« AnteriorContinuar »
atmosphere, in a direction contrary to the trade-winds, towards the polar limits of the latter, or to about 30° N. lat. and descended there.
Then the same principle was extended to dry, hot continental areas, where a powerful ascending current must exist on account of the heating by the sun, and this was proved by the great decrease of pressure in summer time.
Buchan extended the idea af ascending and descending currents further, supposing there was an ascending current over every area of low barometer, not only near the equator and on warm continental areas, but also in high latitudes, as on the North Atlantic, the North Pacific, etc. This air, he supposed, descended over areas of high pressure, as for example those existing in winter in Northern Asia and North America. Thus the supposition is, that the air flowing out of areas of high barometers, to a certain extent, comes from above, and again where the barometer is low, air ascends and flows in the higher strata, towards areas of high pressure.
I must repeat here, that this is a supposition, though a very plausible one, and that the actual facts which would prove the existence of such upper currents, with the exception of the so-called counter-tra-les in the region of the trade-winds, are very scarce. To these principles I would refer the direction of the wind at Dodabetta in the Neilghiris, in Southern India (above 8000 feet), which is nearly opposite to that observed in the lower strata in Central India, being from the N. W. in summer, that is, from the heated regions of the Punjab, where pressure is very low. In the lower regions, the winds on the contrary are S. E. and S., that is, the air is flowing towards Punjab. Another remarkable fact is the strong, constant, and warm W. wind observed in winter on some mountains near Lake Baikal. At that time of the year, the air is generally calm in lower regions, the cold intense, and pressure high. This west wind of the higher regions would thus seem to be a compensating current, flowing perhaps from Iceland towards the region of highest pressure of Eastern Siberia.
The observations on two of the highest peaks of the Rocky Mountains, above 14,000 feet, have failed to show an upper current of air blowing in a direction different from the lower one. As we have said before, our information as to upper currents is very scanty, and thus great caution seems yet necessary in drawing conclusions.
On the other hand, the influence of pressure on the winds near the surface of the earth is so well authenticated and reliable that we need not hesitate to base further conclusions on it.
The greater part of the earth being covered with water, we can first consider what would be the case if there were no intervening continents. What in this case would be the normal arrangement of pressure on the oceans? A belt of low pressure near the equator, a belt of high pressure at about 30° north and south, and a belt of low pressure about from 60' to 65°, after which the pressure would rise again towards the pole. This gives us three systems of winds at the surface of the earth, easterly (polar) in the lowest latitudes, westerly (equatorial) in the middle latitudes, and again polar in the highest latitudes, in each hemisphere. A
reference to the maps shows that, in the main, such is the actual arrangement of pressures on the oceans and on parts of the continents.
It is easy to see that this is the general conclusion arrived at by Prof. Coffin in his “ Winds of the Northern Hemisphere.” The main result is thus the same, the study of the winds, alone having shown that this is the case in a great part of the globe, while what we have said as to the pressure of the air shows at least the proximate cause of the prevailing winds. In how far this normal arrangement of winds is disturbed by geographical features, especially by the influence of the continents, will be shown later.
A further condition is the yearly movements of the belts of high and low pressure with the change of seasons. When the sun is in the zenith over the northern hemisphere, the seas under it will be more heated than the southern seas, and the equatorial belt of low pressure, which is also on the seas, the belt of highest temperature, will move northward. Owing to the great specific heat of the water, and consequently to the longer time it takes to cool, this northward movement will continue nearly to the end of the summer. On the other hand, the belt of low pressure in the higher latitudes will also move northward as the temperature rises near the poles, and the storm-tracks can take a more northerly course. The belt of highest pressure between the two of lowest must also take a more northerly position, as the air flows both north and south out from it. There can be no doubt that it holds an intermediate position between the two.
When the sun is in the zenith over the southern hemisphere, the reverse takes place: the equatorial belt of lowest pressure recedes southward, and also that in higher latitudes of the northern hemisphere, as the polar regions are so much cooled that the condensation of vapor there cannot sustain great barometric depressions. These normal or ideal conditions are realized to some extent on the surface of the present oceans, and are the more striking, the larger the bodies of water are. Generally the southern hemisphere has meteorological conditions which approach more nearly to the normal conditions than the northern. Thus, it will be seen by reference to the map of the isobars that the high pressure in about 30° really encircles the globe in the southern hemisphere, while in the northern, the pressure is highest in January at about latitude from 50° to 53° N. in Asia, and in July the pressure is very low, about 30° L. N. on the same continent. Again the low pressure about from 60° to 65o encircles the globe in the southern hemisphere, the difference of pressure under the different meridians not being great, and further south (especially from 70° to 78°) somewhat higher pressure and easterly winds are found, . In the northern hemisphere, on the contrary, the lowest pressure is found on two elliptical spaces, in the Northern Atlantic, about Iceland, and in the Northern Pacific, about the Aleutian Islands, that is, where a great extent of water prevails at about 60°, and the ocean is abnormally heated by currents of warm water.
We thus see that at a distance from the influence of water, the above-stated normal conditions are very much interfered with.
If the earth consisted mainly of continents without intervening oceans, very different conditions would prevail. As continents are more rapidly heated than oceans, temperature would be highest very soon after the passage of the sun
through the zenith of a parallel. The greatest heat in our summer would be about the Tropic of Cancer, in our winter about the Tropic of Capricorn, and this would also be the belt of low pressure at that time. The S. E. trade would cross the equator into the northern hemisphere when the sun has a northern declination, and the N. E. trade follow into the southern hemisphere during the other half of the year, giving a variation of the inner limits of the trades of perhaps 40°, instead of the 10° or 12° which are now observed. Further, as dry continents cool also more rapidly, the cold in the polar region of each hemisphere during the winter would be more intense than now, extending to the whole polar region, and coinciding with a very high pressure.
These hypothetical conditions are much more imperfectly realized than those I have sketched before, as the extent of continents is much less than that of oceans. The nearest approach to realization is on the greatest continent, that of Asia, where the highest pressure of winter is a little north of 50° N. If it is not found further north, it is because the continent does not extend much beyond 73° N. In summer, on the contrary, we find the highest temperature in N. W. India between 30° and 35° N., and also the lowest pressure there and in N. China.
The larger the continent the more it approaches to the ideal conditions I have supposed. In Africa, for example, there is a belt of lowest pressure in summer at about 17° N., and the highest temperature is probably still more to the north.
The narrower continents of North and South America are more under the influence of oceans than Africa,
As already seen, the highest mean pressure on the surface of the globe is found in winter on the Asiatic continent. It is necessary to mention here a feature of the climate of this continent, explained by geographical conditions, which has a great influence on the winds, namely, the steadiness of pressure in winter. Pressure is so constant here that, though the barometrical range generally increases with latitude, it is not greater at Jakutsk in N. E. Siberia, under 62° N. L., than in Vienna in Central Europe, Lat. N. 48°, or even in St. Louis in North America, under 39° N, L. The coldest and heaviest stratum of air over Eastern Siberia is prevented from flowing towards the south and east, where pressure is low, by the intervening mountains and plateaus, from 3000 ft. to 5000 ft. high. So long as the cold of winter continues, pressure must, therefore, be high over the cold region of Northern Asia. As it is low in the Pacific Ocean and the equatorial regions, air will flow there from the region of high pressure above the mountains and plateaus. But, as above said, the coldest and heaviest lowest stratum cannot flow towards the Pacific on account of the intervening heights; the quantity of air moving in this direction will not be great enough to supply the deficiency. Thus pressure being lower the whole winter in the S. and E., the winds should be regular from the N. and W., and this is really the case.
On the whole southern and eastern slope of Asia we see a mutual reaction of continental and oceanic influences—the great monsoons. The Europeans were first made acquainted with the regular change of wind and weather in India through the campaigns of Alexander the Great. Not only did the Greeks see this change themselves, but they also learned from the natives with how great a regularity.this
change took place; how in all this region the winter was the dry, clear time of the year, and summer the rainy season. The navigators of the sixteenth and seventeenth centuries knew that the monsoons extended much further east than India—to the Indo-Chinese Peninsula, the Sunda Islands, and Southern China.
The cause of the monsoons is this: in our winter the continental regions of Asia are cooler than the surrounding seas, and pressure is higher. The air flows from these towards the equatorial calm-belt in the Indian Ocean, and towards the region of low pressure in the Northern Pacific, as a N. E., N., N. W. or W. wind. As the pressure is continually lower on the seas than on land at this season, this flow of air is very constant. As the air comes from the interior of the continent, and generally also from higher latitudes, i, e., from colder regions, the season when these winds prevail will be a dry season, as the vapor contained in the air will be further and further from its point of condensation the further south and east it flows.
In our summer, pressure is very low over a great part of the Asiatic continent, owing to the heat and ascending current produced by it; therefore the air of all surrounding regions will flow towards Asia, and the movement will be especially rapid in and near Southern and Eastern Asia, as the greatest oceans of the world, the Indian and the Pacific, approach Asia in this direction.
Pressure is higher on the oceans in summer on account of the comparatively cool temperature which prevails there. Thus the movement of air will be reversed, and the wind in summer will blow from the S. W., S., S. E. and E. This summer monsoon will also be very steady, as the difference of pressure is nearly always in one direction during the whole summer-lower on the land.
Not only is the direction of the movement of air different in summer from that prevailing in winter, the influence on the weather is also different. As the air drawn towards Asia has to pass over a great extent of warm equatorial seas, it is laden with vapor, and this vapor will be deposited in copious showers, especially when it meets a mountain chain, which compels it to rise into higher and cooler regions of the atmosphere. Thus the summer monsoon is the time of cloud and rain for all Eastern and Southern Asia, or the wet monsoon. There is no doubt that the condensation of vapor, giving out its latent heat, is a new and powerful cause for the continuance of the movement in the same direction.
The influence exerted by the heated continent of Asia is so powerful that there is no equatorial calm-belt in the Indian Ocean during our summer, but pressure decreases steadily from about 25° S. L., the polar limit of the S. E. trade, till about 30° N. L. in Northern India, the S. E. trade crossing the equator, and being thus converted into a S. and S. W. wind. On the eastern coast of Asia the tendency of air to flow towards the continent similarly acts on the N. E. trade of the Pacific Ocean, which is drawn in as an E., S. E. or S. wind. We see here the normal or oceanic conditions very seriously disturbed by the influence of the great continental mass, Asia.
I must correct here an error which is frequently made, i. e., limiting the monsoons to the tropical part of Asia, i. e., India, Indo-China, and Southern China. Even on the new Pilot Chart published by the British Admiralty in 1872, this
error exists. It can be easily explained thus: in the tropical seas adjoining India and Southern China, the direction of the wind is N. E. in winter and S. W. in suinmer, and seamen were accustomed to consider as monsoon regions those only where winds of this direction were found. The further north we proceed along the coast of Eastern Asia the more the summer winds become S. E. and E., and the winter winds N. W. and W., yet there is good reason to consider Eastern Asia to the 60° N. L. as belonging to the monsoon region, because here also the winds in winter are from the land; in summer, from the sea, they bring dry, clear weather in the first season, and rain in the second; and last, not least, at both seasons they are very constant. (See Maps, Plates 5, 6.)
For these reasons I consider China, Japan, Mantchooria, the basin of the Amoor River, and the western coast of the sea of Ochotsk, as belonging to the monsoon region.
As to the constancy of the winds I would remark, that the inner regions of India, as, for example, the northwest provinces, Oude, Central India, Punjaub, are generally considered as being situated in the monsoon region, yet the winds are not so constant here as in Japan and the Russian Amoor Provinces.
The continent of Australia may also be considered as belonging to the monsoon region, only the periods are reversed, i. e., our winter is the rainy season there, our summer the dry time. At this season regular S. E. winds are experienced in the northern part of Australia; they may be considered as the S. E. trade, strengthened by the comparatively low temperature and high pressure on the continent. They blow towards the Sunda Sea, and, further on, cross the equator, to appear as the S. W. monsoon on the coast of South China. In our winter, on the contrary, pressure being highest in Asia, and very low in the dry, hot interior of Australia, the N. E, monsoon of China crosses the equator and appears as a N. W. monsoon, bringing clouds and rain to the northern coast of Australia. In these meridians the juxtaposition of the continents of Asia and Australia on the north and south of the line, gives additional strength to the monsoons. Here no equatorial calmbelt is found, neither in our summer nor in our winter, while it exists south of India in the Indian Ocean, as there the monsoons can be said to be single, caused by the Asiatic continent alone, while further east they are double, Asia and Australia both exerting an influence. It
may be asked why the whole Asiatic continent, being equally heated in summer and the air rarefied, does not exhibit monsoons of equal magnitude coming from the Arctic and Atlantic Oceans?
The reason is this: on the Arctic Ocean, pressure is also low in summer, though probably not so low as indicated in Buchan's map of isobars, and besides it is not steady, as on the tropical seas. Yet there is a northern wind coming from the Kara Sea, and blowing through Western Siberia to Central Asia, but it is not as steady as the monsoon of India and China. Besides, as this wind comes from a colder region, it does not bring rain, and thus the secondary influence-condensation of vapor, which is instrumental in producing the monsoons of Eastern and Southern Asia-is not effective here. There flows also a current of air, and a very powerful one, from the Atlantic Ocean towards Central Asia; but, as it