, and amount of any change either in the direction or force of the wind. It is by such observations as these, carried on with great care and made at every accessible portion of the oceanic surface, that we may be able to ascertain the continuity of these atmospheric waves, to determine somewhat respecting their length, to show the character of their connexion with the rotatory storm, and to deduce the direction and rate of their progress.

V.—SEASONS FOR EXTRA OBSERVATIONS.

In reference to certain desiderata that have presented themselves in the course of my researches on this subject (see Report of the British Association for the Advancement of Science, 1846, p. 163), the phases of the larger barometric undulations, and the types of the various seasons of the year, demand particular attention and call for extra observations at certain seasons: of these, three only have yet been ascertained—the type for the middle of November—the annual depression on or about the 28th of November—and the annual elevation on or about the 25th of December. The enunciation of the first is as under: "That during fourteen days in November, more or less equally disposed about the middle of the month, the oscillations of the barometer exhibit a remarkably symmetrical character, that is to say, the fall succeeding the transit of the maximum or the highest reading is to a great extent similar to the preceding rise. This rise and fall is not continuous or unbroken; in some cases it consists of five, in others of three distinct elevations. The complete rise and fall has been termed the great symmetrical barometric wave of November. At its setting in the barometer is generally low, sometimes below twenty-nine inches. This depression is generally succeeded by two well-marked undulations, varying from one to two days in duration. The central undulation, which also forms the apex of the great wave, is of larger extent, occupying from three to five days; when this has passed, two smaller undulations corresponding to those at the commencement of the wave make their appearance, and at the close of the last the wave terminates." With but slight exceptions, the observations of eight successive years have confirmed the general correctness of this type. On two occasions the central apex has not been the highest, and these deviations, with some of a minor character, form the exceptions alluded to. This type only has reference to London and the south-eastern parts of England; proceeding westward, north-westward, and northward, the symmetrical character of this type is considerably departed from; each locality possessing its own type of the barometric movements during November. The desiderata in immediate connexion with the November movements, as observed in the southern and south-eastern parts of England, that present themselves, are—the determination of the types for November, especially its middle portion, as exhibited on the oceanic surface within an area comprised between the 30th and 60th parallels, and the 1st and 40th meridians west. Vessels sailing within this area may contribute greatly to the determination of these types by making observations at intervals of three hours from the 1st of November to the 7th or 8th of December. The entire period of the great symmetrical wave of November will most probably be embraced by such a series of observations, as well as the annual depression of the 28th. For the elevation of the 25th of December the three-hourly observations should be commenced on the 21st, and continued until the 3rd or 4th of the succeeding January.

With respect to the great wave of November, our knowledge of it would be much increased by such a series of observations as mentioned above, being made on board surveying and other vessels employed off Scotland and Ireland; vessels navigating the North Sea; vessels stationed off the coasts of France, Spain, Portugal, and the northern parts of Africa, and at all our stations in the Mediterranean. In this way the area of examination would be greatly enlarged, and the differences of the curves more fully elucidated; and this extended area of observation is the more desirable, as there is some reason to believe that the line of greatest symmetry revolves around a fixed point, most probably the nodal point of the great European systems.

It is highly probable that movements of a somewhat similar character, although presenting very different curves, exist in the southern hemisphere. The November wave is more or less associated with storms. It has been generally preceded by a high barometer and succeeded by a low one, and this low state of the barometer has been accompanied by stormy weather. We are therefore prepared to seek for similar phænomena in the southern hemisphere, in those localities which present similar states of weather, and at seasons when such weather predominates. We have already marked out the two capes in the Southern hemisphere for three-hourly observations: they must doubtless possess very peculiar barometric characters, stretching as they do into the vast area of the Southern Ocean. It is highly probable that the oscillations, especially at some seasons, are very considerable, and vessels visiting them at such seasons would do well to record with especial care the indications of the instruments already alluded to. At present we know but little of the barometric movements in the Southern hemisphere, and every addition to our knowledge in this respect will open the way to more important conclusions.

It has been observed in the south-east of England that the barometer has generally passed a maximum on or about the 3rd of every month, and this has been so frequently the case as to form the rule rather than the exception. The same fact during a more limited period has been observed at Toronto. With especial reference to this subject the three-hourly series of observations may be resorted to in all localities, but especially north of the 40th parallel in the northern hemisphere. They should be commenced at midnight immediately preceding the 1st and continued to midnight succeeding the 5th.

CHAPTER IV.

Practical Directions for Avoiding the Centres of Rotating Storms.

Figures 1 and 2, enlarged and printed on narrow rings of stiff cardboard, are employed for this purpose. The letters outside the thick circle are intended to distinguish the points of the compass, and in use should always coincide with those points on the chart. The letters within the thick circle indicate the direction of the wind in a hurricane, the whirl being shown by the arrows between the letters. In the northern hemisphere the direction of the whirl is always contrary to that in which the hands of a watch move, and in the southern coincident thereto. The graduation is intended to assist the mariner in ascertaining the bearing of the centre of a storm from his ship.

Use.

At any time when a severe gale or hurricane is expected, the seaman should at once find the position of his ship on the chart, and place upon it the graduated point which answers to the direction of the wind at the time, taking care that the needle is directed to the north, so that the exterior letters may point on the chart to the respective points of the compass: this is very essential. This simple process will at once acquaint the seaman with two important facts relative to the coming hurricane—his position in the storm, and the direction in which it is moving.

Examples.

A captain of a ship in latitude 35° 24' N., longitude 64° 12' W., bound to the United States, observes the barometer to stand unusually high, say 30·55 inches: shortly after the mercury begins to fall, at first slowly and steadily; as the glass falls the wind freshens, and is noticed to blow with increasing force from the S. so as to threaten a gale. The position of the ship on the chart is now to be found, and the graduated point under the letters E. S. is to be placed thereon, taking care to direct the needle to the north. From these two circumstances, the falling barometer and the wind blowing from the south with increasing force, the mariner is aware of this simple fact, that he is situated in the advancing portion of a body of air which is proceeding towards the N.E.; and if he turn his face to the N.E. he will find he is on the right of the axis line, or line cutting the advancing body transversely. The hurricane circle as it lies on the chart reveals to him another important fact, which is, that if he pursue his course he will sail towards the axis line of the hurricane, and may stand a chance of foundering in its centre. To avoid this he has one of two courses to adopt; either to lay-to on the starboard tack, according to Col. Reid's rules (see his 'Law of Storms,' 1st edit., pp. 425 to 428), the ship being in the right-hand semicircle of the hurricane, or so to alter his course as to keep without the influence of the storm. In the present case the adoption of the latter alternative would involve a reversal of his former course; nevertheless it is clear the more he bears to the S.E. the less he will experience the violence of the hurricane: should he heave his ship to, upon moving the hurricane circle from the ship's place on the chart towards the N.E., he will be able to judge of the changes of the wind he is likely to experience: thus it will first veer to S.S.W., the barometer still falling; then to S.W., the barometer at a minimum—this marks the position of the most violent portion of the storm he may be in, and by keeping the barometer as high as he can by bearing towards the S.E., the farther he will be from the centre—the barometer now begins to rise, the wind veering to W.S.W., and the hurricane finally passes off with the wind at W. It is to be particularly remarked that in this example the ship is in the most dangerous quadrant, as by scudding she would be driven in advance of the track of the storm's centre, which of course would be approaching her.

Assuming that the hurricane sets in at the ship's place with the wind at S.E., the proceeding will be altogether different. At first the wind is fair for the prosecution of the voyage, and it is desirable to take advantage of this fair wind to avoid as much as possible the track of the centre, which passes over the ship's place in this instance, and is always the most dangerous part of the storm. As the ship is able to make good distance from this track by bearing towards the N.W., provided she has plenty of sea-room, she will experience less of the violence of the hurricane; but as most of the Atlantic storms sweep over the shore, it will be desirable to lay-to at some point on the larboard tack, the ship being now in the left-hand semicircle. By moving the circle as before directed it will be seen that the veering of the wind is now E.S.E., E., E.N.E., N.E., the lowest barometer N.N.E., N., and N.N.W., the ship experiencing more or less of these changes as it is nearer to or farther from the axis line.

In latitudes lower than 20° N. the Atlantic hurricanes usually move towards the N.W. Taking the same positions of our ship with regard to the storms as in the two former examples, if the storm set in with the wind E. the proper proceeding is to bear away for the N.E., the most dangerous quadrant of the hurricane having overtaken the ship, the veering of the wind if she is lying-to will be E., E.S.E., S.E., with the lowest barometer S.S.E. and S. Should the storm set in at N.E., her position at the time will be some indication of the distance of the centre's track from the nearest land, and will greatly assist in determining the point at which the captain ought to lay-to after taking advantage of the N.E. wind, should he be able so to do, to bear away from the centre line, so as to avoid as much as possible the violence of the storm. From the proximity of the West Indian Islands to this locality of the storm-paths, the danger is proportionally increased.

The above examples have reference only to the lower and upper branches of the storm paths of the Northern Atlantic in the neighbourhood of the West Indies and the United States. In latitudes from about 25° to 32° these paths usually re-curve, and at some point will move towards the north. The veering of the wind will consequently be more or less complicated according as the ship may be nearer to or farther from the centre. The tables on page 11, combined with the first of those immediately following the next paragraph, will, it is hoped, prove advantageous in assisting the mariner as to the course to be adopted. As a general principle we should say it would be best to bear to the eastward, so as not only to avoid the greater fury of the storm, but to get into the S. and S.W. winds, which give the principal chances of making a westerly course.

We have in page 44 called attention to the fact that the storm paths traced by Mr. Redfield do not extend eastward of the 50th meridian. This by no means precludes the existence of severe storms and those of a rotatory character in the great basin of the Northern Atlantic, especially between the 40th and 50th parallels. A remarkable instance has come under the author's attention of the wind hauling apparently contrary to the usual theory: it may be that the storm route was in a direction not generally observed. We are at the present moment destitute of any information that at all indicates a reversion of the rotation in either hemisphere. The following tables constructed for the northern hemisphere, and for storm routes not yet ascertained, may probably be consulted with advantage on anomalous occasions.

HURRICANE MOVING FROM SOUTH TO NORTH.

Axis line, wind E., barometer falling, first half of storm.
Axis line, wind W., barometer rising, last half of storm.

RIGHT-HAND SEMICIRCLE.

Wind E.S.E., S.E., S.S.E., S., barometer falling, first half of storm.
Wind W.S.W., S.W., S.S.W., S., barometer rising, last half of storm.

LEFT-HAND SEMICIRCLE.

Wind E.N.E., N.E., N.N.E., N., barometer falling, first half of storm.
Wind W.N.W., N.W., N.N.W., N., barometer rising, last half of storm.

HURRICANE MOVING FROM NORTH TO SOUTH.

Axis line, wind W., barometer falling, first half of storm.
Axis line, wind E., barometer rising, last half of storm.

RIGHT-HAND SEMICIRCLE.

Wind W.N.W., N.W., N.N.W., N., barometer falling, first half of storm.
Wind E.N.E., N.E., N.N.E., N., barometer rising, last half of storm.

LEFT-HAND SEMICIRCLE.

Wind W.S.W., S.W., S.S.W., S., barometer falling, first half of storm.
Wind E.S.E., S.E., S.S.E., S,, barometer rising, last half of storm.

HURRICANE MOVING PROM WEST TO EAST.

Axis line, wind S., barometer falling, first half of storm.
Axis line, wind N., barometer rising, last half of storm.

RIGHT-HAND SEMICIRCLE.

Wind S.S.W., S.W., W.S.W., W., barometer falling, first half of storm.
Wind N.N.W., N.W., W.N.W., W., barometer rising, last half of storm.

LEFT-HAND SEMICIRCLE.

Wind S.S.E., S.E., E.S.E., E., barometer falling, first half of storm.
Wind N.N.E., N.E., E.N.E., E., barometer rising, last half of storm.

HURRICANE MOVING FROM NORTH-WEST TO SOUTH-EAST.

Axis line, wind S.W., barometer falling, first half of storm.
Axis line, wind N.E., barometer rising, last half of storm.

RIGHT-HAND SEMICIRCLE.

Wind W.S.W., W., W.N.W., N.W., barometer falling, first half of storm.
Wind N.N.E., N., N.N.W., N.W., barometer rising, last half of storm.

LEFT-HAND SEMICIRCLE.

Wind S.S.W., S., S.S.E., S.E., barometer falling, first half of storm.
Wind E.N.E., E., E.S.E., S.E., barometer rising, last half of storm.

APPENDIX.

TABLE I.—Correction to be added to Barometers for Capillary Action.

TABLE II.—Correction to be applied to Barometers with Brass Scales, extending from the Cistern to the top of the Mercurial Column, to reduce the observation to 32° Fahrenheit.