The first of the accompanying plates, entitled,
WEST INDIAN HURRICANES, AND OTHER NORTH ATLANTIC STORMS,
gives a brief and yet complete résumé of what is perhaps the best modern practice. In these brief statements the attempt has been made to put concisely, intelligibly, and completely (if one will but read each and every sentence as carefully as they were written), the very latest, most important, and best-established facts, with which every navigator should be familiar. The paragraph entitled "Intensified trade-wind belt," for instance, is very important. A close consideration of the caution expressed in these few lines may prevent a serious mistake that might be made by a too rigid adherence to the old rules. The idea is as follows: It has been proved by Meldrum, from his studies of Mauritius hurricanes, that the SE. trade-winds blow toward a part of the track of a hurricane, rather than at right angles to the direction of its center, and it is therefore unsafe to assume that the center bears at right angles to the wind, or that, because the trade wind increases in strength without any decided change of direction, the center is approaching directly toward the vessel. This principle might naturally be expected to hold for similar storms in other regions, and Abercromby, in a thorough study of the whole subject, has shown that such is the case, although he states that "the position of this belt [of intensified trades] differs in every hurricane region, so that a special set of rules are necessary for each country." It seems to me, I must say, that in the absence of such special rules the law may safely be assumed to be general; its importance to navigators is certainly very great, and its principal effect must be to urge the greatest caution in making any attempt to cross the track of a hurricane, from the dangerous to the navigable semicircle.
WEST INDIAN HURRICANES, AND OTHER NORTH ATLANTIC STORMS.
From the Pilot Chart of the North Atlantic Ocean, August, 1889, with Additional Paragraphs.
Explanation.—These diagrams are for practical use in West Indian hurricanes. The upper one will also answer for ordinary storms along the transatlantic route. The small arrows fly with the wind, the direction being stated at the end of each dotted line; the long arrow on each diagram is the STORM TRACK, that is, the probable path of the cyclone through the belt of latitude to which the diagram applies.
| JUNE and OCTOBER, lat. 23° to 55°. JULY and SEPT., lat. 29° to 55°. AUGUST, lat. 33° to 55°. STORM TRACK, N NE. to E NE. Motion of storm center along track, 20 to 30 miles per hour. |
| JUNE and OCTOBER, lat. 20° to 23°. JULY and SEPT., lat. 27° to 29°. AUGUST, lat. 30° to 33°. STORM TRACK, N NW. to N NE. Motion of storm center along track, 5 to 10 miles per hour. |
| JUNE and OCTOBER, lat. 10° to 20°. JULY and SEPT., lat. 10° to 27°. AUGUST, lat. 10° to 30°. STORM TRACK, W. by N. to N NW. Motion of storm center along track, about 17 miles per hour. |
| [Edition of July, 1890.] |
Use of the Diagrams.—When a falling barometer, freshening rain squalls, &c., indicate a hurricane, select the proper diagram (according to the MONTH and LATITUDE), plot your position upon it by means of the direction of the wind, and thus ascertain the approximate bearing of the storm center. The probable storm track is indicated by the long arrow. If the wind shift, plot your position by means of the new wind-direction (nearer the center if the wind has freshened and the barometer has fallen). In this way you can readily observe every change of position relative to the storm center, and decide what action to take, according to the character of your vessel, the lay of the land, &c. These storms vary greatly in size, but are smallest and most violent in the tropics, where the cloud ring averages about 500 miles in diameter and the region of stormy winds 300 miles, or even less. You can therefore only roughly estimate the DISTANCE of the center, although its BEARING can be obtained from the diagrams with a high degree of probability. There is also considerable variation in the direction of motion and the velocity of the storm along its track, but the general tendency is as stated herewith.
Cyclonic Circulation.—One of the most important indications that an approaching storm is of hurricane violence is the marked cyclonic circulation of the wind, lower and upper clouds, etc. This may be easily appreciated by remembering that a cyclone of any great intensity is an ascending spiral whirl, with a rotary motion (in the Northern Hemisphere) against the hands of a watch, as shown on the diagrams. The surface wind, therefore, blows spirally inward (not circularly, except very near the center); the next upper current (carrying the low scud and rain clouds), in almost an exact circle about the center; the next higher current (the high cumulus), in an outward spiral—and so on, up to the highest cirrus clouds, which radiate directly outward. The angle of divergence between the successive currents is almost exactly two points of the compass. Ordinarily, with a surface wind from N., for instance, the low clouds come from N., also; on the edge of a hurricane, however, they come from N NE., invariably. In rear of a hurricane, the wind blows more nearly inward; with a SE. wind, for instance, the center will bear about W., the low clouds coming from S SE. (two points to the right of the wind), etc. Great activity of movement of the upper clouds, while the storm is still distant, indicates that the hurricane is of great violence. If the cirrus plumes that radiate from the distant storm are faint and opalescent in tint, fading gradually behind a slowly thickening haze or veil, the approaching storm is an old one of large area; if of snowy whiteness, projected against a clear blue sky, it is a young cyclone of small area but great intensity.