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THE NATURE AND MOTIONS OF ATMOSPHERICAL
BA N turning from the theoretical view to treat of the
Ef weather conditions experienced over these islands, it So would have been desirable to have illustrated the
subject by means of the “ Daily Weather Charts," but as that would be impracticable within the compass of an article, I must content myself by endeavouring to make my meaning clear without their aid.
It must be apparent to everyone who has given the least attention to our argument, that the depressions with which we are acquainted are quite different in their general character from those formed by the sun and moon within the tropics; and this will be further evident from a variety of considerations. The depressions in the vicinity of the equator have east winds both to the north and south of them, whilst the cyclones in our neighbourhood have always east winds on their northern and west winds on their southern sides. In the calm belt or region of equatorial depressions cyclones or revolving storms are unknown, and the reason of this will readily appear by means of the experiment of the rotating hemispheres. If the two outer hemispheres, A and B,* be made to rotate, the one with, the other against watch hands—and we know that these are the conditions of the circulation north and south of the equator--the central hemisphere C will remain at rest. We thus see that a cyclone circulation is impossible under such circumstances, and that a revolving storm can only happen between two rotating waves which are revolving in the same mannereither both with or both against watch hands. In one word the equatorial depressions produce the circulation, whereas the cyclones are the result of it. The winds do not circulate round the equatorial depressions; a true depression, without any progressive movement, would only give rise to waves or oscillations. If, however, it should possess a progressive movement, it would produce two separate systems of circulation, one on each side of the disturbing cause, and revolving in contrary directions.
* See Nautical Magazine, January, 1879, page 13.
If we form a miniature whirlpool in a cup of tea, it is quite clear that the depression or hollow noticed in the middle is the result of the circulation, for the faster the liquid is made to move the deeper the hollow becomes. The circulation really produces two effects—whilst the centre is depressed the sides are elevated. I am inclined to the opinion that whirlpools are made up of a number of rotating waves as shown in Figures 1, 2, and 3.
Figure 1 represents the falling sides of the waves or motion away from the centre; Figure 2 the rising sides of the waves or motion towards the centre ; Figure 3 shows the whole of the waves, the broken line showing the falling side and the continuous line the rising side of the waves. The large arrows show the direction in which the waves are revolving.
In each section shown in Figure 3 there is therefore a rising and falling motion of the water—the falling motion, I believe, giving
the requisite inclination or gradient for the flow of the current, and the rising motion feeding the stream. In section (a) the falling motion is nearly coincident with the rising, as shown by the small arrows, the broken one representing direction of falling motion and the other the rising motion of the gradients. In section (b) these motions are at right angles, in section (c) nearly opposite and in (d) exactly so. Many of the features of cyclones are visible here. There is this difference, however, that in the whirlpool these rotating waves are stationary, as may be readily seen by any observer, whilst those in the cyclones have a progressive movement. I must not be understood as affirming that the cyclones are formed by the rapidity of the rotation alone. They are the result of the rotatory and progressive movements combined
—the former adding to their depth, the latter extending their area. Near the centre there is a calm area, and the steepest gradients are to be found in its vicinity.
But the most important inference to be drawn from a study of these motions is that every current presupposes both a rising and a falling gradient. It does not require much attention to perceive that to produce a current we must not only have a gradient, but a fountain or head to supply the stream. Of course in the atmosphere it is hardly possible to form a gradient without producing conditions favourable to the development of a current; at the same time it occasionally happens that extensive gradients—in some cases very steep—are formed without winds of very great strength being experienced. In every strongly marked carrent, therefore, we may confidently look for both a rising and a falling gradient. The south-westerly current provides a familiar example. When strong we have a rising barometer over France, and a falling motion from the north-west, the former supplying the wind and the latter forming the gradient along which the southwesterly current flows. A south-westerly current is, therefore, the product of a south-westerly rising and a north-westerly falling gradient. The necessity and utility of the rising and falling motion of the great gradients must now be apparent. I arrived at this conclusion, before I thought of the whirlpool, from an examination of the “ Daily Weather Charts," and its truth will, I think, be acknowledged on a very slight inspection of those charts which show well-developed currents.
As all our waves have a progressive movement, it is plain that the two sides of the wave at right angles to the line of progression are the most important, and we will confine our attention to these. No one who has watched the progress of the waves from the deck of a vessel could have failed to notice that the water was rising in front of the advancing wave and falling behind it, and when two waves met they caused a much higher compound wave, and after as it were a moment's tussle for the victory they passed through each other and left behind them a much deeper compound hollow. Similar effects undoubtedly take place in the atmosphere.
In treating further of these waves or gradients it will be convenient to adopt the nomenclature employed above in speaking of those producing the south-westerly current. When we have a motion from the south-east we will call it a south-easterly rising or falling gradient, according as the barometer rises or falls. A wave from the south-west will be called a south-westerly rising or falling wave, and so on. Now, as the wind, generally speaking, blows at right angles to the wave, by simply observing the direction of the wind, and the rise or fall of the barometer, we can tell how the wave motion is progressing, and also form a fair estimate of the movements of the respective gradients by considering the strength of the current and the rate and amount of the rise or fall of the barometer. Thus, a strong wind from the south-west, with a steady barometer, would indicate that the south-easterly gradient is rising as much as the north-westerly is falling ; with a rising barometer that the south-easterly gradient predominates ; with a falling barometer that the north-westerly wave is the stronger.
To produce a south-westerly curiont we thus see that a northwesterly falling wave is required; a north-westerly current requires a north-easterly falling wave; a north-easterly current a south-easterly falling wave; and a south-easterly current a southwesterly falling wave. These currents will not be strong unless there are strong rising gradients from the opposite point of the compass. No doubt the reaction of the air would of itself, for a short time, supply the current.
From a consideration of the whirlpool, we learn that the air of a south-westerly current must come round from the south-east; of a north-westerly from the south-west; of a north-easterly from the north-west; and of a south-easterly from the north-east. The effects of these currents must therefore be viewed in the light of the region whence they come. A south wind, coming as it does from the east, will, therefore, in winter bring a low, and in summer a high temperature, as in winter the continent has usually a lower and in summer a higher temperature than these islands.
Another principle deserves attention. It is quite evident that a current may become a wave and a wave a current. When the south-westerly current stops for some reason or other the wind usually veers to north-west, and the barometer rises ; these changes indicating that the south-westerly current has become a south-westerly rising wave. A current may change into a wave either on account of the friction of the earth—perhaps more correctly the friction of the lowest stratum of air,—or on account of a stoppage in front, something there checking its course. If the check is only due to friction the current will probably, after a short interval, resume its course. In a south-westerly current the south-westerly wind is seldom of long continuance, and the reason of this is not far to seek. We see the same principle exemplified in the bounding leaps which water takes in coming down a steep hill, also when a vehicle is driven down an incline, the hinder part has a tendency to rise up, and the driver requires to keep the horse's head well up as a kind of counterpoise. The top of the current goes faster than the bottom, and a rising wave is the result. The wind, therefore, cannot continue long in a right line with the force of the current or tendency, but constantly veers or backs according as the wave approaches or recedes, at the same time its natural tendency is to place itself in a line with the current.
If the south-westerly current is thus only stopped by friction the barometer rises, and the wind veers to the north-west, the