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VII. — Notes on the Dynamics of Cyclones and Anticyclones. By John Aitken, F.R.S. 



(With a Plate.) 



PART I. 



(Read March 5, 1900.) 



The vertical movements of the earth's atmosphere from which the energy is derived 

 which causes the horizontal movements of the air which we call winds, and by means of 

 which the moisture evaporated from the surface of land and water is collected and 

 carried to the higher regions of the atmosphere, where it is condensed to cloud and 

 again distributed in the form of rain over the earth's surface, are of great interest, and 

 a thorough knowledge of the laws governing these vertical movements is necessary 

 to enable us to arrive at a correct forecast of the coming weather over any area. 



In the present communication I do not intend entering on a review of the work 

 which has already been done in this field. Many explanations have been offered of the 

 movements of cyclones and anticyclones as a whole, and of the winds within their 

 areas, but any detailed reference to these would far exceed the limits of these notes, and 

 would, I fear, only complicate matters. In what I have to say there will necessarily 

 be much that is old, and I am sorry I must leave to the reader the task of finding out 

 what is new, as in a subject of this kind, on which so much has already been written, it 

 is impossible to say whether any particular point has not been referred to before by some 

 other writer. And further, I shall confine my remarks to what takes place over our 

 area and Western Europe, so as to avoid unnecessary verbal complications ; but the 

 principles can be easily applied to other areas in the Northern hemisphere, and to the 

 reversed direction of circulation in the cyclones and anticyclones in the Southern 

 hemisphere. 



At the outset it will be as well for me to make a few elementary remarks on the 

 formation of cyclonic movements, as I find that many who take an interest in Meteor- 

 ology have rather hazy ideas of how the vortex motion in cyclones is produced. All 

 that some seem to think necessary is to have an area of low pressure and that the air 

 will rush towards it in spiral paths, just as they see water in a wash-hand basin forming 

 a vortex movement whenever the plug is withdrawn and the water allowed to run away. 

 Now it must be clearly understood that no vortex will form in air or water that is at 

 rest before the low-pressure area is formed ; the air or water under these conditions will 

 flow to the low-pressure centre along radial paths and not in spiral ones. The above 

 statement requires qualification. When it is said the air or water is at rest it is not 

 meant that it is at rest absolutely, which would be an impossibility in this rotating, 

 revolving, and space-travelling world of ours. All that is necessary is that the water or 



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