144 



MR JOHN AITKEN ON 



1899. 



Date. 



Hour. 



Oct. 23 



6 p.m. 



,, 24 



8 a.m. 





6 p.m. 



„ 25 



8 a.m. 





6 p.m. 



„ 26 



S a.m. 





6 p.m. 



Position of centre of Cyclone. 



West of Norway, . 

 Over Norway and Sweden, 

 Over entrance of Gulf of Finland, 

 South of Gulf of Finland, 

 West of Moscow, . 



Isobars. 



Steep to south-west. 

 Becoming equal all round. 



Miles. 



400 

 400 

 150 

 300 

 50 

 50 



The history of all these cyclones bears out the conclusion that, when the 

 barometric gradient is steep on one side, the cyclone travels quickly and nearly 

 parallel with the isobars on the steep side, and that, as the gradients become equal 

 all round the centre, the advance becomes slow. 



Dr Buchan, in his Introductory Text-Book of Meteorology, points out that the 

 isobars in cyclones are frequently elliptical. Now that is just the form we would 

 expect them to take when the barometric gradient was steep on one side only 

 — that is, with strong winds blowing in one direction. The centrifugal force of these 

 strong winds will tend to draw out the front of the cyclone, and make the length 

 of the ellipse to point in the direction in which the centre of the storm is moving. 

 When the winds are equally strong on all sides there is little tendency for the 

 isobars to depart from the circular form, which an examination of the Synoptic 

 Charts will show to be the case. 



There is another effect of the inertia of the winds blowing along the steepest 

 gradient which will be observed in these charts. Whilst the air from the anticyclone 

 on the side next the cyclone is drawn into the cyclonic vortex, the air nearer the 

 centre of the anticyclone passes on in the general anticyclonic circulation, and this 

 quickly-moving air in the anticyclonic area seems to force back the high pressure in 

 front. The effect is to aid the advance of the cyclone by lowering the pressure in front 

 of it. This effect will be seen in the chart for the 10th December 1898 (Plate I.). On 

 the morning of the 9th the shape of the isobars was similar to that shown in the chart 

 for the 10th, but the furthest north point of the 29 '9 isobar, instead of being near the 

 eastern limit of our area, was a little to the north of Scotland, and it was driven 

 eastwards to the position shown on the morning of the 10th. The curve of this isobar 

 kept much the same form as it travelled eastwards. 



We showed in the experimental part that the air currents at different elevations 

 coming to the centre of the cyclone move in different paths, the air near the ground 

 moving more radially than the air higher up, and it may be asked — Is there anything 

 corresponding to this in the cyclones in the atmosphere ? To get an answer to this 

 question, the air currents in a number of cyclones have been examined. The position 

 of the centre of the depression was taken from the Synoptic Charts, and the air 



