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Meteorological S\>mons'8 Meteorological No. 630. JULY, 1918. VOL. LIII. SIR NAPIER SHAW ON REVOLVING FLUID IN THE ATMOSPHERE. IN a recent memoir communicated to the Royal Society* Sir Napier Shaw gives an extremely interesting and important summary of his views on the subject of " revolving fluid in the atmosphere " following Lord Rayleigh's contribution to the dynamics of this subject based on the paper by Dr. John Aitken, F.R.S. on " The Dynamics of Cyclones and Anticyclones." Lord Rayleigh in the introduction to his paper remarked that " so much of meteorology depends ultimately upon the dynamics of revolving fluid that it is desirable to formulate as clearly as possible such simple conclusions as are within our reach," with the object of finding their application in the phenomena shown by cyclones and anticyclones. At the beginning of the paper Sir Napier Shaw states that in the twenty years during which he has been practically concerned with the progress of dynamical meteorology and the explanation of the observed phenomena of cyclones and anticyclones he has deliber­ ately avoided the discussion of the theory of revolving fluid because he could not find it in the weather maps which form the starting point for such an investigation. In several previous investigations, of which the most exhaustive was the well known memoir dealing with " The Life-History of Surface Air Currents," it was clearly shown that in our latitudes cyclones and anticyclones are not examples of revolving fluid in the special sense referred to, and for that reason cannot be explained on the hypothesis that they are. The situation is summed up by the statement that " if the motion of the air had really been motion of a revolving fluid symmetrical with regard to a vertical axis it would not have appeared in circular form on the map. If it looks circular it is not a case of revolving fluid." This apparent paradox is explained thus, that the rate * Royal Soc, Proc. ; A, Vol. 94, pp. 34-52, 62 Symons's Meteorological Magazine at which all the depressions of well marked circular form in our latitudes travel is much the same as that of the wind velocity of which they are composed. Stationary depressions are of irregular form and do not show the characteristic features of strong winds with a definite centre." But when the motion of translation is of the same order of magnitude as the winds the instantaneous motion is round a moving centre, and the actual motion with refer­ ence to the centre is the apparent uniform motion round the centre shown on the map combined vectorially with a velocity equal and opposite to that of the translation of the cyclone. Consequently the appearance of uniform and symmetrical instantaneous motion in a cyclone is in itself proof that we have not in that case symmetrical motion, about a centre, of a mass which travels as a whole. In other words the motion of air in cyclones is not the motion of revolving fluid in the special sense referred to." Tn " The Life History of Surface Air Currents," this conclusion was amply con­ firmed for all classes and forms of depressions. Cyclonic motion is, therefore, not analogous to that of a vortex ring travelling in air, but on the other hand the trajectories of a moving cyclonic disturbance show that the air flows through the depression. The vortex, therefore, does not actually breathe or pulsate. Visible examples of revolving fluid whose existence cannot be denied are are to be found in the eddy of dry leaves in a gusty wind at a street corner, the " dust-devil " of Eastern deserts, the water spout, the whirlwind, and the tropical cyclone, which is universally treated in text books of meteorology as a gigantic eddy typically analagous to the vortex ring. These, Sir Napier Shaw says, may (the italics are ours) be cases of revolving fluid, but they do not correspond with the phenomena of the cyclonic depressions of middle latitudes such as those experienced in the British Isles, where, during the ten years ending with 1916, a new cyclonic depression was shown on the weather maps on the average every third day. It is pointed out that a cyclonic disturbance which began as revolving fluid in the tropics is gradually transposed in higher latitudes into an ordinary cyclonic depression '' and yet we have to deny the identity of the two types of phenomenon." Here a short digression containing a mathematical proof is given to show how the revolving tropical cyclone passes into the depression of our own latitudes, and how it is quite possible that in the end " the conditions necessary for maintaining a core of revolving fluid disappear and the whole system degenerates (if that is the correct expression) into cyclonic motion without a core of revolving fluid." There is nothing in the weather maps to contradict such a suggestion, although it is pointed out that owing to the destruction of buildings, instruments and some­ times the human beings who attend to them, the tropical revolving storm is a very unsatisfactory example of revolving fluid for scientific purposes. It is thus impossible to ascertain whether the behaviour Symons's Meteorological Magazine. 63 of a tropical storm is in harmony with Lord Rayleigh's theory or Dr. Aitken's practical illustrations. Sir Napier Shaw in his investigations starts with the assumption which meteorologists cannot deny that temporary whirls are actually visible, and cites a case described by Mr. R. H. Curtis of a whirl which carried off the sheets of music from an open-air band-stand with a spinning motion to a great height and dropped them some miles away. Here there are obviously two elements, the current which carries the whirl, and the whirl within the current. The supposition is made that we have here a mass of revolving fluid with a discontinuity of velocity at the circle bounding the revolving mass. There may be a relation at the margin of the whirl and the wind in the current which carries it, but as shown by Lord Rayleigh, and Dr. Aitken, the velocity in the whirl at any time depends upon dynamical or physical processes within the whirling mass itself. No general assumption can therefore be made of a relation between the velocity in the whirl and the rate of its translation. The difficulty of investigating small whirls is that they cannot be detected on the working weather map, if under twenty miles in diameter. The vortex that appears em such a map it is shown " must be more like a penny than a pin." A method was devised for the investigation and identification of vortices having a temporary existence in the atmosphere, but not to be found directly on the map. This is described on pp. 40-44 of Sir Napier Shaw's paper, but considerations of space render it impossible to go into the matter here with the fulness necessary for a clear understanding of 'the procedure. It will be sufficient to say that in addition to a large number of illustrative cases two special cases are cited, viz., the destructive " secondary " depression of March' 24th, 1895, which in the eastern counties of England was unattended with any rainfall either before, or during, or after the strong wind. Except for the wind it was a fine afternoon and remained so. The second case is the tornado in Devonshire and South Wales on October 27th, 1918, which did not appear on the working charts at 6 p.m. when the greatest damage was done. The first case may " be cited as a good example of revolving fluid carried along in a main current, the velocity of translation being governed by the spacing of the isobars of the main depression outside the area affected by the local rotation, and, in virtue of this, the air which formed the mass of revolving fluid over the south of Ireland at 8 a.m., was carried along bodily to Denmark by 0 p.m." As a good deal of rain fell in the west its existence is -prima facie evidence of considerable convection in the atmosphere. It is thus reason­ able to suppose that the special conditions involved in the mainten­ ance of a column of revolving fluid are satisfied, namely that the wind velocity in the surrounding medium should be the same at all elevations. The second case cited, vis., the tornado in the Taff Valley, Glamorgan, in which the barograph near the centre showed 64 Symons's Meteorological Magazine. a sudden fall and recovery of 10 mh. (-30 in.), within less than one minute, arid where the localities where damage was done were those of heavy rainfall, was evidently associated with a very definite line of discontinuity marking off the area of rotational velocity from its environment. The destruction in South Wales seems to have been occasioned by the conveyance of the air within the moving column of revolving fluid, forming an excellent example of the progression of revolving fluid, although it was impossible to identify it on a map. Sir Napier Shaw sums up his conclusions in the following words, " We are thus able to draw a definite distinction between the cyclonic or anticyclonic motion as exhibited in the atmosphere arid cases of revolving fluid motion. In the latter there is rotation about the vertical axis and the whole mass of revolving fluid is carried along bodily in a current of air which has a velocity corres­ ponding with its position in the main cyclonic system and which can be related to the run and the distance apart of the isobars.
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