4 



Hon. Ralph Abercromby. Relation between 



extends further in front than in rear of the centre of the cyclone, but 

 still both rain and cloud are obviously related to that point (see fig. 1). 

 But, strange to say, the kind and character of the cloud and the 

 relative warmth and dampness of the weather are not related to the 

 centre but to the front and rear of a line drawn through the centre and 

 more or less perpendicular to the line of propagation of the cyclone, 

 marked " trough " or line of barometer change in the diagram, 

 fig. 1. For instance, the whole of the weather in front of that line 

 is warm, muggy, and damp, while all in rear is cold and dry. The 

 effect of this is that when a cyclone drifts past an observer, he expe- 

 riences a sudden change from warm and damp to dry and cold weather 

 the moment the trough has passed over him. 



Then as to the kind of cloud. — A thin ring of halo forming cirro- 

 stratus or cirro-nebula fringes almost all the outside of the front edge 

 of the main cloud ring, but this kind of sky is rarely seen anywhere 

 in rear of the cloud ring of the cyclone. Moreover all the cloud in 

 front of the trough is usually more or less stratiform — cirro-stratus 

 and strato- cumulus — while that in rear is distinctly of the cumulus 

 type. 



The portion of the line of the trough that is drawn southwards 

 from the centre marks out the position of a long line of squalls, and 

 of a very much more sudden shift of wind than the symmetry of the 

 cyclone might have suggested. Instead of veering steadily from S.E. 

 to N.W. the wind jumps during the passage of the trough sometimes 

 as much as from S.S.W. to W.N.W., and then veers more gradually. 



Considering then both cloud and wind, the sequence of weather to 

 a solitary observer as a cyclone is propagated over him, the centre 

 passing to the N., will be as follows : — Cirro-stratus and halo will 

 appear in a blue sky, while the wind comes from about S.E. Gradually 

 the clouds get lower and heavier, as they gather into strato -cumulus ; 

 then rain comes on, while the wind has veered steadily to S.W. 



All this time the barometer has been falling fast, but suddenly just 

 as the mercury has reached its lowest point a violent squall comes 

 on, during which the wind jumps suddenly three or four points from 

 the S.W. up to W. or W.N.W. Almost directly after, unless very 

 near the centre of the cyclone, the clouds take the form of cumulo- 

 nimbus, and then of simple detached cumulus, till the sky is com- 

 pletely blue again. 



Any number of observers, situated on the southern side of the path 

 of the centre, will find their weather change suddenly just as the 

 mercury has begun to rise ; and we, therefore, conclude that a line 

 drawn across a cyclone at any moment, through all points where the 

 barometer has just turned upwards owing to the onward motion of 

 the depression, divides the cyclone into two halves which have 

 different physical characteristics. 



