|
50 TROPICAL REVOLVING STORMS (TRS)
190. GENERAL DESCRIPTION OF TROPICAL REVOLVING STORMS
Tropical Revolving Storms constitute a major hazard to seamen and consequently merit a special study. A tropical revolving storm may be defined as a small area of intense low pressure around which winds of hurricane force blow, inclined slightly towards the centre, in an anticlockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. These storms as a whole (or the centre of the storms) have a forward or progressive motion and often produce mountainous seas and weather conditions of a far more devastating character than the worst storms of temperate latitudes.
Although not so extensive as the depression of higher latitudes, the TRS is often far more violent, especially within 75 miles or so of its centre, where high and confused seas may cause considerable damage to large and well-found ships, while small craft have foundered. The danger is even greater where vessels are caught in restricted waters without adequate room to manoeuvre. Within 5 to 10 miles of the centre of a TRS, the wind is light or moderate and variable, the sky is clear or partially so, and there is a heavy, sometimes mountainous confused swell – this area is known as the eye of the storm. Visibility, near the storm centre (but outside the eye), is almost nil owing to torrential rain and sheets of almost continuous spray. Tropical Revolving Storms are so-named because the wind does not revolve Tropical Revolving Storms constitute a major hazard to seamen and consequently merit a special study. A tropical revolving storm may be defined as a small area of intense low pressure around which winds of hurricane force blow, inclined slightly towards the centre, in an anticlockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. These storms as a whole (or the centre of the storms) have a forward or progressive motion and often produce mountainous seas and weather conditions of a far more devastating character than the worst storms of temperate latitudes.
around the centre of low pressure in concentric circles, but has a spiral movement towards the centre. The tropical revolving storm (TRS) is known in various parts of the world by the local names of cyclone, hurricane, typhoon or willy-willy. Recent investigations into the synoptic situations of low latitudes indicate that there are more depressions in the tropics than was at first appreciated, but that only in a small number does the wind attain hurricane violence (i.e., Beaufort force 12). The name cyclone originated in India because of the likeness between the spiral winds of the tropical storm and the coils of a snake. The word hurricane applied to TRS in the region of the West Indies comes from the vocabulary of an American Indian tribe and means big wind. The word typhoon is from the Chinese tat = great, fung = wind. Willy-willy is pure Australian slang.
The table in fig 38-1 shows the area, season and nomenclature of tropical storms around the world. In addition to the names in the table, there are various local names for them such as bagnios (Philippine Islands) and cordonazos (W. coast of North America). Whatever their name, they are all members of the same family and behave in accordance with certain broad laws and with equal ferocity.
They occur chiefly in the late summer and early autumn months and mostly over the western portions of the great oceans.
The following table shows in round numbers the average number of severe TRS recorded in a period of 10 years in several areas: –
West Indies 50 Western North Pacific Ocean 250
Western South Pacific Ocean 30 Southern Indian Ocean 60
Bay of Bengal 20 Arabian Sea 10
Eastern North Pacific Ocean 30 Western Coast of Australia 10
Variations in any one year amounting to 50% above or below the average are not unusual. Some of the figures quoted may be an underestimate since in the less frequented parts of the world some storms may have escaped detection.
191. THE ORIGIN, DEVELOPMENT AND PATH OF TROPICAL REVOLVING STORMS.
As a general rule, TRS originate in the doldrums, between the parallels of 07º and 15º of Lat. Those which affect the western parts of the Pacific, South Indian and North Atlantic Oceans are first reported in the western third of those oceans, though there are exceptions such as in the North Atlantic during Aug and September when an occasional storm originates near the Cape Verde Islands.
In the northern hemisphere, TRS move off in a direction between 275° and 350°, although most often within 30° of due W.. When in a Lat. of 25° or so, they usually re-curve away from the Equator, and by the time they have reached the 30th parallel they will be moving on an N.-easterly course. In the southern hemisphere they move off from the area of origin in a WSW to SSW direction (usually the former), re-curve in Lat. 15° S. to 20° (approximately), and thereafter adopt a south-easterly path. Some storms, however, do not re-curve but continue in a WNW (or WSW) direction until they reach the mainland, where they quickly die. The speed of these storms is usually about 10 knots in their early stages, increasing a little with Lat. but seldom exceeding 15 knots before re-curving, but thereafter 20 to 25 knots is usual, although speeds of 40 knots or even more have been known.
Occasionally TRS move erratically, the path turning towards the Equator or adopting an easterly component in a low Lat., or even making a complete loop, but on such occasions, their speed is low, generally less than 10 knots while the unusual path is being followed.
The extent of the storm area varies considerably with individual storms, but generally speaking winds of force 7 or more are improbable more than 200 miles from the centre (especially on the equatorial side of the storm area), while winds of force 8 are unlikely to be exceeded at more than 100 miles from the centre in latitudes lower than 20°.
Hurricane-force (12) winds are likely within 75 miles of the centre and gusts of over 150 knots have occasionally been reported within 50 miles (except in the eye of the storm). Thereafter the radius increases with Lat., so that these distances are nearly doubled in Lat. 35°, but the intensity near the centre diminishes. Subsequently, TRS usually acquire the characteristics of deep temperate-Lat. depressions and continue to move N.-eastwards (or south-eastwards), eventually filling up and disappearing.
The following Glossy of the terms used in connection with tropical revolving storms will be found useful (see also fig. 38-2): –
VORTEX – (or eye of the storm) – the central area within the ring of hurricane-force winds, where the barometer is lowest.
VERTEX – (sometimes called the cod), the point of re-curvature or most westerly point reached by the vortex when the storm is moving out of the Tropics.
TRACK – the route along which the vortex has travelled.
PATH – either the route along which the vortex has travelled or the route along which the vortex is expected to travel.
STORM FIELD – the regions covered for the time being by the winds forming the storm system.
DANGEROUS SEMI-CIRCLE – the side of the path towards the usual direction of re-curvature, i.e., the right-hand semi-circle (looking along the path) in the northern hemisphere or the left-hand semicircle in the southern hemisphere. A sailing or low-powered vessel caught in the dangerous semicircle may be blown towards the path along which the storm will pass, or the storm may re-curve and the vortex passes over her.
DANGEROUS QUADRANT – the leading quadrant of the dangerous semicircle. This is the area to avoid at the first warning of the approach of a TRS.
NAVIGABLE SEMICIRCLE – the semicircle which lies on the side of the path farthest from the normal direction of re-curvature, i.e., the equatorial side of the path. A vessel situated within this semicircle will tend to be blown away from the storm centre. After the storm has re-curved, the navigable semicircle is on the polar side of the path.
192. THE WARNING SIGNS OF THE APPROACH OF A TROPICAL REVOLVING STORM
Warning of the position, intensity and probable movements of a tropical storm may be received at any time by radio from a Meteorological Service or from another vessel. In most TRS areas the responsible Meteorological Services take considerable care to issue comprehensive warnings to shipping by radio when such a storm is known to be developing and issue frequent bulletins concerning the storm’s progress. In the TRS season, it is very important, therefore, that a careful radio watch be kept for storm warnings, but as an official warning from a Meteorological Service is often based largely on observations from ships themselves, the navigator should be guided also by his own observations, bearing in mind that a Meteorological Service may not be in possession of sufficient information to issue even a general warning until some message from one or more ships has been received.
It is therefore, very important that in the TRS season every mariner if he suspects or knows of the existence of a tropical storm, should send in a radio weather message to the nearest coast radio station and to other ships as soon as possible, in accordance with the International Convention for the Safety of Life at Sea, and take action forthwith for the safety of his own vessel. An intense storm of small diameter may develop with very little warning, and increased vigilance on the part of mariners may well be the only safeguard against becoming uncomfortably, or even dangerously, involved in such a storm.
Apart from a definite, unusually steep and regular fall of the barometer, other indications, taken individually, are uncertain guides as to the possible approach or development of a TRS. Sea, wind direction and force, cloud, weather and barometer must be considered together and when navigating in any area in which there is even only a remote possibility of a TRS, the mariner will do well to be constantly on the alert for any sign of a change in the weather.
Any cessation of the diurnal range of barometric pressure in the tropics should be regarded with suspicion. At sea in the tropics, the barometric pressure normally varies very little except for diurnal variation. The diurnal range of the barometer results from pressure waves, with a period of nearly 12 hours, which sweep regularly around the Earth from E. to W.
Over a long period, the mean daily range is about 0.8 mb. in Lat. 51° but can be more than 2 mb. in subtropical and tropical regions. Fig. 38-3 shows a typical curve for the tropics against local time from which it can be seen that the barometer rises from about 0400 hrs. to 1000 hrs., then falls until 1600 hrs., rises again until 2200, when it once more falls until 0400 hrs.
In temperate latitudes, irregular pressure changes are usually so much larger than the diurnal variations that the latter need not be taken into account. In tropical regions, however, irregular changes are usually much smaller than the diurnal variation from which they cannot be distinguished until the diurnal variation has been subtracted. This procedure is important in tropical regions because of a fall of the pressure of 2 or 3 mb. below the value appropriate for the locality, after allowing for the diurnal change of pressure, maybe the first indication of the approach of a TRS. The normal value of pressure for a locality and the diurnal variation can be obtained from the Admiralty Sailing Directions or Admiralty Routeing Chart for the area, or from Meteorological Office Climatological Atlases.
Of all the indications of the proximity of a TRS, the fall of barometric pressure is by far the most reliable within 20° of the Equator. When proceeding through an area liable to be affected by these storms it is advisable to take hourly readings of the barometer, even when conditions appear to be normal. In such areas, if the barometer reading, corrected for height, Lat., temperature and diurnal variation, is 3 mb. Or more below the mean pressure for the time of year, the mariner should be on his guard. If the reading thus corrected is 5 mb. Or more below the normal, there can be little doubt, that there is a TRS in the vicinity, probably not more than 200 miles away, and it is time to take avoiding action. At this distance, the wind has usually increased to about force 6.
When a TRS passes fairly near to a vessel there are usually three distinct phases in the fall of the barometer:
A slow fall, with the diurnal variation still in evidence, usually occurs 500 to 120 miles from the storm’s centre.
A more marked fall, during which the diurnal variation is almost completely masked, usually occurs between 120 and 60 miles from the vortex.
Throughout this phase, the barometer is sometimes very unsteady.
A rapid fall occurring at from 60 to 10 miles from the vortex.
In the rear of a TRS, the barometer rises as rapidly as it fell in advance of the storm. It is not uncommon for the barometer at the centre of a storm to fall 60 to 70 mb.
Lower than in the region just outside the storm field. The steepest barometric gradient normally encountered is 11 mb. In 15 miles. Fig. 38-4 shows a barograph trace when the centre of a TRS passed within a few miles of a vessel near Bermuda.
Of the other warning signs of the approach of a TRS, swell from the direction of the storm, providing there is no intervening land between the vessel and the storm centre, will probably give the first indication, since it travels at a greater speed than the storm itself. With a fully developed TRS, the swell can usually be relied upon to make itself felt 400 miles from the centre. The swell moves out from the centre of the storm and its direction thus gives a good idea of the bearing of the centre.
TRS is frequently preceded by a day of unusual clearness and remarkable visibility. The atmosphere at such times is oppressive.
These conditions are followed by extensive cirrus cloud, often V-shaped and pointing towards the storm centre, and showing no disposition to clear at sunset but instead reflects sickly greenish-yellow colourings and again at sunrise. It later becomes reinforced by a thick layer of altostratus and ultimately by cumulus fractus and scud. The progress of the scud is marked by rain squalls of increasing frequency and violence. Rain is one of the most prominent features of a TRS; in the outer potions, it is intermittent and showery, whilst in the neighbourhood of the centre it falls in torrents. The rain area extends further in advance of the centre than at the rear.
Any appreciable change in the direction and/or appreciable change in the wind strength during a TRS season may indicate the presence or formation of a tropical storm and should be regarded with suspicion.
Once a mariner considers that he is in the vicinity of a TRS, although it is appreciated that he will be preoccupied with the safety of his own vessel, in accordance with Article 35 of the International Convention for the Safety of Life at Sea, it is his duty to inform other vessels and shore authorities with all means at his disposal. A concise weather report radioed from a vessel will almost certainly result in timely advice being given to other vessels who may be in the path of the storm and also to the inhabitants of island and coastal communities where life and property may be threatened. Such a priority message, addresses to the nearest Coastal radio Station and repeated CQ (to all ship’s) might read as follows:
Sécurité, Sécurité, Sécurité. Storm. Appearances indicate approach of hurricane 1300 G.M.T. 10th Jul. My position is 12° 22.0 N., 72° 36.0 W. Barometer corrected 994 mb. tendency down 6 mb. Wind N.-E. force 7, frequent rain squalls. My course 220°, 10 knots.
Thereafter weather reports should be made by radio at frequent intervals, giving as much information as possible, especially barometer readings.
193. PRACTICAL RULES FOR AVOIDING TROPICAL REVOLVING STORMS
If a mariner is caught at sea in the path of a TRS, he must do his best to avoid the storm centre and manoeuvre his craft to sectors of the least intensity. His course of action should, in a general way, be based on the Law of Storms devised in the last century by Henry Piddington and endorsed by later authorities on navigation and seamanship such as the British Admiralty. Very briefly, the basic principles of the law of storms are first to establish the vessel’s position relative to the storm centre, whether she is in the navigable or dangerous semicircles and then to adopt a course of action which will best avoid the dangerous sectors. The various methods of doing this will now be discussed.
To decide on the best course of action, if a TRS is suspected to be in the vicinity, the mariner should:
Determine the bearing of the vortex and endeavour to estimate its distance from his vessel.
Plot the probable path of the TRS.
Determine the semi circle in which the vessel is situated.
The bearing of the vortex may be ascertained by the application of Bays Ballots Law, namely, face the wind and the centre of the storm will be on you right if you are in the northern hemisphere, and on your left if you are in the southern hemisphere. In northern latitudes the centre will bear about 12 points to the right at the beginning of the storm, i.e., when the barometer begins to fall.
When the barometer has fallen 10 mb. the vortex will bear about 10 points to the right, and when it has fallen 20 mb. the vortex will bear about 8 points to the right (to the left in the southern hemisphere). The nearer the observer is to the centre of the storm, the more nearly does the angular displacement approach 90° i.e., the direction of the wind more closely follows the isobars.
The distance of the vortex from the vessel will depend on so many factors that it is almost impossible to estimate without the aid of information from other sources. It is quite impossible to estimate the distance of the vortex by the height of the barometer, or by its rate of fall, alone. From the barometric pressure and force of the wind one can, however, arrive at certain broad conclusions. For instance, if the corrected barometer is 5mb. below the normal for the time of year, the centre of the storm is probably not more than 200 miles away. At this distance the wind will probably have increased to force 6. If the wind is force 8, the centre is probably within 100 miles.
The path of the storm may be approximately determined by taking two bearings of the vortex (as described above) with an interval of from two to three hours between them, provided that allowance is made for the vessel’s movement. As it is difficult to determine the movement of a TRS if the vessel continues to make headway, the best and surest way of obtaining an accurate impression of the storm’s movement is to stop the vessel between the two bearings.
It can normally be assumed that the storm is not travelling towards the equator and if in a Lat. lower than 20° it is unlikely to have an easterly component. On the rare occasions when neither of these statements applies, the storm will be moving very slowly.
Determination of the semicircle in which the vessel is situated depends on a true appreciation of whether the wind is veering, backing or remaining steady in direction. Unless the relative speeds and directions of the vessel and the storm field are known, (and, in the early stages, it is unlikely that they will be), the prudent mariner will at once heave-to and watch for a shift of wind, carefully watching the barometer at the same time. If he does not heave-to, the mariner must work out a relative motion problem to determine exactly the true wind shift.
Heaving-to for two or three hours to make careful observations should enable the mariner to make fairly accurate predictions of both the path of the storm and his own position in relation to this path, so that the best subsequent course of action to avoid danger can be determined.
It is a matter of vital importance to avoid passing within 50 miles or so of the vortex of a TRS. It is preferable to keep outside a radius of 200 miles or more because at this distance the wind does not exceed force 7 (and is generally not more than force 6) and freedom to manoeuvre is maintained. Sometimes a TRS moves so slowly that a vessel if ahead of it, can easily outpace it; or, if astern of it, can overtake it. Since, however, she is unlikely to feel seriously the effects of a storm so long as the barometer does not fall more than 5 mb. (corrected for diurnal variation) below the normal, it is recommended that frequent readings should be made if the presence of a TRS in the vicinity is suspected or known and that the vessel should continue on her course until the barometer has fallen 5 mb. or until the wind has increased to force 6 when the barometer has fallen at least 3 mb. If and when either of these events occurs, she should alter course in accordance with the following paragraphs until the barometer has risen again above the limits just given, and the wind has decreased below force 6. Should it be certain, however, that the vessel is behind the storm, or in the navigable semicircle, it will evidently be sufficient to alter course away from the centre?
In the northern hemisphere, if the wind is veering, the vessel must be in a dangerous semicircle. A power-driven vessel should proceed with all available speed with the wind 1 to 4 points (depending upon her speed and that of the storm) on the starboard bow. A sailing vessel should heave too on the starboard tack. Either type of vessel should haul round to starboard as the wind veers, thereby tracing a course relative to the storm centre as shown by track (1) in fig. 38-5 (a). If the wind remains steady in direction. Or if it backs, so that the vessel seems to be nearly in the path of the storm, or in the navigable semicircle, a power-driven vessel should bring the wind well on the starboard quarter and proceed with all available speed, while a sailing vessel should run with the wind on the starboard quarter. Either type of vessel should alter course to port as the wind backs, thus tracing out a course relative to the storm track (2) in fig. 38-5 (a). (mb. It is sometimes difficult to determine satisfactorily if indeed the vessel is nearly in the path of the storm, particularly if in a dangerous semicircle, because the wind does not always behave according to the rule).
In the southern hemisphere, if the wind is backing, the vessel must be in a dangerous semicircle. A power-driven vessel should proceed with all available speed with the wind 1 to 4 points (depending on her speed and that of the storm) on the port bow. A sailing vessel should heave too on the port tack. Each type of vessel should haul around to port as the wind backs, thereby tracing a course relative to the storm, similar to track (3) in fig. 38-5 (b). if the wind remains steady in direction or if it veers, so that the vessel is either in the path of the storm or in the navigable semicircle, a power-driven vessel should bring the wind well on the port quarter and proceed with all available speed, while a sailing vessel should run with the wind broad on the port quarter. Either type of vessel should alter course relative to the storm similar to track (4) in fig. 38-5 (b).
In either hemisphere, if, owing to the presence of land or some other navigational hazard, there is insufficient room to run when in the navigable semicircle and it is not practicable to seek a safe and effective shelter before the storm begins to be felt, all vessels (sail or power) should heave-too with the wind on the starboard bow in the northern hemisphere and on the port bow in the southern hemisphere.
Fig. 38-6 illustrates the Law of Storms to be followed by all vessels in the northern hemisphere. A piece of tracing paper on which is drawn the diagram in fig. 38-6 representing the average winds and tendency of the barometer in a TRS will be found useful for studying the behaviour of the wind in the storm field. One can lay off the vessel’s course and speed on a chart and manipulate the tracing paper as necessary to indicate the relative motion of the vessel and the storm. Many examples which may be experienced according to the vessel’s position, course and speed relative to that of the storm can thus be illustrated, but it must be remembered that the forces and directions of the wind shown on the diagram are average or approximate.
As the storm approaches. Long low swell, the pressure falls more than 3 mb. Below normal value and diurnal variation ceases, sometimes remarkable visibility, extensive Ci cloud with lurid sunrises and sunsets. Wind and sea increase, the pressure falls more rapidly; cloud thickens to as and scud.
Squalls become more frequent and violent, torrential rain, sky covered with dense Cb cloud. Wind and sea continue to increase, and visibility is reduced to near zero.
After the storm has passed. Wind and sea decrease, slowly at first, rain lessens, cloud cover thins, and pressure rises rapidly, then more slowly and resumes its diurnal variation and average height, and swell decreases.
Vessel A: Is in the right-hand semicircle with a falling barometer. Heaves too on the starboard tack or steams with the wind ahead or on the starboard bow and is thus heading away from the storm field. The barometer will cease to fall when she is in position A2 and when at A3 she will be able to resume her course again.
Vessel B: Is in the left-hand semicircle with a falling barometer. Runs with the wind on the starboard quarter whether steam or sail, hauls round as the wind backs and traces a course relative to the storm, as shown by the pecked line, with barometer rising slowly.
Vessel C: Is in the direct path of the storm and acts as vessel B.
Vessel X: Is overtaking the storm and converging on its centre. If the master obeys the rules and heaves too, he will find the wind is veering and the barometer rising.
He will thus know he is in the rear quadrant of the right-hand semicircle and will, by keeping the wind on his starboard bow for a few hours in accordance with the rules, let the storm draw ahead before resuming his original course and getting astern of the storm.
If, however, he does not heave too, he will find that the wind is hauling to the left, relative to the ship, and with a falling barometer he might incorrectly assume that he is in the advanced quadrant of the left-hand semicircle. If he acts on the assumption and, in accordance with the rules, puts the wind on his starboard quarter he may eventually find himself in the dangerous quadrant of the storm.
The most difficult situation is encountered when a vessel finds herself at or near the point of re-curvature of a TRS, and fig. 38-7 illustrates how necessary is a constant watch on the weather, even when it has been decided in which semicircle the vessel is situated. In fig. 38-7 (a) it is assumed that a vessel is in the path of an advancing TRS in the northern hemisphere and that, unknown to the meteorological authorities or the vessel’s master, the storm has reached the point of re-curvature. From a succession of observations made when the tile vessel was hove-too, it was found that the wind was shifting to the right and it was accordingly assumed that the vessel was in the right-hand semicircle of a TRS moving to the NW, and in accordance with the Rules, the course is altered to place the wind on the starboard bow.
During the interval between fig. 38-7 (a) and fig. 38-7 (b) the wind was observed to change very slightly, a little to the right at first, then back to the previous direction again but gaining in force. From this observation, it was correctly assumed that the vessel was now in, or nearly in, the direct path of the storm and accordingly course was altered to bring the wind on the starboard quarter as in fig. 38-7 (b), From that time onwards the wind was observed to shift progressively to the left as in fig. 38-7 (c), suggesting that the vessel was now in the navigable semicircle. The wind continued to shift to the left more rapidly, with the barometer rising, showing that the storm was clearing to the eastward (fig. 38-7 (d)
Had the manoeuvre shown in fig. 38-7 (a) been continued without paying careful and frequent attention to the shifts of wind being experienced, the vessel might have been dangerously involved in the direct path and subsequently in the vortex of the storm and suffered severely in consequence.
Although the standard advice set forth in the law of storms described above is generally accepted as being sound even today, it was originally intended primarily for power-driven vessels and large square-rigged sailing vessels.
One original concern, which probably does not have the same significance for the modern for-and-aft rigged boat, was the danger of being caught aback by the suddenly shifting winds in a TRS.
Furthermore, advice intended for ships does not necessarily apply in all cases to small craft; thus slight modifications of the standard rules may be necessary for small boats. In fact, one of the most experienced hurricane observers, Robert H. Simpson, believes that in many instances a small craft will fare better in the rear quadrant of the dangerous semicircle than in the navigable semicircle. Simpson’s advice should not be taken lightly for he was chief of the National Hurricane Warning Centre in Miami, a top-ranking meteorologist, and formerly an associate director of the U.S. Weather Bureau. Furthermore, he is an experienced yachtsman and boat owner, and has flown through hundreds of hurricanes.
Simpson warns against a small boat being in the navigable semicircle in a large, severe TRS. In general, he agrees with the standard advice to run away from the centre in the navigable semicircle in small storms, but in large storms, he maintains that seas are more tenable for small craft in the right rear quadrant in the northern hemisphere (or the left rear quadrant in the southern hemisphere) see fig. 38-8. In this location, the waves are long, less confused and flattened by the high winds. However, Simpson definitely agrees with the standard rule that the leading quadrant of the dangerous semicircle is the most dangerous sector of all, because this area has the combination of steep, confused seas with the highest winds, swells of the greatest magnitude, and the wind in this quadrant tends to blow a vessel towards the storm centre.
194. PRECAUTIONS AGAINST A T.R.S. IN HARBOUR
If in harbour, whether alongside, at moorings or at anchor, the mariner should, in the TRS season, be just as careful as at sea in watching the barometer and the weather and, if a TRS is threatened, in watching the shifting of the wind and estimating the movement of the storm relative to himself, so that he can make timely and seamanlike precautions well before the storm reaches his area. If the storm is known to be heading towards the harbour, it is often preferable to put it to sea provided there is plenty of sea room outside, rather than encounter the storm in a harbour.
A major danger in many harbours may come from the extremely high tides which normally accompany a TRS. The largest waves originate in the rear right-hand quadrant of a TRS and travel through the storm field, eventually reaching the shore where they cause a rise of water in front of, and 100 to 200 miles to the right of, the line of advance of the storm. This rise begins when the centre of the TRS is 300 to 500 miles away and continues until the TRS crosses the coast. The height of the flood level reached at the shore near the centre of the storm is sometimes as much as 5 metres above the predicted tide level. Slow-moving storms of large diameter create the highest storm tides. It is therefore important to pay out all the scope possible when the vessel is at anchor, and to make allowance for a considerable rise of water when she is made fast to a quay or pier.
Riding out a TRS the centre of which passes within 50 miles or so, in a harbour or anchorage, even if some shelter is offered, is an extremely unpleasant and hazardous experience, especially if there are other vessels in the vicinity. The extreme violence and gustiness of the wind and its sudden shifts of direction involve great risk of the anchors dragging. The torrential rain and driving spray may impair visibility to such an extent as to make it very difficult to see if such is in fact the case. Whether or not the vessel is moored, anchored or alongside a quay, all loose gear should be taken off her decks, she should be stripped as much as possible of running rigging, dodgers, weather cloths, sails or anything which may cause windage, and the engine should be made ready for use.
At moorings or an anchorage it is a good idea to buoy both anchors with brightly painted drums since at the height of the storm it is often necessary to use the engine to maintain position between the anchors. Major considerations are ample swinging room and good holding ground. Soft, oozy mud is a poor bottom during a storm, unless perhaps a large, deeply imbedded mushroom anchor is used with sufficient scope to allow for the storm tide. When the vessel is made fast to a wharf, her flooring lines should be set out on both sides in order to hold her away from the wharf. If there are no pilings to which the lines can be secured, then anchors should be set out, on the side without pilings. Without lines on both sides, the storm tide might lift a small craft above the wharf or pier and seas could drop her on top of a piling punching a hole through her bottom. Mooring lines should be doubled and well wrapped for chafe prevention and plenty of fenders should be rigged – old tyres are effective, and normally they are readily available.
In the case of a small craft having insufficient warning to enable her to gain a reasonable distance from the storm or from the shore by putting to sea, it may be preferable to remain in a reasonably sheltered harbour. If such a vessel receives warning of an approaching TRS when at sea, and there is considered to be insufficient time or sea room to avoid the dangerous part of the storm field, it may be advisable to seek shelter. In the China Sea, for example, there are so-called Typhoon harbours which are listed in the Admiralty Sailing Directions. An ideal TRS anchorage should afford protection on all sides, but especially on the sides from which the winds are expected. The wind will seldom swing entirely round the compass but it may shift through a semicircle arc. Protecting shores should be sufficiently high to be above the storm tide level, and if there is any choice in the matter, the anchors should be dropped behind a stand of new trees rather than old ones, which are more apt to blow down and drift down on the anchored vessel.
In all cases, however, the mariner must use seamanship and initiative. It would, for instance, be imprudent to make for harbour and attempt to pick up TRS moorings if the storm was already being felt. It would be equally imprudent to remain in harbour, even in a sheltered harbour, without taking all the precautions listed above, and having the engine ready for use, as soon as the first warning of an approaching storm is given.
|