IV.—STORMS, HURRICANES, AND TYPHOONS.
The solution of the question—How far and in what manner are storms connected with atmospheric waves?—must be extremely interesting to every one engaged in either the naval or merchant service. As we have in the former chapters directed attention to their connexion, our great object here will be to endeavour to mark out such a line of observation as appears most capable of throwing light, not only on the most important desiderata as connected with storms, but also their connexion or non-connexion with atmospheric waves. We shall accordingly arrange this portion of the instructions under the following heads:—Desiderata; Localities; Margins; Preceding and Succeeding Accumulations of Pressure.
Desiderata.—The most important desiderata appertaining to the subject of storms, are certainly their origin and termination. Of these initial and terminal points in the course of great storms we absolutely know nothing, unless the white appearance of a round form observed by Mr. Seymour on board the Judith and Esther, in lat. 17° 19' north and long. 52° 10' west (see Col. Reid's 'Law of Storms,' 1st edit. p. 65), may be regarded as the commencement of the Antigua hurricane of August 2, 1837. This vessel was the most eastern of those from which observations had been obtained; and it is the absence of contemporaneous observations to the eastward of the 50th meridian that leaves the question as to the origin of the West Indian revolving storms unsolved. Not one of Mr. Redfield's storm routes extends eastward of the 50th meridian; this at once marks out, so far as storms are concerned, the entire space included between the 20th and 50th meridians, the equator and the 60th parallel, as a most suitable area for observations, under particular circumstances hereafter to be noticed, with especial reference either to the commencement or termination of storms, or the prolongation of Mr. Redfield's storm paths.
Localities.—The three principal localities of storms are as follows:—I. The western portion of the basin of the North Atlantic; II. The China Sea and Bay of Bengal; and III. The Indian Ocean, more particularly in the neighbourhood of Mauritius. The first two have already been marked out as areas for the three-hourly observations; to the latter, the remark as to extra observations under the head of Desiderata will apply.
Margins.—Mr. Redfield has shown that on some occasions storms have been preceded by an unusual pressure of the atmosphere; the barometer has stood remarkably high, and it has hence been inferred that there has existed around the gale an accumulation of air forming a margin; barometers placed under this margin indicating a much greater pressure than the mean of the respective localities. With regard to the West Indian and American hurricanes—any considerable increase of pressure, especially within the space marked out to the eastward of the 50th meridian, will demand immediate attention. Upon the barometer ranging very high within this space, three-hourly observations should be immediately resorted to; and if possible, hourly readings taken, and this is the more important the nearer the vessel may be to the 50th meridian. Each observation of the barometer should be accompanied by an observation of the wind—its direction should be most carefully noted, and the force estimated according to the scale in page 21, or by the anemometer. It would be as well at the time to project the barometric readings in a curve even of a rough character, that the extent of fall after the mercury had passed its maximum might be readily discernible by the eye. A paper ruled in squares, the vertical lines representing the commencement of hours, and the horizontal tenths of an inch, would be quite sufficient for this purpose. The force of the wind should be noted at, or as near to the time of the passage of the maximum as possible. During the fall of the mercury particular attention should be paid to the manner in which the wind changes, should any change be observed; and should the wind continue blowing steadily in one direction, but gradually increasing in force, then such increments of force should be most carefully noted. During the fall of the barometer, should the changes of the wind and its increasing force indicate the neighbourhood of a revolving storm, (independent of the obvious reasons for avoiding the focus of the storm,) it would contribute as much to increase our knowledge of these dangerous vortices to keep as near as possible to their margins as to approach their centres. The recess from the centre towards the margin of the storm, will probably be rendered apparent by the rising of the mercury; and so far as the observations may be considered valuable for elucidating the connexion of atmospheric waves with rotatory storms (other motives being balanced), it might be desirable to keep the ship near the margin—provided she is not carried beyond the influence of the winds which characterize the latter half of the storm—until the barometer has nearly attained its usual elevation. By this means some notion might be formed of the general direction of the line of barometric pressure preceding or succeeding a storm.
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.