28 



Hon. Ralph Abercromby. Relation between 



Conclusions. 



The conclusions as to the relation of tropical to extra- tropical cyclones 

 which the author has derived from the researches of which this paper 

 gives an account may be stated thus : — 



All cyclones have a tendency to assume an oval form ; the longer 

 diameter may lie in any direction, but has a decided tendency to 

 range itself nearly in a line with the direction of propagation. 



The centre of the cyclone is almost invariably pressed towards one 

 or other end of the longer diameter ; but the displacement may vary 

 during the course of the same depression. 



Tropical hurricanes are usually of much smaller dimensions than 

 extra-tropical cyclones ; but the central depression is much steeper 

 and more pronounced in the former than in the latter. 



Tropical cyclones have less tendency to split into two, or to develop 

 secondaries than those in higher latitudes. 



A typhoon, which has come from the tropics, can combine with a 

 cyclone that has been formed outside the tropics, and form a single 

 new, and perhaps more intense, depression. 



No cyclone is an isolated phenomenon ; it is always related to the 

 general distribution of pressure in the latitudes where it is generated. 

 The concentric circles, which are usually drawn to represent a cyclone, 

 ignore the fact that a cyclone is always connected with and controlled 

 by some adjacent area of high pressure. 



In all latitudes pressure often rises over a district just before the 

 advent of a cyclone. The nature of this rise is at present obscure ; 

 but the character of the unusually fine weather under the high 

 pressure is identical both within and without the tropics. 



In all latitudes a cyclone which has been generated at sea appears 

 to have a reluctance to traverse a land area, and usually breaks up 

 when it crosses a coast line. 



After the passage of a cyclone in any part of the world there is a 

 remarkable tendency for another to follow very soon, almost along the 

 same track. 



The velocity of propagation of tropical cyclones is always small, 

 and the average greatly less than that of European depressions. 



There is much less difference in the temperature and humidity 

 before and after a tropical cyclone than in higher latitudes. The 

 •quality of the heat in front is always distressing in every part of the 

 world. 



The wind rotates counter-clockwise round every cyclone in the 

 northern hemisphere ; and everywhere as an in-going spiral. The 

 amount of incurvature for the same quadrant may vary during the 

 course of the same cyclone; but in most tropical hurricanes the 

 incurvature is least in front, and greatest in rear, whereas in England 



