began, gradually and intermittently breaks away throughout the whole of southern and eastern Asia, and the north-east monsoon of the northern hemisphere presently asserts its predominance, until we again reach in winter the conditions shown in Chart I. 136. In the summer of the southern hemisphere an irregular oval area, having barometric pressures below 29.8 in. (757 mm.), and having its longest axis in an east-west direction, exists over Central and Northern Australia. In the winter, on the other hand, an area of high pressure is noticeable over the more southern and south-eastern parts of this continent. 137. Looking now at the polar areas of depression, we find the south polar area to be fairly uniform, so far as observation extends, over all meridians. There is reason, however, to believe that in very high southern latitudes the gradients diminish in steepness; and some authors consider that there is a slight increase of pressure near the south pole. Although we regard the south - polar area of depression as practically uniform, centres of local depression course rapidly along this area from west to east, the winds shifting from north-west to south-west as they pass. Over certain meridians some of these depressions have a more northerly course, and the strong east winds of their southern segments are from time to time experienced. The north - polar depression is very greatly dislocated by the large amount of land surface in high latitudes, being almost interrupted where the two northern continents approach over Behring's Straits, and being thrown considerably to the southward by an area of high pressure lying in winter over Greenland. This area is marked at this season of the year by the passage of local depressions, some of small, others of great dimensions, some of which sweep across the comparatively narrow continent of North America, while others are formed, and many expend their fury, on the North Atlantic and its coasts, a few travelling over Northern Europe and part of Northern Asia, but very few penetrating to any extent the great area of high pressure over Eastern Asia. It is principally the southern segments of these systems which are encountered on the steamship lines between Europe and America, the discomforts of whose south-west to north-west gales are only too well known to most readers. 138. The prevailing upper currents over the actual globe, while they follow somewhat more nearly than the surface winds the arrangement indicated in the previous chapter, are yet modified by the actual distribution of land and sea. For example, over the North Atlantic in winter the current which carries the highest visible clouds flows, with remarkable strength and persistency, from westerly points on the south side of the area of low pressure lying to the south and south-east of Greenland and Iceland, over somewhat lower latitudes than it would occupy over an ocean-covered sphere. The statement, however, that this current is permanently from the west-a statement which has been again and again repeated -is due to want of careful observation; for, as a matter of fact, the highest Cirrus is sometimes, though rarely, noticeable moving from eastward points even in winter over North-western Europe and over Canada and the Northern States. Nevertheless, the slope of the gradients towards the pole in the higher regions of the atmosphere exercises so predominating an influence that even over Eastern Asia in winter the upper currents generally travel towards the east, not being governed by the great barometric maximum over the Asiatic continent. In an analogous manner over the northern segments of the non-periodic depressions of the higher latitudes the upper currents rarely move from any east point, and the effect of these depressions is, as we shall presently notice, to cause a southward inflexion of the lower isobars in the higher parts of the atmosphere. Much has been written in recent years on the distribution of pressure at various heights in the atmosphere, and on the general slopes of the isobaric shells. A good deal of this is purely theoretical, and much is based on observations which are too sparsely distributed over the globe to be treated by us with full confidence at present. But even were it otherwise the subject does not apply in detail to the topic with which we are especially dealing in this volume. CHAPTER IX CYCLONES AND ANTICYCLONES AND THEIR CLOUD-FORMS 139. WHAT We have above regarded as the permanent and the periodic systems of atmospheric circulation represent the mean positions of the atmospheric pressures and wind-systems derived from observations taken during a number of years, and do not correspond with actual distributions of pressure and winds at any given moment of time. The variations from the mean, though considerable over intra-tropical regions, are much more distinct in the higher latitudes of the northern hemisphere, and they are somewhat more marked over the Atlantic and North Pacific. It is in these regions that the variable and intermittent systems generally come most distinctly into play, although in intra-tropical regions even more important irregularities are, at comparatively rare intervals, produced. It will be necessary to describe briefly the general characteristics of these local systems. Cyclones 140. For our purposes a cyclone may be defined as any non-permanent area of low pressure into which winds blow in a curved direction. It will be at once seen that such a definition embraces not only the small whirl and the large "depression," but also those more destructive and terrible occurrences, the violent hurricane and the tornado. Moreover, this definition takes no account of the origin of the centre of low pressure or of the weather usually accompanying it. Thus, as we shall presently see, a true cyclone, though generally of feeble dimensions, may occur without any particular formation of cloud or precipitation of rain. 141. Now from what has been said in the two previous chapters we know that round the permanent areas of low pressure on our globe the winds blow counter-clockwise in the northern and clockwise in the southern hemisphere. The reason for these motions is attributed to the rotation of the earth. For, if we have from any cause a centre of low pressure, the winds would blow from all sides directly into this area if the earth had no rotation (Fig. 7). But in the northern hemisphere, owing to the earth's rotation, and according to the law stated in § 121, the wind from the south will be deflected to the east of the centre of low pressure, the wind from the east will be deflected to the north, that from the north to the west, and that from the west to the south of this centre. Also, as we shall see when we come to speak of the origin of these cyclones, just as in the equatorial continent of our hypothetical sphere (Fig. 9), these winds will be sucked upwards as they approach the centre, and, rising higher and higher, they will eventu M |