W
20
N
E
1100 miles
Typhoon of July 17
1890
21
§ 2.-HOW TO MANAGE YOUR SHIP IN A TYPHOON.
337
Long before the nature of hurricanes was understool it had been remarked by mariners that it was safer to heave to is a storm than to run, but the reason of that was not apparent. When the
r of storms was first discovered it did not all at once mend matters, for, according to the old t-point rule, all you had to do when a typhoon approached was to run with the wind on the tarboard beam under as much canvas as your ship could carry, except when on the path right in front and the centre moved quicker than the vessel. That should have taken you out of the neigh- bourhood of bad weather, but it did not always do so, and since typhoons have been investigate, we know that the wind never blows round the centre in a circle, and that the eight-point rule, laid down by Redfield, Reid, Thom, and Piddington, is very far from the truth. It is just as much out
as Espy's rule, according to which the wind blows always straight towards the centre. According to the twelve-point rule, which I have proved to obtain in typhoons (see "Observations and Researches ade at the Hongkong Observatory in 1883") you must still ran on the starboard tack, but you hust keep the wind only four points from the stem. hossible to keep a vessel going ahead under square sails nearer than about six points to the wind. A steamer can do so easily enough, but it is not She therefore cannot within two points run straight away from the centre and her risk is much greater than it would be if the old rule had proved true. The twelve-point rule may be stated
as follows:-Stand with your back to the wind and you will have the centre on your left and about
four-points in front of your left-hand. It appears that a ten-point rule obtains in about 30 degrees
atitude and a sailing vessel can then just manage to steer, fall and by, a course that will take her straight away from the centre; but in a tropical hurricane we see that the danger is much greater eren if the force of the wind were not stronger.
The force of the wind and the appearance of the weather do not furnish a trustworthy guide to determine the distance of the centre of a typhoon. The dimensions are different in almost all typhoons and near land the strong winds are often so irregularly distributed that in a place near the centre less wind may actually be experience than at some distance farther away from it. cases the approach of the centre is judged of by the fall of the mercury in the barometer.
In such But on the high sea the increase or decrease in the strength of the squalls, and the state of the weather and sen, enable the approach of the centre to be known, and this, together with its direction which is known from the direction of the wind and perhaps also from the clouds, enables the master of a vessel at sea The average rate of progress of the centre of a typhoon in 11 deg. latitude is 5 miles an fa
lay down the path of the typhoon, with reference to his vessel, with more or less accuracy. In 20 deg. it is 9. In 25 it is 11.
In 30 deg, it is 14 great difficulty is, that the typhoon may be increasing or decreasing in violence and the barometric The In 13 deg, it is 64. In 15 deg. it is 8. in 324 it is 17 miles an hour. South of 13 deg. the speed does not vary perceptibly, so it is a depression getring steeper or filling up.
In 324 deg. Nitores the former is likely to take place. while the latter would probably happen near the western shores To the eastward of the Philippines and near their western for mariners to know it, but it is more variable the farther north you go. from 6 to 36 miles an hour, so that you cannot be sure that a typhoon encountered there will of the China Sea.
In such cases it may become more or less impossible for a seaman to know how the at anything like the average rate of speed. We have not traced the centre of a typhoon nearer tre moves, except in so far as he can judge by published paths of previous typhoons.
Equator than about 9 deg. N. But a very slightly falling barometer, a squally SW wind, a les sea, and some swell, may at times be traced nearly all the way down to the Equator,
Once the bearing of the centre has been ascertained from the direction of the wind, you require know in which semi-circle your vessel is situated if in the right-hand semi-circle the wind will The prevailing wind not only carries the centre along with it, but combines with the rotary set, t., shift with the sun, and in the left-hand semi-circle it will back, ie., shift in the opposite cansing the wind in the right-hand (the dangerous) semi-circle to be stronger and to blow more levetion. But this rule fails if your vessel is moving with the typhoon and quicker than the centre round the centre than in the left-hand (the manageable) semi-circle, where the wind is more male moving. Then the rule may be reversed. Masters of vessels are therefore advised to heave to curly It also causes the wind to blow and observe how the barometer behaves and how the wind changes; but it is so dangerous to lose straight in towards the centre behind the typhoon and to blow more across the path in front of my time in a storm that carries you into the centre, that this should not be done except when and blows with greater incurvature towards the centre.
was while the centresolutely necessary. centre. It also makes the weather heavier after the centre is past than
You may happen to be right in front of the centre and lose your chance of getting out of its track. approaching.
Here it is that a knowledge of the paths of past typhoons such as have been ually published from this Observatory during the past fourteen years, becomes so useful. That Less than half a mile up in the air the incurvature of the wind towards the centre disappy enable you to know at once in which semi-circle your vessel is situate, eg, with NE wind in the the average of the different quadrants, but it still blows in towards the centre in the rear. Its in Sea you are pretty certain to be in the right-hand semi-circle. The wind shifts faster the the wind at this altitude that carries the typhoon along, for late in autumn there are every dearer the centre you are, but the direction of the sea does not change so fast as the wind, If the typhoons that move along against the NE monsoon, but we know that that monsoon is at the mills eye of the typhoon overtakes your vessel, the wind bursts again from the opposite quarter, and These typhoons disappear sometimes suddenly; et perhaps greater violence than before, that is, when the centre is past. Very deceitful lulls shallow and there is SW wind above it.
secur during the raging of a typhoon and last sometimes long enough to be mistaken for the central when the NE monsoon increases in depth and intensity.
calm, but in such a case the wind bursts again from about the same direction as before.
At a still higher level the air, which has been carried in towards the centre and raised eve area where it is raining, blows away from the centre, and as the friction of air against air under pressure is insignificant, it sometimes rushes away with such speed as to cause the upper act upon the rate at which the latter is travelling. You cannot therefore safely draw conclusions sucked down into the central calm. This is the reason why the sky clears over the bull's-eye.
fierce.
The rate at which the barometer falls depends upon your approach to the centre, and in con- oncerning the amount of wind to be expected from the rate at which the barometer is falling, but to e extent that may be done. Remember that at sea when the barometer has fallen to the lowest ye and is beginning to rise again, you may expect as much, if not more, bad weather than yest ave already gone through although it will be, on the whole, improving.
Typhoons originating in the Pacific in a low latitude (say 13 deg. N) are very small and The isobars are nearly circular, as the centre moves very slowly, and the incurvature is 45 all directions; but there is this important difference between à typhoon and a tornado-that the
The right-hand semi-circle is called the dangerous semi-circle; there you are carried not only in As typhoons reach a higher wards the centre, but also towards the path in front of the centre; besides, the force of the wind is is taller than it is broad, whereas the former forms a flat disc.
is is di tensions become greater, the violence of the wind and then nothing to distinguish them from storms originating in northern latitudes. This makes unlikely that the latter originate from causes at all different from those which give rise to a ty
createst in
me direction as it veers with the progress of the typhoon. Here you must make up your mind at
No comments yet.
Private notes are available after approval.