Tropical and Extrcir-tropical Cyclones. 



29 



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 central 

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

 winds and steepest gradients are often some way from the centre. 

 The portion of a cyclone which is of hurricane violence forms, as it 

 were, a kernel in the centre of a ring of ordinarily bad weather. In 

 this peculiarity tropical cyclones approximate more to the type of a 

 whirlwind 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 alline- 

 ments of the stripes of cirrus appear to lie more radially from the 

 centre in the tropics, instead of tangentially to the isobars, as indicated 

 by the researches of Ley and Hildebrandsson in England and Sweden 

 respectively. 



The general character of the cloud all round the centre is more 

 uniform in than out of the tropics ; but still the clouds in rear are 

 always a little harder than those in front. 



Everywhere the rain of a cyclone extends farther in front than in 

 rear. Cyclone rain has a specific character, quite different from that 

 of showers or thunderstorms ; and this character is more pronounced 

 in tropical than in extra- tropical cyclones. 



Thunder or lightning are rarely observed in the heart of any 

 cyclone, and their absence is a very bad sign of the weather. 

 Thunderstorms are, however, abundantly developed on the outskirts 

 of tropical hurricanes. 



Squalls are one of the most characteristic features of a tropical 

 cyclone, w r here they surround the centre on all sides ; whereas in 

 Great Britain squalls are almost exclusively formed along that portion 

 of the line of the trough which is south of the centre, and in the right 

 rear of the depression. As, however, we find that the front of a British 

 cyclone tends to form squalls when the intensity is very great, the 

 inference seems justifiable that this feature of tropical hurricanes is 

 simply due to their exceptional intensity. 



A patch of blue sky in the centre of a cyclone, commonly known as 

 the "bull's-eye," is almost universal in the tropics, and apparently 



