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§ 5.-WINTER TYPHOONS IN THE SOUTHERN PART OF THE CHINA SEA.
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In the typhoons of the summer months, the SW monsoon period,-which move towards WNW or NW in the N part of the China Sea and enter the coast in the neighbourhood of the Gulf of Tongking, the area over which the winds become strong with a decided fall in the barometer in front of the centre is generally small. This is accounted for by the low pressure prevailing over the Gulf of Tongking and the continent beyond it. For the same reason the winds in rear are not only stronger but also long continued, the more particularly as it seems that depressions on entering the coast frequently become diffused over a large area. In the autumn months (September and October)-the NE monsoon period, these conditions are reversed and we find that in front pressure begins to give way and strong winds usually blow at great distances, while in rear the area over which winds are governed by the depression is comparatively small. Moreover, at this period the depression fills up rapidly on entering the mainland, which is then dry. In consequence a vessel in front of a typhoon, moving as stated, will usually get much shorter notice of the advance of a typhoon from the barometer in summer than in autumn, and while in summer the bad weather lasts a long time in rear, in autumn it improves rapidly when the centre is past and a strong NE monsoon sets in. For instance, N gales frequently blow late in the season off the Annam and Cochin China coasts with a typhoon centre a long distance off on the E side of the China Sea,
Typhoons of classes IVa, IVb or Ib occur especially late in the year in an unusually low latitude over the China Sea, during the height of the NE monsoon which blows much stronger in these seas than the SW monsoon. Taking as an example of such, the typhoons of the 15th November, 1891, of the 20th November, 1891, and of the 13th November, 1895, it is seen that there is very little W, SW or S wind except within perhaps at most 50 miles of the centre. And to the S or SSW of the centre, where SW gales might be expected their place is taken by dead calms or light variable winds. No stronger argument could be adduced to show that the winds round a typhoon-centre are composed of the cyclouic winds on one hand and the prevailing wind on the other. In these cases where the cyclonic SW gales are combined with the NE monsoon gales, calms are the result. Unfortunately, I have not succeeded in drawing the isobars, as the anëroid observations on board ship are too rough to be of much assistance for this purpose, and the currents are strong, but the isobars might be inferred from the wind-directions laid down on the maps.
When all the wind forces round the centre of a typhoon are resolved into N and E components, positive or negative as the case may be, and the resultant direction and velocity are computed and compared with the direction of motion and velocity of the centre, it is found that the latter moves from a greater azimuth (counting from N) than the wind. This might be expected as it then agrees with the wind at a greater altitude. These November typhoons move from a direction from 2 to 7 points different from the resultant wind. As the NE monsoon is shallow, this probably agrees with the direction of the wind at an altitude of about half a mile, but the speed is only a fraction of the resultant wind velocity.
1891 Nov. 15.
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