THUNDER AND LIGHTNING 215 



As contrasted with lightning they usually move with un- 

 canny deliberation, floating along so slowly that they have 

 sometimes been followed by a curious crowd. They usually 

 end by exploding, sometimes gently, but sometimes with 

 destructive violence. Genuine fireballs as part of the pheno- 

 mena of a thunder-storm are seldom seen ; most of the 

 reports of their observation are due to a confused and 

 startled impression of some particularly near and vivid 

 lightning-flash. 



Our occasional thunder-storms in winter are more closely 

 connected with sharp changes of weather than the storms 

 brewed locally in summer by the heat of the sun. The con- 

 flicts of temperature which cause them are produced by 

 eddies and squalls, especially the line-squalls which advance 

 rapidly across a wide front, and leave the air many degrees 

 colder behind them. Turbulent south-westerly weather in 

 autumn produces yet another variety of thunder-storm, due 

 to the same fundamental opposition of varying temperatures. 

 On stormy October evenings, with a wild, warm air drawn 

 from far southward on the Atlantic, there is often a flash of 

 lightning and a crash of thunder in the clouds racing swiftly 

 to the east ; and sometimes an autumn thunder-storm rages 

 for some hours at night. These storms are part of the gale, 

 and have not the independent solemnity of summer storms, 

 which so frequently work up on a different current to the air 

 prevailing on our level. They occur most frequently on the 

 outskirts of a great cyclonic depression, and at night ; and 

 the reason in both cases is the same. On the edge of the 

 cyclone the warm Atlantic wind meets the colder air of our 

 autumn climate, and at night the temperature falls, so that 

 the inrushing air meets with a sharper contrasting chill. 

 Such storms often sweep past so rapidly, and amid such 

 roaring of the gale, and play of starlight and moonlight in 



