185
The approximate hourly intensity of rainfall i. e. the hourly rainfall divided with the number of hours, during portion of which it rained, or heavy dew fell, was calculated for the mean of 1884 and 1885 and indicates, that the rain is heaviest in the middle of the day and lightest about midnight:-
1 a. 0.103
7 a. 0.127
2
.154
8
92
.175
3
.143
9
.184
22
.109
""
6
.118 .134
10 11
.116
.202
Noon .140
1 p. 2
0.165
.120
7.p. 8
0.127
.112
11
3
.107
9
.080
22
.080
10
.091
.098
11
.087
,
6
.141
Midt.
.070
The true mean hourly intensity came out as follows in 1884 :—
January, 0.000; February, 0.037; March, 0.117; April, 0.072; May, 0.116; June, 0.148; July, 0.150; August, 0.155; September, 0.252; October, 0.119; November, 0.045; December, 0 000; Year, 0.101.
and in 1885 :—
January, 0.029; February, 0.028; March, 0.035; April, 0.179; May, 0.085; June, 0.373; July, 0.175; August, 0.218; September, 0.104; October, 0.100; November, 0.063; December, 0.039; Year, 0.119.
This was obtained by dividing the monthly rainfalls in Table XI with the duration. The intensity is a maximum in June, a minimuin in January and was greater in 1885 than in 1884.
Table VIII exhibits the velocity of the wind expressed in miles per hour and the excess of the hourly values above the mean at the Observatory. The velocity at the Peak has been computed from the force estimated there at 10 a., 4 p. and 10 p. The usual daily variation (maximum at 11 a. and minimum at 10 p.) is seen to be greater in summer than in mid-winter during the months when the velocity was greatest, while it was least in September. The velocity is much greater at the Peak than near sea level even taking into account that it is observed principally during the day, and the annual variation is scarcely seen, though there appears to be a maximum in June. The daily variation is probably also small. The increase in wind-force with the height above sea level being small in winter and great in summer agrees with the circumstance that the SW monsoon extends so much higher up than the NE monsoon, which in consequence is subject to a certain amount of friction at its upper surface where the direction of the wind is veering.
In order to throw light on the maximum of wind-force mentioned in the monthly reports of January and February, 1884, as occurring in the early morning hours, and which is often noticed during strong East winds early in the year, the mean force of the wind at South Cape (Formosa) was computed from the three-hourly observations made from the 1st of August, 1884 till the 31st July, 1885 inclusive. These observations are made carefully by the staff of the lighthouse. The observers stand about 150 feet above sea level, but as they are occasionally guided in their estimations by the amount of sail carried by sneh sailing vessels as happen to be visible, the figures may be taken' to represent the force of the wind at a lower level.
Mean Force of Wind at South Cape (Formosa).
Year. 1884. August,
Mouth.
3 a. 6 a.
9 a..
Noon.
3 P.
6 p.
9 p.
Midt.
Mean.
3.4 3.0
2.9
2.9
3.2
3.0
2.9
3.0
3.0
"}
September,....
3.7 3.2
3.1
3.2
3.1
3.1
3.2
3.6
3.3
11
October,....
4.2 3.8
3.8
3.8
3.2
3.5
4.1
4.3
3.8
November,.
4.8 4.6
4.3
4.3 3.8
4.0
4.4
4.6
4.4
December,
4.1 4.0
4.0
4.1
3.8
3.4
3.7
4.2
3.9
1885. January,
3.1 3.2
3.0
2.9
2.6
2.6
2.7
3.0
2.9
"}
>>
February, March,. April,
4.0 4.0
4.0
3.7
3.4 3.3 3.4
3.7
3.7
3.1 3.2 3.0
2.8 2.8 2.7 2.8
3.1
2.9
2.8 2.5
2.6
2.5
2.2 2.3 2.3
2.6
2.5
2.6 2.6
2.7
2.8
2.6
2.5
2.1
2.5
2.6
2.3 2.4
2.2
2.2
2.5
2.1
2.0
2.2
2.2
""
Mean:
2.8 2.9 3.4
3.3 3.2 3.2
2.8
2.8 2.9
3.0
2.7
2.8
2.8
3.0
3.0
3.0
3.3
3.2
May,
>> June, July,
It is seen, that the maximum force of the wind occurs at 3 a. and the minimum at 6 p. which no doubt closely correspond with the epochs of extreme force over the open sea, the effects of which during strong Easterly wind were traced in our records.
Table IX exhibits the mean direction of the wind at the Observatory and at the Peak. The direction at the Peak is to an ebserver facing the wind about a point to the right in winter and over two points in the same direction in summer. The excess of the hourly direction above the mean, at the Observatory expressed in degrees, is counted from North through East towards South. The veering of the wind during the day is small but greater during the southerly monsoon. It was greatest in September.