SOUTHERLY BURSTERS. 19 



order, it is followed by an overcast sky composed of nimbus from 

 which patchy rain descends. (See Plates i to 4.) 



As the cloud roll approaches it gradually loses its symmetrical 

 aspect, and careful observation reveals a light cirrus fringe rising 

 from underneath it in front, falling over the top, and trailing 

 behind the advancing cloud. Up to this time the wind has been 

 blowing with steadily increasing force from a northerly direction, 



INTERVAL OP CALM 



but at this stage it drops suddenly and a profound calm prevails, 

 broken only at intervals by a few fitful puffs. This state of things 

 lasts for a varying space ; if the southerly arrives during the heat 

 of the day, it endures but a very short time ; if at night the calm 

 is of longer duration. 



THE BURST. 



Meantime the cloud roll is seen rapidly approaching, clouds of 

 dust rise in the southern part of the city four miles away, and 

 gather volume as they come, until they almost hide the city as 

 viewed from Observatory Hill, while from immediately under the 

 roll light clouds rush forward with great velocity only to be thrown 

 back over the top of it as they reach the front, the wind vane on 

 the Time Ball tower flies to the south and the wind reaches us on 

 the ground a moment later, and in a few moments is blowing 

 with the force of a gale. 



VERY LITTLE DEW. 



It is noteworthy that the night preceding a burster, however 

 clear it may appear, seldom precipitates any measurable quantity 

 of dew. This affect is, no doubt, owing to the excessive dryness 

 of the northerly current which absorbs any moisture obtainable 

 by radiation from the earth, or by condensation from the upper 

 strata. 



A BURSTER FOLLOWS A FOGGY MORNING. 



But it is a curious circumstance that during a period of hot 

 weather, should a fog exist at daybreak a southerly change is 

 almost certain to follow within twenty-four hours. Such a con- 



