« ForrigeFortsett »
atmosphere, the heated air which rises from them; and the pressure is still farther increased by the upper current flowing from the region of calms. Now the North Atlantic is the ocean which most fully meets these conditions, being surrounded by the heated land of North America, South America, and North Africa. Consequently this ocean shows the greatest excess of atmospheric pressure as compared with other parts of the earth's surface in the same latitudes. The South Atlantic shows also a pressure as high ; but this pressure is due both to the upper currents from South America and Africa, and to the increased pressure which prevails over the whole of the southern hemisphere at this season.
100. The extension of the area of high pressure from the North Atlantic into South-Western Europe is instructive. Taken in connection with what has been said, it brings before us in an impressive manner the subordinate position of Europe as a simple peninsula thrown out from the great Asiatic continent. The effect of the Mediterranean, Black Sea, Caspian Sea, North Sea, and Baltic Sea, in deflecting and otherwise determining the position of the curves, is very interesting. On a larger scale, and with the isobarometric lines drawn for every half-tenth of an inch of pressure, the influence of these seas on the atmospheric pressure, and thence on the summer climate of Europe, would more clearly appear.
101. But moist air, or air charged with the vapour of water, is considerably lighter than dry air; consequently when moist air accumulates over any region, an ascending current takes place. But as moist air ascends into the upper regions of the atmosphere, it is cooled below the point of saturation, condensation follows, and rain is precipitated. In the act of condensation heat is liberated, which, by heating the air in the higher parts of the atmosphere, tends still further to diminish the pressure, and so accelerate the ascent of the current. This influence of the vapour on the isobarometric curves is illustrated in different places in Plate I.
102. In the North Pacific, where at this season the two tradewinds meet and mingle, there occurs a belt of low pressure,
caused by the vapour accumulated by these constant winds.
In the above two cases the belt of lowest pressure marks out the region of calms and of constant rains, which will be re. ferred to further on.
103. The crowding together of the isobarometric lines in the south and east of Asia is caused by the vapour which the summer monsoon brings to these regions, and which is there precipitated in a copious rainfall. The low pressures which prevail during all seasons in the Antarctic Ocean are no doubt due to the saturated state of the atmosphere resulting from the N.W. winds, which blow thither from an almost unbroken sheet of waters, which embraces the South Pacific, the South Atlantic, and the Indian Ocean, and which meet with little land to condense the vapour till they flow within the antarctic circle.
104. MEAN ATMOSPHERIC PRESSURE FOR JANUARY—PLATE II.—Since, speaking generally, the same conditions which bring about a high atmospheric temperature in summer from the effects of solar radiation, also bring about a low atmospheric
* These figures are the means of the observations made by Captain Henry Toynbee, R. N., during five voyages to India, the results of which are given in an excellent paper which was published in the 'Proceedings of the Royal Society,' June 15, 1865.
temperature in winter from the effects of terrestrial radiation, we should expect to find the relations of the two hemispheres to each other in respect of atmospheric pressure reversed in January as compared with July. An inspection of Plate II., giving the isobarometric lines for January, shows that this is the case.
Here we see the red lines, which indicate a pressure above the average, almost wholly confined to the northern hemisphere; whereas the blue lines, showing pressures below the average, are, except in two small patches, the only pressures prevailing in the southern hemisphere.
105. An extraordinary excess of atmospheric pressure extends over Central and Northern Asia, rising in the centre of that region to upwards of 30.400 inches. It is here that at this season the greatest degree of terrestrial cold known to exist takes place, the mean temperature falling to –440.0 at Yakutsk, which is 285 feet above the sea. The highest atmospheric pressure does not occur at this point, but at a considerable distance westward, near the sources of the Obi and Yenisei. The non-coincidence of the areas of greatest cold and greatest pressure may be explained by the proximity of Yakutsk to the area of low pressure in Kamtchatka, which may properly be regarded as a drain lowering the pressure of Eastern Asia. The effect of the mountain-systems of Asia, in particular the Yablonoi or Stanovoi Mountains, in shutting in the cold dense air within that region, and thus tending to increase the pressure, may be here referred to. Indeed, but for this high mountain-range, the singular distribution of the pressure which is seen to obtain from Yakutsk eastwards to Kamtchatka could not be maintained, and in all probability the whole winter pressure of Central Asia would be materially reduced. The sources of this high mean pressure must be upper currents flowing towards it and over it from the four regions of low pressure,-lying respectively to the east, the south, the south-east, and the west.
106. This area of high barometer is continued westward through Europe south of the North Sea and the Baltic; the north of Africa; the North Atlantic between 15° and 45° lat.; North America, except the north and north-west; and the Pacific as far west probably as 150° W. long. Its breadth, as was to be expected, is doubled in North America, as compared with the Atlantic. As I have no observations from the central parts of the North American continent, the winter pressure of that region is given only hypothetically. It is, however, probably correct, since there is no mountain barrier stretching east and west across the continent damming up the dense cold air and preventing it from flowing over the United States. The effect of the Mediterranean and neighbouring seas, which are at this season warmer than the land surrounding them, in lowering the mean winter pressure, and thus breaking the continuity of the isobarometric line of 30.2, and preventing its extension from the Pacific to the mouth of the river Lena in Siberia, is well deserving of attention.
107. In addition to the Antarctic and Equatorial depressions, there are four areas of diminished pressure: two caused by their high temperature-viz., South America and South Africa ; and the other two by their being vast reservoirs of moist air-viz., the northern part of the Atlantic, and the northern part of the Pacific, and the parts of the continents adjoining. In the South Atlantic, between the two regions of low pressure, there is seen a space of high pressure, which has its origin doubtless in the overflow of air from the upper currents which set in towards it from the heated regions of South Africa and South America. The influence which this peculiar distribution of atmospheric pressure exercises on the storms of the Cape of Good Hope and of the southern parts of the Indian Ocean at this time of the year will be afterwards pointed out.
108. Over the North Atlantic occurs an extensive diminution of pressure, which deepens northwards till the greatest depression, 29.5, is reached in Iceland, or perhaps in a slightly lower depression nearly midway between that island and Spitzbergen. The widening of the iso barometric curves of 29.6 and 29.7 inches to the westward over Greenland, and to the eastward over the north of Norway and Russia, is an
interesting feature of this area of low pressure. The low pressure of this region is due to the saturated state of its atmosphere and to the copious rainfall resulting from it. The flow of the Gulf Stream north-eastwards through the Atlantic to at least beyond Spitzbergen, and the larger amount of vapour poured into the atmosphere from its warmer waters, tends still further to lower the pressure. It is this low pressure over the North Atlantic, together with the high pressure to the eastward over Asia, which forms the key to the explanation of the winter climate of Europe.
109. Another, and even a more remarkable depression, occurs in the North Pacific, having its curve of greatest depression, 29.6 inches, in the ocean between Kamtchatka and Sitka, in what was formerly Russian America. The singularness of this depression consists in its close proximity to the high pressure of Siberia.
110. The position of the lowest equatorial pressure in the Indian Ocean, which determines the region of calms and constant rains, is peculiar in this, that it does not lie parallel to the equator, as might have been expected, but slanting from Tamatave in Madagascar, 18° S. lat., to the coast of Sumatra, in 5° S. lat. This is one among
valuable results arrived at by Mr Charles Meldrum, Mauritius, in his laborious researches into the meteorology of the Indian Ocean. Moreover, it is in this trough that nearly all the tropical storms of the Indian Ocean have their origin.
111. MEAN ATMOSPHERIC PRESSURE FOR THE YEAR-PLATE III.-In Plate III., which gives the mean annual pressure of the atmosphere, the sums of the disturbing influences at work throughout the year in increasing or diminishing the weight of the atmosphere, are seen at once in the areas of high and the areas of low mean pressure in different parts of the globe.
112. Regions of High Pressure. There are two such regions -the one north and the other south of the equator-passing completely round the globe as broad belts of high pressure. They enclose between them the low pressure of the tropics,