Thirty-first Annual Meeting 203 
““‘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.: 
oe 
DURAND-GREVILLE’S CLASSIFICATION OF SQUALLS® 
1. Sudden increase in 
Wan VeElOGIEYi ss ie. <2 4.5% 4 
Sudden change in 
Witla GireCtion...........- | squall 
Sudden rise in 
PREGSUTE. £5206 Pe bed. 
Sudden fall in Wind 
DRCSSULCss War ha waus es ae}geeds: . squall 
Sudden rise in 
Petitivie Mianidity, «<6... 2-<02 ~~. | Rain, 
Rapid increase in | hail, 
Blodelimesss 2 hie 2 eee be: or 
TS Se ee LN 
AAR ceeesce (eM Sie eh Pout IE Maes 62 at squall 
