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another from the place of observation to that of least pressure, the angle is generally from 60 to 80 degrees. This is unquestionably the general direction of the wind in storms, but the angle is frequently as small as 45 degrees, especially where the winds become lighter on approaching the central space of least pressure, and on rare and peculiar occasions it exceeds 80 degrees.* The same general characteristic of the wind is also illustrated by the storm which at the same time prevailed in Lapland and the north of Norway. In reference to these two storms, the winds in the south of Norway and in Denmark are very interesting.
518. From this it follows that in these cases the wind blew round the area of low barometer in a circular manner, and in a direction contrary to the motion of the hands of a watch, with, and be this particularly noted, a constant tendency to turn inwards towards the centre of least pressure. It will be observed that the greater the force of the wind was at any place, the more nearly was the direction here indicated approximated to; and that where the direction showed any material departure from the general law, such winds were light, and consequently more under local influences, which tend to turn them out of their course. Hence in these storms the winds circulated round the region of least pressure; or, to state it more accurately, the whole atmospheric system appeared to flow in upon the CENTRAL area of low pressure in an in-moving spiral course. This peculiarity is common to all European storms I have yet examined ; and it should be
* Of these rare occasions, the storms of 6th February 1867 and 1st December 1867 are illustrations. In the former case the direction of the wind at places on the Channel and in the north of France, and in the latter case on the east coast of Scotland, as far as the north of Shetland, formed angles from 90° to 100° ; in other words, the winds followed the isobarometric curves, or even to a slight degree crossed them. In every one of the occasions in which I have observed this peculiarity, the barometric lines were disposed in lines nearly straight, were closely crowded together, and the anomalous direction of the wind was strictly confined to a very small portion of the storm. Elsewhere, the winds were in accordance with Buys Ballot's law, and the storm, taken as a whole, was consequently in agreement with it.
particularly noted that it is no mere theory or opinion, but a simple statement of what has been constantly observed.
519. Professor Taylor first applied Dove's law of rotation to explain the direction of the rotation of storms round their centre, This
may be illustrated by referring to the storm of the 2d November. On that morning, the pressure over England being much less than in surrounding countries, if the earth had been at rest, it is assumed that air-currents would have flowed from all directions towards England, in straight lines. The earth, however, is not at rest, but revolves from west to east; and as the velocity of rotation diminishes as the latitude increases, it is evident that the current which set out, say from Lyons to the north, would, on account of its greater initial velocity, blow when it arrived at Paris, no longer directly towards the north, but to a point a little to the east of north ; in other words, it would be no longer a south, but a south-west wind. Again, since the current from the north of Scotland had a less velocity than those parts of the earth's surface on which it advanced, it lagged behind, and, consequently, by the time it arrived at Silloth in the north of England, had changed from a north to a north-east wind. Similarly the north-west current changed to a north, the south-west to a west, &c. The west and east currents, since they continued to blow in the same latitude, would have blown in the same direction, if they had not been disturbed by contiguous currents. Hence in a storm the whole system of winds, as may be expected, flows in and round upon the centre. As a further confirmation of the correctness of this explanation, it is observed that, when a high barometric pressure covers a limited space, the wind is always observed gently whirling out of this area of high barometer, but in an exactly opposite direction from that assumed when it blows round and in upon an area of low pressure. This, it will be observed, is the same relation that has been already shown to obtain between the mean barometric pressure of neighbouring regions and their prevailing winds; but, in the case of storms, the relation is more strongly marked, inasmuch as the barometric differences are greater,—the whole phenomena being, as it were, condensed into smaller space.
520. Apparent Exceptions.—(1.) The influence of local causes in changing the direction of the wind, especially when light or moderate, has been already referred to. Suppose a W. wind to prevail in the west of Scotland, then a S.W. wind would prevail in the upper part of Loch Fyne, Argyleshire, which lies from S.W. to N.E. ; in other words, the wind would be diverted from its general course into that of the valley of Loch Fyne. This peculiarity of the wind is well known and acted on by the fishermen and sailors in the west of Scotland. In inland situations, the influence of hills and valleys in changing the direction of the wind is everywhere recognised.
521. (2.) The irregular outlines of some of the isobarometric lines (Plate VII.) suggest that the increase of pressure from the lower to the higher is not in all cases uniform, but that there occasionally occur interruptions in its regular fall or rise over the space covered by the storm. Thus, on the morning of 20 November 1863, the pressure in the valley of the Clyde was a little less than what prevailed to the east, south, and west, as well as to the north of it. The following will show, in inches, the nature of this depression : Glasgow, 29.053 ; Douglas Castle, 29.072; Girvan, 29.092 ; Paisley and Balloch Castle, 29.148; Oban, 29.175; and Edinburgh, 29.192. The influence of this small local depression is seen in diverting from the normal directions the wind at Aberdeen, in East Lothian, and at Glasgow. At the same time the pressure on the coast of the Gulf of Genoa was lower than it was elsewhere in the vicinity; and the influence of this small local disturbance is plainly seen in the direction of the wind at Marseilles, Toulon, Leghorn, Venice, Milan, and Turin, these all being seen to flow round and in upon
this patch of less
pressure. Similar areas of small local disturbances in the pressure occurred near Geneva, in the north-east of Austria, and near Odessa in the Black Sea, and at all of these places corresponding deviations of the wind may be ob
served. These local disturbances are thus noticed by Francis Galton in his “Meteorographica,' page 7: “In addition to the great barometric fluctuations, there are numerous smaller ones, and it occasionally happens that the barometric readings have a widely-spread tendency to show marked though minor irregularities in adjacent places ; or, to use an intelligible though inaccurate metaphor, they are violent ripples on the great barometric waves." If the expression “or eddies" be inserted after ripples in the above extract, these smaller barometric disturbances would be more completely described. *
522. (3.) Since the relations of pressure at great heights over different places may be, nay often are, very different from what obtains near the level of the sea, the direction of the
these heights cannot legitimately be referred to isobarometric lines showing the pressure at the level of the sea. It is thus we explain the anomalous direction of the wind as not unfrequently observed at Madrid, and at the higher of the stations in Austria. The value of observations from such high stations, as previously stated, lies in the earlier notice they give of the prevalence of the equatorial current and saturation of the atmosphere by means of it, and consequently of coming changes of weather, including the approach of storms. In critically examining Buys Ballot's law of the winds, it is indispensable that these three classes of apparent exceptions be taken into account. From the
extensive examination of the law which I have made, I am convinced that if all apparent deviations from it be examined they will turn out, if the critic has the whole facts before him, to be only strong confirmations of its correctness.
523. This view, which considers the direction of the wind as brought about by the drawing force arising from differences of atmospheric pressure, together with the influence of the earth's rotation on its axis, is sometimes objected to as insufficient-it being urged that the rotation of the earth is inadequate to produce so large an angle as 60 or 80 degrees. But storms cover a large portion of the earth's surface, and since the part of the earth's atmosphere involved in the storm is simultaneously moved as one mass by the difference of pressure, it is evident that, as the storm extends over many degrees of latitude, the influence of the earth’s rotation must be very considerable. Another force is also, no doubt, brought into action in this flow of the winds round and in upon the region of least pressure, resisting the direct flow of the winds towards it, such as we see exemplified in the movements of water let off from a bath of stagnant water, resulting in whirling currents which flow round and in upon the centre in an in-moving spiral course. According to this view the rotation of the earth will account for the direction in which storms revolve round their centres of least pressure, and for part of the angle of 60° or 80° of deviation from the centripetal direction of the winds; whilst the rest of the angle of deviation will be due to the elastic force exerted by the air-currents themselves as they are pressed inward towards the centre, or forced upward into the higher regions of the atmosphere by the greater pressure all round. In this way it is easy to understand, from the unequal distribution of vapour and heat through these air-currents, and their unequal and varying tensions, why the angle of deviation from the drawing force may on rare occasions, and over limited districts, amount to, or exceed, 90°.
* This interesting feature of storms can only be properly investigated when observations from numerous stations are available.
524. The Force of the Wind. It will be observed from Plate VII., that everywhere in Scotland, except in the extreme south, the winds did not exceed in force a moderate breeze (represented on the chart by two feathers on the arrows). Over this space the isobarometric lines are far apart; and it will be observed in the other parts of Europe where these lines are far apart, that there the wind did not reach the force of a gale.
525. In the south of Scotland, the north of England, and in Ireland, where the isobarometric lines 28.9 and 29.1 inches approach close to each other, the wind was strong. Thus gales