SUPPLEMENT
To the HONGKONG GOVERNMENT GAZETTE of 18th July, 1885.
GOVERNMENT NOTIFICATION. -No. 287.
The following Report, by the Government Astronomer, on the Progressive Motion of Typhoons in 1884, is published for general information.
By Command,
Colonial Secretary's Office, Hongkong, 18th July, 1885.
FREDERICK STEWART,
Acting Colonial Secretary.
ON THE PROGRESSIVE MOTION OF TYPHOONS IN 1884.
Hongkong is situated in the region of the trades, but the winds are greatly affected by the neighbouring continents, principally by the immense Asiatic mainland, but to some extent also by Australia,the influence of which is the cause of the monsoons. Thus we find, that although the average
direction of the wind here is E,-a direction to some extent caused by the trend of the coast,- it still exhibits a regular annual variation.
The air is impelled from a region where the barometric pressure is higher, towards one where it is lower, its motion being however deflected toward the right in the northern hemisphere, owing to the rotation of the earth.-In winter, when the pressure is high over China and low over Northern Australia, E or NE winds blow almost without interruption over the China Sea. In spring, when the barometer is falling over Southern Asia, the direction of the wind veers toward S and reaches SW, when about midsummer the lowest pressure lies over Central Asia and a high pressure over Australia. In autumn the direction of the wind backs by degrees to NE.
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The SW monsoon does not however blow so steadily as the NE monsoon, possibly because the summer area of low pressure is not so regular as the winter area of high pressure in Asia, and the Easterly trade wind, supported by the-at that season-comparatively high pressure over the North Pacific, intrudes even in midsummer.
It is explained in the "Annual Weather Report for 1884," how the changes in the height of the barometer increase with the latitude. North of Hongkong the barometric pressure is subject to much greater changes than south of it. In consequence the E wind in winter increases in force with a rising barometer, and the SW wind in summer increases in force with a falling barometer, except in the presence of a typhoon.
During the winter season depressions originate within the area of Asia, which is covered by the high pressure, and pass Eastward. These are analogous to the depressions, which originate in Nebraska and cross the Atlantic, and which also have their maximum frequency and intensity in winter. Those depressions lie outside the field of our investigations, and will no doubt continue to have the attention of the Observatories in the North of China.
The typhoons appear to have their origin E or SE of the Philippines in the trough of low pressure between the two high-pressure areas in the North Pacific and in Australia. Their paths are determined according to the law, first enunciated by the Rev. CLEMENT LEY, according to which an atmospheric depression moves so as to keep the high-pressure area on its right. This law was origin- ally proved only in the case of depressions in the neighbourhood of the United Kingdom, but it applies equally to the typhoons. Now the application of this law would greatly facilitate forecasts concerning the progress of a typhoon, if the telegraphic information were sufficient to give a correct idea of the position and shape of the area or areas of high pressure, as it is known, that these are subject to comparatively little change.
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Typhoons may be divided into three classes according to the paths, which they generally follow. No doubt abnormal instances will occasionally present themselves, in China as well as in other coun- tries, but probably they will be of rare occurrence.
The first class of typhoons is common at the beginning and at the end of the typhoon season. Typhoons belonging to this class cross the China Sea, and pass either in a WNW direction from the neighbourhood of Luzon towards Hainan and Tonquin, as Typhoons II and V, or, if pressure is high over Siam and Annam, they pass first Westward and subsequently SWestward, as Typhoon XVIII.- Their life is generally between 5 and 6 days.
The second class of typhoons is perhaps the most frequently encountered, and their paths can be traced the farthest. They generally move NWestward in the neighbourhood of Luzon, and recurve owards NE in about 26°,or rather between 22° and 32°,-northern latitude. They either strike the coast before recurving, as Typhoons III, IV, IX and XII, in which case they generally at once lose the character of tropical hurricanes, or travel along the coast up through the Straits of Formosa,
* Typhoon IV struck the coast travelling NWestward, but passed North of Formosa, which appears to be an unusual path. It is remark- e to note, how Typhoons III and IV struck the coast near Fouchow, Typhoon IX in a lower latitude near Swatow, and Typhoon XII lower ll, near Macao.