SUPPLEMENT TO THE HONGKONG GOVT GAZETTE OF 2ND OCT., 1886.
As the angle between the direction of the wind and the gradient does not change much wh wind is strong on approaching to or on receding from the centre, it follows that the air moves the centre in logarithmic spirals or rather (as it is at the same times ascending) in screw horizontal projections are such spirals. This is quite correct in case of a stationary typhoon, but typhoon changes its position, new portions of air are constantly set in motion while oth stopping behind, and an air particle describes therefore with variable speed a curve of double cur whose horizontal projection is a kind of curve of pursuit, its path being constantly directed tow point about half a rightangle distant from the centre of the typhoon, which may for a short pe supposed to move on a parabola.
As the deflection of the wind towards the right increases with the sine of the latitude, south of the centre must ceteris paribus blow more straight in towards the centre than nort The difference between the amount of inblow north and south of the centre is proportional to the of the latitude of the centre, and is therefore largest in a low latitude, but it increases of cour together with the dimensions of the typhoon, and this is the reason, why by far the greater nu typhoons move in a northerly direction and with increasing velocity on account of their expan
The path actually followed by a typhoon appears to depend upon the wind, that prevails time. Typhoon XVIII of 1884 was blown SWestward by the NE monsoon, while in the sum 1885 when the SW monsoon was strong, typhoons moved Northwards. Whence also typhoons upon the season of the year. This explanation likewise agrees with the fact that depressions i motion onwards keep a permanent high pressure area on the right, with more or less steeper
nd stronger wind on that side. Probably the wind is on the whole stronger behind than inf The centre and it stretches farther away behind it. A typhoon moving SWestward is generally f strong NE wind that keeps blowing for some time, and when a typhoon has passed Nort as followed by strong and more or less persistent SW winds. Typhoons are likewise deflecte heir previous course when exposed to strong winds blowing out of open channels in which peed of the progress is frequently abruptly increased.
Very low clouds in a typhoon move with the wind. When clouds are observed at a high the anterior semicircle their direction forms generally an angle with the gradient, that is abo ints larger than the angle between the wind and the gradient. But at some distance behi
hoon they are frequently observed to move almost straight towards the centre.
It is probable, that the smaller angle which the wind forms with the gradient in front of the not altogether depend upon the increased friction in case of offshore winds. The he air would account for it in case of a typhoon on the open sea. We have seen, that w id rises after a calm, such as precedes a typhoon, the air must in the first instance blow str
rds the centre while the wind behind moving with less accelerated speed would have the dency there. It would at first sight appear, that the wind in front, blowing more straight
centre, must cause the centre to be filled up in front and pushed backwards, but this wo pensated by the greater altitude of the disturbance behind the centre. If the vertical height Moon behind the centre is to the height in front in inverse proportion to the cosines sponding angles, no effect on the progressive motion of the typhoon would follow.
es probably much higher up behind, so that the centre is filled up quicker there than in fr entre is in consequence pushed forwards.
When the centre has entered on dry land it frequently moves faster owing to the disturb case being much greater behind i. e. over the open sea.-Owing to the rotation of the e have a tendency to raise the air especially in low latitudes, which would on the whole com rease the disturbance behind the centre. The same tendency would cause the force of ands to be smaller than the force of E and NE winds for the same gradient.
hat the principal part of the disturbance is situated high above the surface of the earth is fact, that the centres of typhoons pass across mountains several thousand feet high, and cumstance that the difference between the temperature at this Observatory and the Peak ibly affected by the approach of a typhoon, for we cannot well presume, that the average
vertical column of air is lower near the centre than outside the cyclone. That on e cyclone does not attain the height of the cirrus clouds is made probable by the observa rection, although unfortunately it is impossible to see upper clouds near the centre and of the upper clouds in the eye of a typhoon are a desideratum. Over the beginning of area the cirrus back towards NE and they then sensibly preserve this direction. B of cirrus cloud preceeding a typhoon show that frozen water vapour is carried miles up ugh the action of a typhoon.
average rate of progress of the centre of a typhoon in 11° latitude is 5 miles an hour. in 15° it is 8, in 20° it is 9, in 25° it is 11. in 30° it is 14 and in 321° latitude it is l The rate of progress does not vary perceptibly in case of typhoons south of 13°1 more variable the farther north we go. In 32° latitude it varies from 6 to 36 miles an
typhoons while E or SE of the Philippines are found to move towards a direction b NNW. Subsequently they recurve and pass away in a direction between NNE and not all recurve, but about two thirds of them are found to do so. They recurve betwe Latitude and between 115° and 130° longitude. The average place of recurvature lies
about the position of the Middle Dog Light-house.
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