THE BRITISH ISLES. 



201 



often nearly tangential to the edge of the cloud-bank itself, and nearly 

 opposite to the direction of the wind which is presently about to spring 

 up at the earth's surface, and in nearly all cases it makes a greater angle 

 than 90° with this wind. It is important to observe that when this move- 

 ment is very rapid, the approaching depression may be expected to be 

 " deep," and probably attended by strong winds at the earth's surface. 

 As the sheet extends over us, the upper current backs very quickly, and 

 continues to do so over the whole front half of the advancing system. 



cift^ 



BLUE 



UMULUS 



BLUB 



WINDY CiKRUS 



Distribution of Clouds and Weather in a typical Cyclonic Disturbance. 



As the centre advances towards us, we commonly observe composite 

 cloud (nimbus), the rain or snow being accompanied by the new current of 

 air belonging to the depression. This cloud-bank extends in most instances 

 nearly over the whole front half of the system, both on the right hand and 

 left hand of the centre's path, but its character differs very considerably 

 on the two sides. 



If the centre is passing so as to leave our station on its right, the nimbus 

 usually continues until the centre has almost reached its nearest point to 

 us. Then, usually after a sudden increase of precipitation, the sky clears, 

 the cirro-stratus terminating in an abrupt line, while the wind at the 

 earth's surface begins to veer. Up to this time the barometer will have 

 continued to fall, except in those cases in which the whole system is be- 

 coming rapidly shallower ; and the pressure now begins to rise, except in 

 those instances in which the depression is becoming rapidly deeper. 



The appearance of the sky in the rear of the disturbance, but still on 

 the right hand side of the centre's course, is usually very unlike that in the 

 front. Comparatively few upper clouds are observed, and the movement 



N. A. 0. 27 



