« AnteriorContinuar »
southerly winds in winter. The following are the percentages in winter on the northern coast of Nova-Zembla.
S. E. S. S. W. W. N. W. 75° 55' N., 590 0' E. 21 17
20 11 14 21 11 4 Here it seems that the winds blow from the land towards the partially open sea, with its low pressure and high temperature. By winds from the land I mean here local winds from the island itself, as also those from the cold Siberian continent.
We have seen before that prevailing westerly winds extend to the Jenisei. Farther north and east we have but very few observations. It seems that we have here the region of polar calms in winter. The number of calms increases towards the interior and N. E. of Siberia, till at last there can be said to be no prevailing wind. This is the region of highest pressure in winter, as shown on Plate 14, and of also the greatest cold. Here, unlike the American polar regions, the cold of winter is very permanent, and also high pressure. The cold is not brought by winds, but is generated on the spot by radiation.
I give below the percentages of winds as observed at some few stations.
In the first three places, situated in the vicinity of the Arctic Ocean, there is a decided prevalence of monsoon winds—from the land in winter, from the sea in
The mean direction at Nijnikolymsk' is inSummer, N. 58° E. .48: Winter, S. 16° W. .48: Year, S. 11° E. .25.
The direction of the winds in autumn and spring is probably nearest to that of winter, as may be expected from so high a latitude, where the land is colder than the sea a great part of the year. Thus the mean yearly direction is nearly S. The direction of winds on the northern coast of Siberia is about the same as on the shores of the White Sea (Archangel and Kem).
It is difficult to determine the reason of the frequent N. winds at Yacoutsk, if the air flows towards the Pacific Ocean and is deflected from its true course by the direction of the valley. At any rate, calms are the prevailing feature in win
| The detailed calculations on the winds at this place were published by Spassky in his “Sibirski Vjestnik,” year 1823. have used here only the figures given by Wesselowski, p. 231, as I could not obtain the original.
In the summer, winds from N., E., S., and W. are about equally frequent. It seems that in September and October, when westerly winds are so prevailing in Western Siberia, warm and moist currents of air from the Atlantic can extend to Yacoutsk. At least westerly winds reach the maximum of their frequency in October (20 per cent.).
In this month the flow of air towards Central Asia has ceased, while pressure has not risen high enough at Yacoutsk to prevent westerly winds from the Atlantic. October is also the cloudiest month of the year, the amount of clouds being 6.9, while March has only 2.6. The number of rainy days then is also the greatest in the year.
At the mines of Nertschinsk calms are more prevalent than at any other station we know of. In the winter months 65 to 70 observations out of 100 show no movements of the air, and the recorded winds are generally weak. In spring and summer there are less calms and more strong winds. The basin of the Upper Amoor is thus shown to belong yet to the region of Siberian calms (in winter).
While this is the case in the lowlands and valleys, it seems that the conditions are different in higher regions of the atmosphere. At Mount Alibert, 200 miles west of Irkutsk, and over 7000 feet high, a very constant and strong W. N. W. wind
is observed. This place was inhabited some years on account of rich mines of ✓ graphite, and it was necessary to erect a wall to protect the inmates from the violence
of this wind. The mean temperature was found to be much higher in winter than in the same latitude in lower levels. This wind is probably the upper current flowing towards the Siberian pole of highest pressure. It has been supposed that such upper currents flowed towards all regions of high pressure, but this has been proved only for the polar limits of the trades.
MONSOON REGION OF EASTERN ASIA.
Southeastward from the coldest space of Siberia, towards the Pacific Ocean, we have the region of Asiatic monsoons. I have already explained the cause of the movement of air in this region, and it is only necessary to show how far it extends and how small our knowledge of the northern part of the monsoon region was until the last year. The percentages of the winds in winter and summer are given in the annexed table:
The mass of air which is drawn towards the Asiatic continent in summer is so great that the ordinary conditions prevailing over extensive areas of the oceans must be disturbed, as shown on Plates 5 and 14. As there is also a great mass of air drawn towards India and Indo-China, we must here consider Eastern and Southern Asia together.
The summer monsoon of Asia is a deflection of air already in motion, that is of part of the S. E. trade of the Indian Ocean and part of the N. E. trade of the Pacific Ocean. It is easy to prove this for the Indian Ocean, as the observations there are numerous and well discussed. This is not the case for the Pacific Ocean. Yet seeing a region of high pressure about 30° N. to the E. of China, it is impossible to conceive how the air from above it should not be drawn towards the heated Asiatic continent with its low pressure. Probably at the beginning of the summer monsoon, only the air over the nearest parts of the ocean is drawn towards Asia, and the circle extends as long as the pressure continues to sink over the continent.
The direction of the winds in summer on the coast of E. Siberia, as well as in China and Japan, shows that they cannot have come from the southern hemisphere, as they otherwise would have a direction from the S. W. as in India, and not E., S. E., or S. It seems that the air from the Pacific supplies the northern part of this region, from about 250 to 60° N. In Southern China the prevailing winds are already S. W., so that this is probably air from the southern hemisphere. (See Plates 5 and 6.)
As in summer the Asiatic continent attracts the winds, so, on the contrary, in winter a continuous stream of cold dry air pours out from it towards the surrounding
It takes mostly two directions: towards the depression in the northern part of the Pacific as S. W., W., and N. W. winds, and towards the equatorial region as a N. E. On the coast of E. Siberia, in northern China and northern Japan the winds are mostly N. W., in southern Japan and middle China they are N., and near the tropics they have a direction from the N. E.
The climate of the whole monsoon region is characterized by a great regularity. This is not only the case in the tropics, but also in the temperate zone. The periodicity of the change of monsoons is the leading feature, taking place at more or less fixed periods, with slight changes from year to year. The N. monsoon of winter is the dry time of the year, the summer or S. monsoon the time of clouds and rain. So, for example, at Pekin the amount of clouds is 2.5 in January and 6.3 in July, at Ochotsk, Ajan and Nikolaievsk (Amoor) 2.5 in January and 5.0 in August (an entirely clear sky = 0, an entirely overcast = 10). At Pekin the quantity of rain in July is more than fifty times greater than in January.
As this distribution of rain and clouds is caused by the monsoon, which brings the dry, cold air of the continent in winter, and the vapor-laden air of the sea in summer, thus causing the above-mentioned periodicity, we have means of judging of the character of the climates of this region even without having observations of winds. For a great extent of country, in China and Mantchooria as well as in eastern Siberia, we have no long-continued observations, yet the general character of the climate is known. Thus we must include in the monsoon zone, besides the tropical countries of India and Indo-China, all of China and Japan, Corea,
Mantchooria, the Amoor provinces and the western coast of the sea of Ochotsk, till about 60° N. L. (See I'lates 5, 6, 7.)
As this last extension of the monsoon zone is not generally accepted, it is necessary to give some further details. I have already stated that on the lastnamed coast the cloudiness is double in summer of that of winter. The E. winds of summer and the W. winds which set in September or October lasting all winter are so well known to the inhabitants that they sail in July and August from Kamtschatka to Ajan or Ochotsk and return in September or October, having in each passage
favorable winds. The rains have also a marked monsoon character at Ajan, only they are somewhat delayed, the largest amount falling in August and September. This is due to the great masses of ice in the sea of Ochotsk, which disappear only in the end of summer. So long as the sea is colder than the land, precipitation can not be copious, which is the case until August and September when the sea is warmer than the land.
As to the upper Amoor, the small amount of snow falling in winter and the abundant rains of summer also tend to show that this region is under the influence of the monsoons.
I give below the percentage of the prevailing winds of the different months at Hakodade (42° N. L.) and Nikolaievsk (53° N. L.) to show with how much regularity the change takes place in these northern latitudes, which were till now considered as not belonging to the monsoon regions.
India and adjacent regions have been long known to the Europeans as the classical country of the monsoons, though as we have seen their course is not less regular in China and Japan. There is a reason why the mind is more impressed with their regularity in the Indian Seas; owing to the low latitude, there is scarcely any dif . ference of temperature between winter and summer. The change of the season from wet to dry and vice versa is then the only conspicuous feature in the course of the year. In China and Japan the difference of temperature is greater between the two seasons, and these changes more attract the attention. The inhabitant of a temperate zone finds here the habitual difference between winter and summer, and thus considers this climate as resembling his own, different as it may be in the course of the winds and the period of rains. The atmospheric pressure of the monsoon region is illustrated on Plate 14, the winds on Plates 5, 6 and 7.
SUNDA AND PHILIPPINE ISLANDS.
In the seas south of Indo-China there is a double system of monsoons. The S. E. trade crosses the equator in our summer, and gradually is changed to a S. and S. W. wind, while during our winter the N. E. trade crosses into the southern hemisphere, by and by assuming a direction from N. W. This last movement is caused by the heating and rarefaction of the air over Australia.
The Sunda Islands, being situated near the equator, are under the influence of both monsoons. The one or the other of them can bring rain, and this depends much more on local causes than on the situation north or south of the equator. The direction of the wind in this Archipelago and the surrounding seas is not only governed by the flow of air towards Asia and Australia (the great monsoons), but also by the heating and rarefaction of the air on the islands themselves, especially on the largest, Borneo and Sumatra. Even on the island of Java, narrow as it is, there are great irregularities in the course of the monsoons caused by day and night winds, at least at some seasons.
I give here the mean direction of the winds at Batavia, from the elaborate discussion of the observations made at this place by Dr. Bergsma, director of the Observatory.
It will be seen that the west monsoon (in our winter) is much more regular than the east monsoon. Besides, in the last season, the mean direction of the wind is to the N. of E., while the S. E. trade should be expected.
This is probably due to sea and land winds, which blow more regularly and strongly, as this is a comparatively dry season.
I give next some percentages from this region, adding the Philippine Islands, where the extreme regularity of both monsoons is remarkable, while the Sunda Islands show more local deflections.
1 An excellent sketch of the winds of Java, by Lieut. Jansen, is published in Maury's “Physical Geography of the Sea.”