Sessional_Paper_1906 — Page 184

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The diagrams for the other districts show much the same sequence of fog occurrence, the maximum however, occurring later as we go North. Thus we see that in June when fogs have practically ceased in the South, the maximum has not yet been reached around Shantung. Although the absolute maximum occurs both in the Formosa Channel area and the Yangtze area in April, the decline is a rapid one in the former while the latter has a prolonged period of fog extending towards the end of June before any rapid decrease takes place. The diagram shows the Chusun Archipelago and the estuary of the Yangtze to be the foggiest part of the coast, not only as to the absolute maximum but as to the length of time a large percentage of fog is recorded.

With the complete establishment of the hot weather condition over the Continent fog practically ceases to occur, except the purely local fogs, which occasionally occur at individual stations in the summer, and which are usually caused by the cooling of the ground by radiation. This class of fog occurs usually in the evening or early morning hours and is quickly dissipated by the warming up of the air after sunrise.

It might be assumed that in autumn when the NE monsoon sets in a second period of foggy weather would result. But this is not the case. For while the change, commenting in spring, from winter to summer conditions is a gradual process, that commencing in autumn to the opposite condition is comparatively abrupt. Moreover the extreme dryness of the air, as yet of a relatively high temperature, hich now comes from the Continent, does not favour the formation of fog, and in fact we see by the chart that it is at this time of year-September and October--that the minimum occurs along the coast, perhaps slightly earlier in the North than in the South.

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Towards the end of the year fog begins to increase again. The air over the Continent has now become much colder and any inrush towards the coast of relatively warm air, such as happens when a depression forms inland, is liable to cause fog.

There is a slight secondary maximum in January and a dip in the curve in all districts in February. This tends to show that areas of low pressure were more frequent over the Continent in January than in February, or that the anticyclonic condition was more marked in the latter month. A rapid survey of the pressure conditions prevailing in these two months indicates that this was the case. For although the central areas of anticyclones in Northern Europe are usually characterized by fog, the effect in this respect, of the passage eastwards over the coasts of N. China of such areas, appears to be snall, But the outflow of air here comes from a vast continental area, the air accordingly being exceedingly dry, while in N. Europe it passes to a great extent over sea areas.

To the navigator the practical importance of the results arrived at is this, that about the beginning of the year whenever it is known that low pressure areas exist over China or adjacent waters he may expect to find more or less fog along the coast, its occurrence be- coming more and more frequent and its character probably denser, until the period of the when the maximum of fog obtains, is reached on the different parts of the coast.

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On the other hand when China is covered by high pressure areas he may be tolerably sure, that he will not meet with fog along the coast, but he will, at times, find a good deal of haze. For example, in February, 1905, during the period 8th to 17th inclusive, anticyclonic conditions prevailed over China, and not a single station on the coast recorded fog. The mean height of the barometer in Hongkong for the period named will be a sufficient index as to ressure conditions over China.

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It was 30.26 or 0.11 inch above normal. From the 18th to 25th fogs were general over the S. coast of China, the Formosa Channel and the E. coast as far North as the Chusan Archipelago, corresponding to a period of low pressure over S. China and the Eastern Sea. The mean height of the barometer in Hongkong for this period was 29.93 or 0.17 below the normal.