Nov., V.)2\ 



THUNDERSTORMS 



29 



"heat" thunderstorm — are but an accessory result of a body 

 of extremely complex phenomena — an organism someone has 

 called it'' — the squall, which is subject to fixed laws and forms 

 an integral part of certain lows. This so-called "squall zone" 

 in which, according to Durand-Greville^, nearly all "cyclonic" 

 thunderstorms, or as he calls them, "thundersqualls," occur, 

 starts some where near the center of the barometric depression 

 ■or "low" and usually extends out to its boundary, thus having 

 a length of a thousand miles or more, while its width may vary 

 from 10 to 60 miles or more. This zone moves, advances or 

 recedes, with the "low" of which it is a part, as a rule remain- 

 ing parallel with itself. Should the "low" remain stationary, 

 the squall zone may, and usually does, swing round the center. 

 The passage of the "squall zone" over any given place, shown 

 by the familiar "squall hook" of the barograph trace, is 

 attended by the concomitant production of certain phenomena 

 that occur only within the limits of the zone. They begin at 

 the moment the "squall front" of the squall zone reaches the 

 place of observation, they rapidly attain their maximum, inten- 

 sity, and then gradually weaken and finally die out as the rear 

 of the zone passes and normal conditions become established. 

 These accompanying phenomena may be more or less numerous, 

 thus giving rise to a variety of "squalls," each characterized 

 by its appropriate phenomena. These squalls have been clas- 

 sified by Durand-Greville as follows, viz.: 



Durand-Greville's Classification of Squalls* 



2. 



3. 



Sudden increase in 

 wind velocit3^ . . 



Sudden change in 

 wind direction. . 



Sudden rise in 

 pressure 



Sudden fall in 



Whitel 

 ' squall 



•6. 



7. 



S. 



pressure 



Sudden rise in 



relative humidity. 

 Rapid increase in 



cloudiness 



Downpours of 



rain 



hail 



_ Wind 

 squall 



snow 



Lisfhtning: and thunder. 



Rain, 

 hail, 

 or 

 snow squall 



Thunder 

 squall 



