« AnteriorContinuar »
LAW OF STORMS.
BY. W. R. BIRT. Author of the "Article on Atmospheric Waves." in the Admiralty Manual of Scientific
Enquiry; " The Hurricane," and "Sailor's Guides," Etc., Etc.
The object of the following remarks on Revolving Storms, is to exhibit the importance of gaining such a knowledge of the “ Law of Storms,” that the commander of a vessel may know instinctively what part of a Cyclone he may be in; for
1 this, nothing more is requisite, than that he possess a competent knowledge of the bearing of the centre from the ship, as determined by the direction of the wind; and the result of the hauling of the wind with or against the sụn, as indicating on which side of the axis line he may be placed, the axis line coinciding with the path of the centre; with this knowledge all instruments may be dispensed with, except the barometer.
1. Within the last 30 years the assiduity of meteorologists has developed a most important and highly interesting department of meteorology. This department has immediate reference to, and must exert a most beneficial influence on the Commercial and Maritime interests of the Country. It is now popularly known as the Law of Storms, and on no class of men can the study of it tell with more effect than on the mercantile marine; not that Her Majesty's Navy stands less in need of the important knowledge contributed by an investigation of storms, but the education of its officers fits them more readily to appreciate and apply such knowledge when overtaken by a hurricane or cyclone.
2. The primary idea or fundamental notion of a cyclone is, that of a vast body of air in a state of rotation, more or less rapid. This rotation appears to be immediately connected with the rotation of the earth, or rather with the apparent course of the
sun in the heavens, arising from the earth’s rotation on its axis. The rotation of the air around the azis of the cyclone producing the hurricane wind, is always contrary to, or against the apparent course of the sun, and
apparent course of the sun is reversed in the opposite hemispheres, so the rotation of the air in the cyclone is in opposite directions on either side of the equator. A very simple rule is deducible from these beautiful facts. In the northern hemisphere the cyclone rotates in a direction contrary to that in which the hands of a clock move, but in the southern hemisphere the rotation coincides with the movement of the hands.
3. This whirling of the air in a cyclone, enables us to characterize certain portions of the storm by certain hurricane winds ; thus, in the northern hemisphere the northern margin of the storm always exhibits an easterly wind, the castern margin a southerly wind, the southern margin a westerly wind and the western margin a northerly wind; we shall also further find upon dividing the storm into quadrants by diameters drawn from the northern to the southern, and from the eastern to the western margins, that upon the northern semidiameter, or radius, the wind will be east; on the eastern, south; on the southern, west; and on the western, north; each portion of the cyclone will possess its appropriate wind.
4. The relation of the winds to the margins and semidiameters in the southern hemisphere, will be exactly the revérse of their relations in the northern ; thus it is the southern semi-diameter and margin of a storm, south of the equator, that exhibits an easterly wind, the western a southerly, the northern a westerly, and the eastern a northerly.
5. This arrangement of the winds in a hurricane will conduct us to a very simple rule for determining the position of a vessel in a cyclone, and as a consequence the bearing of the centre of the storm from the ship. From an easterly wind in the northern hemisphere, the centre will bear south, or eight points, reckoned in the same direction as the apparent course of the sun, an easterly wind characterising the northern margin; from a northerly wind the centre will bear east; from a westerly wind it will bear nurth; and from a southerly wind, west ; thus the direction of the wind only in a revolving storni, will announce to the commander of the vessel, two very important points,-bis exact position in the cyclone, and the bearing of its centre from his ship.
6. The same simple and very perspicuous rule holds good in the southern hemisphere. From an easterly wind, the centre of the storm bears north or eight points, reckoned in the same direction as the apparent course of the sun, the rising in the east, culminating in the north, and setting in the west. From a southerly wind the centre bears east; from a westerly, south; and from a northerly, west. These bearings are precisely the reverse of those in the northern hemisphere, but as the apparent motion of the sun is also reversed, the rule is applicable to both hemispheres. THAT THE CENTRE OF A REVOLVING STORM BEARS EIGHT POINTS FROM THE DIRECTION OF THE WIND AT THE SHIP, RECKONED WITH THE APPARENT
COURSE OF THE SUN.
7. While the atmosphere within the cyclone is in so rapid a state of rotation, that the moving air frequently attains a velocity of about one hundred miles an hour, the exterior zone is strikingly characterized by certain meteorological appearances, which herald, as it were, the approach of the coming storm. The rapid motion of the air within the whirl, combined with the sucking in of the exterior air comparatively at rest, produces an immense condensation of vapour generally seen on the horizon in the direction of the cyclone, as a dense, dark, lofty wall or bank of cloud. As the vessel approaches the storm, this bank of cloud appears to advance,
and draw down closely upon the ship, so that she becomes involved, and then the clouds present so appalling an aspect, they appear to be so close to the vessel, and so solid in their structure, that a commander may almost fancy he can from the vessel, put his hand on them.
8. When the ship approaches so near the cyclone, as to experience the effect of the outward gyration, the weather becomes still more significant, the proper wind of the hurricane generally characterized as strong and squally, carries over the vessel portions of the great bank of cloud peculiar to the storm, these portions are torn into rags and shreds, while the bank still marks the locality of the cyclone. From this point a run of two hours toward the centre will generally involve a ship in an impetuous and terrific hurricane.
9. The feature next in importance to the rotation of a cyclone, is its progressive motion, and this in all ordinary cases is reducible to the same order and regularity as we have seen characterizing the rotation; commencing at a point a few
; degrees north of the line, the cyclone moves bodily forward towards the west, its course is however soon directed a little north of west, and as it approaches towards 20°N. lat. its course is more or less towards N.W., at 30°N. lat. its course for a short time is due north, here it recurves, and afterwards is directed towards the N.E. This course is peculiar to the western portion of the basin of the Northern Atlantic. The usual storm paths in this locality, may be divided into ordinary and extraordinary. The ordinary conforming to the course above mentioned, and the extraordinary, departing from this type.
10. Upon combining the rotatory with the progressive motion, some very valuable rules for the guidance of commanders may be deduced. The path which the axis of gyration describes, is not inappropriately termed the axis line, and
this divides the cyclone into two semi-circles, the right or star. board semi-circle, and the left or port semi-circle ; we have consequently three divisions of a storm, each characterized by different phenomena. In the right hand semi-circle, the hauling of the wind resulting from the passage of a cyclone, ' in the northern hemisphere, is in the same direction as the apparent course of the sun, but in the left hand semi-circle it is reversed, being opposite to or against the sun. On the axis line there is no change of wind until the centre has passed, when after a short lull or calm, the wind springs up with great fury from the opposite quarter.
11. The rules deduced from the progressive motion of storm, combined with its rotation, are probably best enunciated as well as elucidated by a series of examples, of which the first has reference to the western portion of the basin of the Atlantic, where the ordinary storm paths follow more or less the course of the Gulf stream.
(a.) A vessel pursuing, the usual course to the West
Indies, shortly after passing 50° W. long. observes unmistakeable meteorological signs of a hurricane bearing down upon her, i.e. the dense bank of cloud, &c. is seen astern, not ahead; when she becomes involved in the scud, and the jagged and torn clouds skirting the cyclone are flying swiftly past her, the steady N.E. trade is replaced, not by a wind from a different quarter, but by a wind still from the N.E. of greater intensity, and characterized by strong and sudden squalls; she is now upon the N.W. margin, or rather just within the N.W. verge, the centre bears S.E. of her, and if she scud before the wind, she will approach the axis line of the storm. If however, she should heave-to on the starboard tack, and allow the cyclone to pass over her, the wind will haul by E.N.E.-E.-E.S.E.-S.E. and S.S.E.