Sessional_Paper_1898 — Page 110

106

20

W-

£

کیا

سلام

7

N

下

365

653

IJA

L

100 miles

n

Typhoon of July 17

1890

The average rate of progress of the centre of a typhoon in 11 deg. latitude is 5 miles an hour. In 13 deg. it is 6. In 15 deg. it is 8. In 20 deg. it is 9. In 25 it is 11. In 30 deg. it is 14, and in 32 it is 17 miles an hour. South of 13 deg. the speed does not vary perceptibly, so it is useful for mariners to know it, but it is more variable the farther north you go. In 324 deg. N it ranges from 6 to 36 miles an hour, so that you cannot be sure that a typhoon encountered there will travel at anything like the average rate of speed. We have not traced the centre of a typhoon nearer to the Equator than about 9 deg. N. But a very slightly falling barometer, a squally SW wind, a lumpy sea, and some swell, may at times be traced nearly all the way down to the Equator.

The prevailing wind not only carries the centre along with it, but combines with the rotary storm, causing the wind in the right-hand (the dangerous) semi-circle to be stronger and to blow more nearly round the centre than in the left-hand (the manageable) semi-circle, where the wind is more moderate and blows with greater incurvature towards the centre. It also causes the wind to blow nearly straight in towards the centre behind the typhoon and to blow more across the path in front of the centre. It also makes the weather heavier after the centre is past than it was while the centre was approaching.

Less than half a mile up in the air the incurvature of the wind towards the centre disappears in the

average of the different quadrants, but it still blows in towards the centre in the rear. It is really the wind at this altitude that carries the typhoon along, for late in autumn there are every year typhoons that move along against the NE monsoon, but we know that that monsoon is at times very shallow and there is SW wind above it. These typhoons disappear sometimes suddenly; evidently when the NE monsoon increases in depth and intensity.

At a still higher level the air, which has been carried in towards the centre and raised over the area where it is raining, blows away from the centre, and as the friction of air against air under low pressure is insignificant, it sometimes rushes away with such speed as to cause the upper air to be sucked down into the central calm. This is the reason why the sky clears over the bull's-eye.

Typhoons originating in the Pacific in a low latitude (say 13 deg. X) are very small and very fierce. The isobars are nearly circular, as the centre moves very slowly, and the incurvature is 45 deg. in all directions; but there is this important difference between a typhoon and a tornado-that the latter is taller than it is broad, whereas the former forms a flat disc. As typhoons reach a higher latitude their dimensions become greater, the violence of the wind near the centre abates, and then there is nothing to distinguish them from storms originating in northern latitudes. This makes it most unlikely that the latter originate from causes at all different from those which give rise to a typhoon.