1887.] Tropical and Extra- Tropical Cyclones. 



139 



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. 



An area of excessive pressure, with unusually fine weather, precedes 

 most cyclones. Though the nature and origin of this high barometer 

 is very obscure, the general character of the formation, and the 

 weather associated with it, appear to be the same everywhere. 



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 ingoing 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 

 the greatest incurvature is usually found in the right front. Some 

 observers think that broadly speaking the incurvature of the wind 

 decreases as we recede from the Equator. 



The velocity of the wind always increases as we approach the centre 

 in a tropical cyclone ; whereas in higher latitudes the strongest winds 

 and steepest gradients are often some way from the centre. In this 

 peculiarity tropical cyclones approximate more to the type of a 

 tornado ; but the author does not think that a cyclone is only a 

 highly developed whirlwind, as there are no transitional forms of 

 rotating air. 



The general circulation of a cyclone, as shown by the motion of the 

 clouds, appears to be the same everywhere. 



All over the world, unusual coloration of the sky at sunrise and 

 sunset is observed, not only before the barometer has begun to fall at 

 any place, but before the existence of any depression can be traced 

 in the neighbourhood. 



Cirrus appears all round the cloud area of a tropical cyclone, instead 

 of only round the front semicircle, as in higher latitudes. The stripes 

 of cirrus appear to lie more radially from the centre in the tropics, 

 than tangentially, as indicated by the researches of Ley and Hilde- 

 brandsson in England and Sweden respectively. 



