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Very low clouds in a typhoon move with the wind, but if the clouds are higher they move round the centre in front and to either side, but more in towards the centre in rear. A heavy nimbus cloud passing to leeward causes heavy squalls, veering or backing regularly; and to windward it has the opposite effect. The cause of this is obvious.
The wind blows from a region where the air pressure is higher towards one where it is lower. It is, however, deflected towards the right in the northern hemisphere. The force of the wind depends upon the difference of pressure between one place and another situated in the direction where the barometric slope or gradient is greatest. The gradient is measured in hundredths of an inch per 15 nautical miles. The force of the wind corresponding to a certain gradient is greater the hotter the air is, and is different in a typhoon from what it is in the trade, owing to the path of the air particles being curved. They are almost logarithmic spirals, but somewhat different from such curves. A gradient of 0.01 corresponds to force 4, 0.02 to 6, 0.03 to 7, 0.04 to 8, 0.05 to 9, 0.06 to 91, 0.07 and 0.08 to 10. 0.09, 0.10 and 0.11 to 11, 0.12 and above this to 12. The steepest gradient usually met with is a third of an inch in 15 nautical miles. Gradients above this are rare, but sometimes they are much greater. The steepest gradient (1 inch in 15 miles) ever met with occurred in a low latitude in the Pacific. That corresponds to a wind velocity of perhaps about 160 miles per hour at sea level. Such velocities are not uncommon at an altitude of 2,000 feet in severe typhooùs. Anything above 80 miles per hour is called a typhoon. It is seen that there is as great difference between the force of one typhoon and another as between a calm and a storm which nearly reaches typhoon force.
When a typhoon is blowing it is of great importance to have a house well shut-up. Windows and doors should be firmly locked, bolted and barred. Damage is frequently caused by shutters being out of repair. Once the wind enters a broken window, it begins to blow through and its force is then quickly felt. As long as all apertures are thoroughly shut on both sides a fearful howling and whistling is heard, the rain blows in through the smallest openings and the house may shake, but damage is seldom done. Should a fierce squall get the chance to blow into a house, the roof is often the first part to give way. It is believed that pressure falls so quickly outside that the air confined in the house bursts through the roof like an explosion, but there is no foundation for that belief; it is more likely that a fierce squall would break through the windows and doors and through the roof as well. But if any fear is entertained of the air being confined inside, it is merely necessary to leave the chimneys open so that pressure inside will be nearly the same as on the outside.
In many typhoons the barometer, reduced to the temperature of freezing water and to sea-level, does not fall below 28.80 inches. In others it falls as low as 28.50. Lower readings are rare, but sometimes it falls much lower.
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No typhoon ever stands still. As soon as it is formed, it is carried forward by the prevailing wind. That is why the isobars are elongated, except near the centre where the force of the prevail- ing wind is of no account. The isobars could be circular only in a stationary typhoon. That is also why typhoons move so as to keep the areas of high barometer on their right, and so as to recede from areas where the barometer is high, and so as to approach low-pressure areas. Most of the typhoons that originate in the Pacific to the East of the Philippines or Formosa move Westward at first, then NW. then N., then they recurve to the NE, and beyond Japan they move Eastward. is under the influence of the high-pressure area in the Northern Pacific, which they rotate around in the same direction as the hands of a watch. When there are two typhoons about at the same time, they rotate round each other in the opposite direction, that is, abstracting from the influence of the high-pressure areas, which may cause them to move somewhat differently from this simple rule. In the China Sea there is sometimes a low-pressure channel between high pressures in China and in the Southern part of the China Sea. A typhoon in the Pacific at such times is attracted towards the China Sea and passes along the low-pressure channel, because the winds blowing to either side of this channel agree with the winds round the centre of a typhoon, and they move according to the principle of least action. During the typhoon season typhoons follow each other quickly, and there are often several at one time raging in different parts of the Far East. Then they cease, and there are none maybe for several weeks; but during the height of the season in August and September that is most unusual.
As explained above, the paths of typhoons in the Pacific look often like parabolas, but those in the China Sea are quite different and the difference must be due to the distribution and land and sea. The latter do not as a rule recurve, i.e., move North-eastward after having moved North-westward and Northward. Some of them, in fact, disappear in the China Sea after turning to the SW. Others recurve between 20 deg. N and 40 deg. Ñ, and between 115 deg. E and 130 deg. E. The Middle Dog lighthouse at the Northern entrance to the Formosa Channel is the centre of the region of recur
* Curved lines drawn on a map through places from which the same height (corrected and reduced) of the barometer is reported or between those that report a slightly higher and lower pressure are called isobara. The gradient lics at a right-angle to the isobar.
These are the most important elements for forecasting the weather. The curvature of isobars indicates the existence of depressions beyond the area where the telegraphic reporting stations are situated, but it is of course impossible to lay down the centre accurately from such data, e.g., while over the sea its position and motion can only be guessed at, not known with certainty.