578 



KNOWLEDGE & SCIENTIFIC NEWS. 



[November, 1906. 



in character, and in their most acute form, termed in 

 ordinary languacfe, "storms." The chief features of the 

 first class of air mov(>ments have been determined both from 

 observation and theory. The movement undoubtedly de- 

 pends upon larg-e temperature differences between the 

 equatorial and polar ret:^ions, or between sea and land areas. 

 With respect to the air movement in cyclones and anti- 

 cvclones there is, on the other hand, much diversity of 

 opinion. Meteorologists, for example, differ as to the con- 

 ditions necessary for the initiation and development of 

 cyclonic storms, the transformations of energy which 

 occur during the storms, and also of the actual features of 

 the air movement. Farrel, on the whole, the most dis- 

 tinguished .American meteorologist, resolves the air move- 

 ment of a cyclonic system into circular movement about a 

 moving centre on a rotating surface. On this assumption 

 he explains the incurvature of the winds and the contrast 

 of the intensity of movement in different quadrants of the 

 storm. The initiation of cyclonic storms is ascribed by him 

 to temperature differences, and he classifies them into storms 

 with a central warm area, and with a central cold area. 

 The leading German meteorologists, we believe, now hold 

 that cyclonic storms are mere incidents in the larger atmo- 

 spheric circulations, just as a whirl in a stream of running 

 water, is a mere eddy in the general stream movement, and 

 due to some slight local obstruction or other cause. 



With respect to the diversity of opinion on this point, Dr. 

 Shaw, in his memoir, remarks : " I think I am justified in 

 saying that meteorologists are even not quite clear as to 

 whether the circulation of a revolving storm as shown on 

 weather maps is the primary element of the problem, the 

 fans et orhjo mulurum, or is merely an exaggerated bye-pro- 

 duct of more general atmospheric currents, possibly in the 

 upper regions, just as the eddy or the whirlpool is the bye- 

 product of the flowing stream and its boundaries." Dr. 

 Shaw's criticism of the vague and careless employment of 

 meteorological terms by meteorologists generally is both 

 just and deserved. In connection with his investigation he 

 especially instances the use of the terms barometric mini- 

 mum and movement of a barometric minimum, pointing 

 out that they are used to designate phenomena of different 

 orders, ;md hence to suggest similarity when there is no 

 similaril)-. It is, on the one hand, universally admitted that 

 pressure is never absolutely uniform over any large area, 

 and that it is in a state of constant flux or change by 

 amounts differing from place to place due to a variety of 

 actions, of which one is the movement of storm areas or 

 barometric minima of a definite type. It is, for example, 

 possible in a hot country like India for pressure on one day 

 to be lowest in, say, Sind, on the next day in North-Eastern 

 India, and on the following day in Burma or the Deccan. 

 Weather charts drawn for short intervals of, say, 

 one hour, might indicate the gradual transference 

 of the seat of lowest pressure from Sind to Chola 

 Nagpur and the Deccan. But to treat this apparent 

 transfer by the same methods and subject to the same 

 general laws as the movement of the centre of an intense 

 cyclone is absurd, as it assumes similarity of conditions and 

 actions where no such similarity exists. The changes of 

 pressure accompanying the advance of cyclonic storms 

 appear to be mainly due to internal actions, whereas in the 

 other class of barometric minima referred to they are 

 chiefly the result of external actions. Hence, to employ Dr. 

 Shaw's words, "Treating the apparent displacement of all 

 barometric minima generally in the same manner tends to 

 obscure rather than to elucidate the difficult problems as- 

 sociated with the origin and march of storms." 



How far this confusion taints the results of many meteoro- 

 logical investigations can only be surmised. It is, however, 

 certain that more exact definitions and methods of investiga- 



tion arc essential for the progress of the science of meteoro- 

 logy. .\ssuming that there are cyclonic storms having a 

 definite continuous existence and associated with certain 

 peculiarities of air movement, the investigation of the 

 motion of the air masses within the storm area 

 or passing through it is evidently a problem of 

 the greatest importance. Hitherto, chiefly due to 



the almost exclusive study of tlaily weather charts, 

 little or nothing has been done to trace the con- 

 tinuous movement of the air masses affected by the cyclonic 

 storm. The movement of the centre of the storm or of the 

 isobars or the direction of the winds in relation to the 

 position of the centre with respect to that of the position of 

 observation, have hence been carefully studied from the charts, 

 but these results tell us nothing of the previous or future 

 history of the actual air mass passing over the place of ob- 

 servation, and hence throw little or no light on the condi- 

 tions which determine the changes of temperature, humidity, 

 and rainfall at the place of observation. Meteorologists are 

 now agreed that what are called circular storms are not 

 circular in the proper sense of the word. There is not 

 merely a flow of air round, but also convergence towards a 

 central area. There is also a large upflow over and near 

 the central area, and also an outflow above and a descent 

 probably in the outskirts of the storm area. The motion is 

 hence successively complicated. It is also very variable, as 

 squalls and gusts of the most violent character may 

 alternate with strong but less destructive winds. Also in 

 the cyclonic storms of the temperate regions of the 

 northern hemisphere, the winds in the same quadrant of 

 cyclonic storm, and hence from the same general direction, 

 differ largely in their meteorological character (that is, 

 temperature, tS;c.). These differences are, as Dr. Shaw has 

 now shown, due to the previous history of the particular air 

 mass in the given position of the storm in question. Hence 

 the value and importance of Dr. .Shaw's investigations. 



The chief method of investigation employed is simple, but 

 requires almost continuous registration of the air movement 

 at a large number of stations in the area passed over by 

 the storm. This is possible in the British Islands, as self- 

 recording anemographs are in operation at many places. 

 The path of a given mass of air is worked out from their 

 observations by a slep-by-step method. The following 

 gives a statement of tlie method by means of a particular 

 case. Suppose at a place the 8 a.m. observation shows 

 that 20 miles of wind from W..S.\V. has passed over it 

 during the previous two hours. Half of this is assumed to 

 indicate the movement of a mass of air during the previous 

 hour {7 a.m. to 8 a.m.), and the remainder the movement 

 of the same mass from 8 a.m. to 9 a.m. Suppose, then, 

 an instrument observing at the new position shows that 

 during the two hours preceding 10 a.m. 24 miles of wind 

 from S.W. has passed over it. It is then assumed that the 

 previous mass of air will have moved 12 miles from S.W. to 

 N.E. between g a.m. and 10 a.m., and the same amount from 

 10 a.m. to II a.m. These values are plotted on a chart, and 

 inferences from similar observations from noon onwards are 

 made. The broken line thus obtained will give an approxi- 

 mation to the motion at the earth's surface of a definite air 

 mass. The paths thus obtained are termed " trajectories " 

 by Dr. Shaw. The method is undoubtedly only approxi- 

 mate, and subject to limitations. So far as we can judge, 

 its employment is legitimate, and the conclusions thence 

 established by Dr. .Shaw and Mr. Lempfort are valid. 



They have worked out a large number of trajectories for 

 each of a series of typical storms which have passed over 

 the British Islands in recent years, and formulated infer- 

 ences of great interest and importance respecting the move- 

 ment of air into or out of storm areas. The following gives 

 a brief summary of what appears to us to be the most im- 



