101
Table II.
Mean Percentage of Days in each Month when Fog was noted at groups of Stations along
the China Coast during the five Years 1901-1905, inclusire.
January,
February,
March,
April,
May,
Jane,
July,..
August,
September,
October,
November,
December,
Month.
Hoihow to Breaker Pt.
Lamoeks to Wenchow.
Pei-ru-shan to
Wo
Foosung.
Shantung Pro- montory and Gulf of Pechili.
12
13
13
7
8
6
3
18
18
27
10
15
32
341
11.
4
20
31
15
3
8
26
18
1
11
27
2
+
11
I
1
1
1
1
1
2
3
1
The means thus arrived at have been plotted on a chart (Plate IV) which is attached. This shows at a glance the mean percentage of days in each month when fog was recorded in the four sections of the coast above mentioned.
As to the conditions under which fog is produced, it is found that the existence of areas of low pressure over the Continent and adjacent waters are the chief factors. In Spring the normal high pressure over the Continent is gradually disappearing and from time to time low pressure areas take its place. The North-east Monsoon under the latter condition becomes light or ceases altogether. At the same time there is an inflow of warm humid air from equatorial regions. We thus frequently have a surface current of light NE winds blowing along the coast of a relatively low temperature as compared with the warm moisture laden current coming from the South, the latter however not necessarily being surface currents but of low altitude. This is frequently seen by the wind records at Victoria Peak (1,816 feet above M.S.L.) and from the direction of the lower clouds. In such cases it is found that the vertical temperature gradient ordinarily obtaining between sea level aud the Peak is diminished or even reversed, the Peak sometimes having a higher tmperature than that at sea level. And it is in March that the least mean difference between the air temperature at the signal station and that at the Observatory (109 feet above M.S.L.) is found, the April difference being very little greater. The intermixture of these two currents of air of quite different temperatures is the chief factor in the production of fog along the coast,
The temperature of this air mixture is too low to enable it to contain all the water vapour present, hence it results that a part of the water vapour is condensed into fog or mist. The difference in these terms is one of degree only. In fog the water vesicles are smaller and more closely packed than in mist. Thus objects are visible at a greater distance when mist prevails than when fog prevails.
From what has already been statel it appears that fog is liable to occur whenever a warm humid air current coming from sea areas impinges on the much colder air lying over the coast, and that the occurrence of fog will practically cease as soon as the tempe- rature of the air over the coast areas has attained a temperature more in accord with the air currents coming from equatorial, regious. And on consulting the marine data furnished in the meteorological logs supplied by shipmasters, it is seen that fog rarely occurs in the China Sea at any considerable distance from the coast, Cases where fog was encountered beyond 100 miles from the coast are rare. But it more frequently happens that vessels run into fog on approaching the coast within about 50 miles.
A
A reference to the chart indicates that this is the case. We see the fog occurrence reaching its maximum in March on the S. coast and then slowly declining in April. rapid descent is then shown by the diagram coinciding with the partial establishment of the hot weather conditions in the South by the middle of May.. Thereafter very little fog is noted, although the absolute minimum is not shown till October.