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The earliest signs of a typhoon are clouds of the cirrus type-looking like fine hair, feathers or small pale white tufts of wool-travelling from the east or thereabout, their direction backing towards the north, a slight rise in the barometer, clear and dry but hot weather, calms or very light winds. If cirrus come from W, they prove that there is no typhoon. If they come from the S, then there may be a typhoon more than 600 miles to the southward. This fine weather lasts for days and the existence of a typhoon at a great distance is a cause of fine weather all round and contributes therefore to the safety of ships at sea, a fact that is not sufficiently appreciated by mariners.
The cirrus clouds, which frequently assume fantastic shapes, make their appearance within 1,500 miles of the centre of a typhoon. They show that water-vapour has risen about 6 miles in the atmosphere through the air rising near the centre. The barometer is generally rising beyond 600 miles of the centre, and the mean of the twenty-four hours' temperature rises in Hongkong above 81 deg. Halos round the sun and the moon, phosphorescence of the water and also glorious sunsets with grand twilight rays, appear to be frequently noticed before typhoons.
Cirrus radiate from various directions but when there is a typhoon, and the radiation is very distinct, it is usually from the same direction as the bearing of the centre.
An increasing swell in the sea is noticed from 300 to 600 miles from the centre, or farther off, but this depends greatly upon the situation of the nearest land, and particularly upon whether land intervenes between the observer and the typhoon centre. The swell arises from the heavy seas that accompany a typhoon, and form the real danger to ships, which are usually well fit to stand the force of the wind without too serious damage. As the velocity of waves in the sea is much greater than the velocity of the centre, the swell is frequently of great help in forecasting a typhoon, but it appears that the bearing of the centre is not well determined from the swell; besides the swell would indicate where the typhoon was at a certain time previous when the waves that caused the swell were raised by the wind. For instance, N of Formosa, i... between the E coast of China and SW Japan there is usually a heavy E to SE swell, when a typhoon centre is approaching from SE or E, and the sea there gets very high when the centre is yet at a great distance. A heavy cross swell is a certain indication of a typhoon, except near rocky land, where cross swell may be caused by reflection of the dir et swell. The cross swell arises from the heavy confused cross seas raised by strong winds from different directions round the centre.
Within 600 miles of the centre the sky is often half overcast with cumulus, above which cirro- cumulus is seen, the sky being frequently paled by high and faint cirro-stratus. S and SW of and beyond 200 miles of the centre, thunderstorms and cumulo-stratus are seen. They may also occur to the W and E of and as close as 250 miles to the centre, but there they are rare, at least during the NE monsoon. In fact, the belief of the Chinese that where there is a thunderstorm there will be no typhoon appears to be well founded. If thunder and lightning should appear to the N of the centre, little or no rain falls at the time. The old accounts of typhoons are probably sometimes to be explained as mere thunderstorms, while on the other hand during a typhoon the noise of the wind and waves might easily be mistaken for thunder.
On approaching nearer than 500 miles to the centre, the cloudiness increases, and the mercury begins to fall slowly (seldom as much as a tenth of an inch in 24 hours) in the barometer. Then the air becomes oppressive, a slight haze is observed during the morning hours, and the sky presents a threatening and vaporons appearance. The weather is then most unhealthy and depressing. Many people find it impossible to get any sleep owing to the very high night temperature.
All sorts of vermiu, including snakes, spiders, beetles, and typhoon flies (dragon flies), are unusually active.
Within 300 miles (or in different typhoons between 200 and 400 miles) of the centre there is a heavy cross sea, which therefore gets up some time before, and lasts longer than the wind. Within 250 miles in front of the centre the sky becomes overcast, and the temperature falls in consequence.
Within 200 miles of the centre the temperature falls quickly, owing to the heavy roll-cumulus with which the sky is densely overcast. About this distance, just in front of the centre, the air becomes sometimes abnorinally dry, and the sky at the same time presents a peculiar black and ominous appearance. And meantime the wind has risen and blows generally with the force of a strong breeze rising to a moderate gale in the squalls. But this depends also upon the bearing of the centre, the wind being usually strongest in the right-hand semi-circle. Within 200 miles of the centre there is usually a mountainous cross sea.
Within 200 miles to the N, and within 150 miles in front of and to the S of the centre, heavy rain begins to fall, and within 60 miles (or from 60 to 150 miles) it pours down in torrents. The temperature near the centre in Hongkong is often about 78° and over the China Sea 76°, but on board ship a temperature as high as 83° has been registered, but that is very unusual.
The dimensions of different typhoons vary much and near land the strong winds are often so irregularly distributed than in a place near the centre less wind may actually be experienced than at some distance farther away from it. The approach of the centre is judged by the fall of the mercury in the barometer and by the increase in the strength of the squalls.
No conclusions can be drawn with certainty from the reading of the barometer concerning the distance of the centre. The readings differ in different typhoons occurring during the same month and are also different in different months (owing to the annual variation in monthly mean pressure). Taking the mean of several typhoons I obtained at 40 miles 29.20, at 50 miles 29.30, at 100 miles 29.40, and at 200 miles 29,50.
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