l62 



NATURE 



[June 14, 1906 



illustrated it were particularly good, which, seeing that 

 .Mr. Bedford began his work in this direction so long ago 

 as ]Sqo, is not, perhaps, to be wondered at. 



The last lecture, on Saturday morning, June 9, to which 

 the teachers of the district were invited, was given by 

 Mr. Wilfred Mark Webb, on nature-study. As two years 

 ago Mr. Webb presented a formal paper to the union, he 

 contented himself, after a few brief remarks, with show- 

 ing by means of lantern-slides what directions the pursuit 

 in question has taken or might take. 



A number of interesting specimens were brought together 

 to form the usual congress museum under the direction of 

 Mr. E. W. Swanton, and the photographic surveys of 

 .Surrey, Kent, and Susse.x contributed a selection of photo- 

 graphs. 



The business done included the election of Prof. 

 Silvanus Thompson as president for 1907, and the accept- 

 ance of an invitation to visit Woolwich for the twelfth 

 congress in that year. Dr. .Abbott, the founder of the 

 union, its first secretary and late treasurer, was added to 

 the list of vice-presidents, of whom besides Dr. Hutchin- 

 son, Mr. F. Merrifield, Mr. K. W. Rudler, the Rev. 

 T. R. R. Stebbing, Dr. Treutler, and Mr. W. Whitaker 

 attended the congress. 



The perfect weather made the four excursions to Mickle- 

 haTTi Priorv and elsewhere a complete success, and a 

 pleasing feature of the meeting was the votes of thanks 

 to local secretaries, Mr. J. J. HoUway and Mr. Sparks, 

 and their coadjutors, as well as that to the general secre- 

 tary, the Rev. R. .Ashington BuUen, which was emphasised 

 by the whole company rising in their seats. 



THE Sl'RFACE TRAJECTORIES OF MOVING 

 AIR.' 



"THE Meteorological Office has just published the results 

 of an investigation into the movements of the air 

 during storms and periods of barometric depression affect- 

 ing the North Atlantic and western Europe. The authors 

 deal, not so much with the discussion of theories about 

 cyclones as with the results of direct observations on the 

 direction and force of the winds as recorded at as manv 

 stations and as. often as possible. Apart from ships' logs, 

 the records from about 200 stations have been utilised. 



The attempt has been made to trace the path of any 

 body of air from the point where it descended from the 

 upper regions of the atmosphere along the surface of the 

 earth to the place where it ascended again, and the method 

 used is briefly as follows : — Using hourly observations 

 whenever possible, arrows have been drawn on a map 

 through the position of the recording station showing the 

 direction of the wind, and the length of the arrow is equal 

 to the distance which the recorded velocity suggests as 

 being the journey of the air during the half-hour pre- 

 ceding and the half-hour following the time of observ- 

 ation. By this method the trajectories arc made up step 

 by step through station after station as the hourly maps 

 are made up. .Anemometer records are consulted to decide 

 where the velocity of the wind has been sufificientlv con- 

 stant to carry the trajectory propcrlv from one hour to 

 the next. 



In the discussion of certain circular storms and baro- 

 metric depressions which have passed over the British 

 Isles, and which have been selected as typical examples, 

 162 trajectories were examined, and also the changes in 

 the meteorological conditions along them. Naturally many 

 of these trajectories do not represent the full course of the 

 particular current considered, only the beginning, the 

 middle or the end coming within the region under observ- 

 ation. 



These trajectories have been divided into five classes. 

 Class i. — Final-stage trajectories terminating generallv, 

 but not always, near the centre or the trough of the de- 

 pression. These are marked by diminishing pressure, in- 



' "The Life-history of Surface Air-current';; a Study of the Surface 

 Trajectories of Moving Air." By Dr. W. N. Shaw, F.R.S., and R. G. K. 

 I.empfert. (London : Published by the Authority of the Meteorological 

 Commiltee, Wyman and Sons, Ltd.) Price 71. 6if. 



NO. 1911, VOL. 74] 



creasing velocity, and falling temperature. These currents 

 end under cloudy skies and with rainfall. In some cases 

 the end is caused by the meeting with a cross-current. 

 The conclusion is that this air has fed the ascending 

 current, and that the rain has been caused by the ex- 

 pansion and consequent cooling. These trajectories are 

 always from almost due south, and show very little 

 curvature. 



Class ii. — Initial-stage trajectories, commencing in 

 regions of fairly still air which may be quite near to the 

 centre of the depression. The meteorological conditions 

 and changes which characterise this class are the reverse 

 of those for class i., even to this extent, that they flow 

 from low pressure to high. There are many cases of this 

 class shown ; too many to suggest a mistake. 



Class iii. — Looped trajectories generally cross the track 

 of the depression twice, once in front of the storm and 

 once behind, and may be taken as a continuation of 

 classes i. and ii. 



Class iv. — Spiral trajectories generally represent cold 

 currents blowing from the east or north round the west 

 of the centre of the depression to replace the southerly 

 currents of class i. 



Class V. has three subsections : — (a) trajectories from a 

 point in front of the trough ; (b) from a point in the rear ; 

 and (c) in the line of the trough. The meteorological con- 

 ditions accompanying (a) and (b) are similar to those for 

 classes i. and ii. Trajectories in the line of the trough 

 are remarkable for the strength of the wind and for the 

 small and irregular changes of pressure. The accompany- 

 ing weather is generally cloudy, but without rain. 



The trajectories over the Atlantic are obtained from 

 observations made between .August, 1882, and September, 

 1883, and, as only daily records are used, the investi- 

 gation is on a much coarser scale. Moreover, they are 

 more open to criticism, for there may be many changes in 

 the meteorological elements in twenty-four hours. Some of 

 the trajectories traced are remarkable for their length ; for 

 example, between December 23 and 30, 1882, one is traced 

 from West Africa to North Russia, and another from 

 Florida to the British Isles, and between November 13 

 and 17 one is followed from Hudson's Bav to the 

 Adriatic. 



In seeking to locate the positions of ascending and 

 descending currents and the connection between these and 

 the distribution of rainfall, it has been taken as proved 

 that an ascending current of air is necessary for the pro- 

 duction of measurable rainfall, and we are reminded that 

 it is not necessary or usual for these ascending or descend- 

 ing currents to be vertical. They are generally very 

 oblique. The approximate positions of ascending currents 

 are located by noting the convergence of air to such places, 

 divergence denoting descension. 



Convergence inay be produced by the trajectories being 

 directed towards one point, or by air overtaking air which 

 is preceding it in the same direction, or by the wind blow- 

 ing towards a persistent cross-current. These are obvious 

 and typical cases. 



If two sets of isochronous points or trajectories be joined 

 by lines, then the ratios of the enclosed areas will indicate 

 convergence or divergence according to whether the second 

 area is smaller or larger than the first. 



The greater convergence takes place almost always in 

 front of the centre of the depression, and this agrees with 

 the area of greatest rainfall. -As, however, the rain is 

 generally brought by southerly winds, the rainy district 

 is somewhat to the north of the area of convergence, the 

 current evidently having continued its onward course 

 whilst rising. 



Some of the general conclusions deduced during the 

 investigation may be given : — 



(1) In the front portion of travelling storms there is air 

 moving from high pressure to low and to lower tempera- 

 ture and rainfall, while in the rear, even quite close to 

 the centre, there is movement from low pressure to high 

 and towards improving weather conditions. 



(2) Fast-travelling storms receive air from the right 

 hand (south) of the path in front of the storm, and lose 

 an equivalent amount from the rear at the same side. 

 Slow-travelling storms receive air from the south direct to 



