NA TURE 



525 



THURSDAY, JANUARY 22, 1920. 



WIND AND BAROMETRIC GRADIENT. 

 Manual of Meteorology. Part iv. : The Relation 

 of the Wind to the Distribution of Barometric 

 Pressure. By Sir Napier Shaw. Pp. xvi+i66 + 

 3 plates. (Cambridge : At the University Press, 

 igig.) Price J2s. 6d. net. 



THE other parts of this manual are not 

 yet published, but Part iv. deals with the 

 question of the extent to which the motion of the 

 lower strata of the atmosphere can be inferred 

 from the ordinary working- chart prepared on 

 receipt of the telegraphic information of the sur- 

 face conditions by a meteorological office, a ques- 

 tion of great importance to the aircraft service. 



As the author informs us in the preface, many 

 inquiries of this sort were addressed to his office 

 in London during the war, and he sets out here 

 the extent to which and the means whereby 

 answers to such questions can be given. 



The matter is a complicated and difficult one, 

 but Sir Napier Shaw is to be congratulated on 

 the mass of information he has brought together 

 and on the clear way in which he has arranged it 

 and correlated together the different parts, which 

 are sometimes more or less contradictory. 



In chaps, i. to iii. the relation of the surface 

 wind to the isobars is set out. The principle that 

 the motion of the air will be at right angles to the 

 direction of the pressure gradient and have the 

 velocity deduced from the gradient equation is 

 accepted as a working hypothesis, and the reasons 

 why the rule does not hold close to the surface are 

 explained. The principle was set out by the 

 author thus in 1913 : "In the upper layers of the 

 atmosphere the steady, horizontal^motion of the 

 air at any level is along the horizontal section of 

 the isobarjc surface at that level, and the velocity 

 is inversely proportional to the separation of the 

 isobaric lines in the level of the section." How 

 far this principle holds is a fundamental question 

 in meteorology. Admittedly, it only applies to 

 first order terms, and the author, in chap, x., 

 shows that there is a systematic departure from 

 the rule near the centre of a travelling cyclone. 

 On the other hand, if a meteorologist is asked to 

 give the velocity and direction of the wind at 

 1500 ft. height at any given time and place, it 

 has been found, in the absence of information 

 from pilot balloons, that the best answer he can 

 give is to quote the gradient wind as shown by 

 an isobaric chart. 



In chaps, iv. and v. the author discusses the 

 increase of wind with height in the lower strata, 

 and gives G. I. Taylor's theory of the diffusion 

 NO. 2621, VOL. 104] 



of eddy motion and its effect on the wind and on 

 the formation of thin sheets of low cloud. Tay- 

 lor's formula takes the form W/G=cos a — sina, 

 where W denotes the actual surface wind, G the 

 geostrophic wind, and o the angle between them. 

 It will be noticed that this formula makes a value 

 of a exceeding 45° impossible. 



Chaps, vi. and vii. deal with the variations of 

 the wind in the upper layers and their dependence 

 on the form of the distribution of temperature. It 

 is shown how the cessation of the lapse rate, i.e. 

 the fall of temperature with height, in the strato- 

 sphere, and the higher temperature over the 

 cyclonic area that is found above 10 km., produce 

 the rapid falling off of the wind that is also shown 

 by direct observation. 



Chap. X. is perhaps the most suggestive in 

 the book, and throws fresh light on the well-worn 

 theme of the mechanics of a travelling cyclone. It 

 is there shown that in what is called a normal 

 cyclone there are three centres : the instantaneous 

 or kinematic centre; the "tornado centre," which 

 is the centre of the supposed rolling disc ; and the 

 dynamic centre, which is the centre shown by the 

 isobars on the chart. This representation leads to 

 a systematic difference between the true and the 

 gradient wind in the parts that are near the centre, 

 a difference that has been noticed, but was sup- 

 posed to be accidental, on some working charts. 



In chap. xi. Sir Napier Shaw discusses Rayleigh's 

 and Aitken's papers on revolving fluid, and gives 

 diagrams and reproductions of instrumental 

 records relating to some noted storms of the last 

 twenty years or so. Synoptic charts are repro- 

 duced which show a good agreement with the 

 method of treatment in chap. x. The diagrams 

 Figs. 5 and 6, on p. 154, are especially striking; 

 they refei" to the storm of September 10, 1903, at 

 Holyhead, and show the velocity and direction of 

 the wind on that occasion corrected for the known 

 peculiarities of the exposure due to the local con- 

 figuration. 



In theoretical discussions of the mechanics of a 

 cyclone, especially if there is much mathematical 

 analysis, one is unavoidably compelled to make 

 hard-and-fast suppositions, and the point arises as 

 to what extent the real cyclone will submit to be 

 bound by such suppositions. The author questions 

 how far the " normal cyclone " of chap. x._ is the 

 real cyclone as shown on the charts, and one 

 v/onders how far Rayleigh's conclusions are 

 vitiated by his leaving out the effect of the rotation 

 of the earth. Doing so greatly simplifies the 

 equations of motion, but it is the earth's rotation 

 which ensures that every cyclone in the northern 

 hemisphere without exception shall rotate in one 

 direction only, and every cyclone in the southern 



