660232-1888-Report-on-Typhoons-Observatory — Page 3

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THE HONGKONG GOVERNMENT GAZETTE, 7TH JANUARY, 1888.

11

South of 11° latitude the typhoons move on an average NWestward. Between 11° and 15° they move

NN Westward between 15° and 22° Northwards, between 22° and 30° NNEastward and north of 30° about NEastward. But it must be remembered that there is comparatively little information available about the paths to the East of the Philippines. If the data were as extensive there as in the China Sea the southern half of the mean path would probably be shifted to the East, and the mean curve would look more like a parabola. The mean curve given above must therefore be considered to represent the mean path of typhoons encountered, not the actual mean path of typhoons. By and by we may be able to lay down the mean path of each class of typhoons, and of typhoons in each month of the year. The longitude in which typhoons are most frequently encountered, is 125° E. The latitude is 1810 N. But there is a slight secondary maximum in 331° N.

2

The gradients, expressed in parts of an inch in 15 miles, corresponding to different forces (f) of the wind in typhoons have been calculated (g.), and compared with the values (g.) deduced from a comparison of the observed values by aid of Ferrel's formula in the following form :-

Se

sino sec

1667

V

873 r

-:-)

B v

30 [1 + 0.002 (t−32)]

in which designates the latitude, i the complement of the angle between the wind and the gradient, v the velocity of the wind in miles an hour, r the distance from the centre in miles, B the height of the barometer in inches, reduced to 32° and mean sea-level, and t the temperature of the air in degrees Fahrenheit. B is assumed-29, t=80, 4—20° and i-45°:-

до

f

V

2

go

0.02

6

34

200

0.015

.03

7

40

160

.02

.04

8

48

125

.03

.05

9

56

95

.05

.07

10

65

70

.08

.10

11

75

50

.13

.30

12

90

35

.26

The first part of the expression for go exceeds the second as long as the force of the wind is below a moderate gale. Thereafter the second part, depending upon the centrifugal force generated by the rotation, exceeds the first, which may be neglected when storm-force is reached and has no appreciable influence when it is blowing with full typhoon force

Assuming :----

sino sec i B

B

and b=

50010 [ 1 + 0.002 (t-32)]

26190 [10.002 (t-32).]

the velocity of the wind corresponding to a certain gradient in a certain latitude is determined by a quadratic equation having only one positive root :-

ar

1 +

1 + 2 2

2b

bg

a2r

The strongest typhoon during the past three years and perhaps as strong as any hurricane ever en- countered was that experienced on board of the S. S. Airlie in 12° N on the 16th July, 1885, when the gradient rose to about an inch in 15 miles. To gain an idea of the velocity of the wind in that typhoon the simple formula:-

V

gr 2b

suffices. According as r is assumed to have been 50 or 60 miles, we obtain v equal to 158 or 173 miles, and the velocity of the wind must therefore have reached about 150 miles an hour.

Hongkong Qbservatory, 13th December, 1887.

W. DOBERCK, Director.