30 WILLIAM H. ALEXANDER Vol. XXII, No. 1 



The basis of this classification is, as you see, increasing 

 complexity. Note also that the phenomena observed during 

 the passage of a squall are actually the results of two causes, 

 one of these, the "squall wind," is purely dynamic, pre-existant, 

 and may be of distant origin; the other is the local condition of 

 the atmosphere and is static. 



8. The Mechanism of the Thunderstorm. 



Thus far we have considered the thunderstorm in a more 

 or less general way — its definition, its causes, its recurrence, its 

 distribution, its relation to areas of high and low pressure, etc. 

 Let us now consider a typical thunderstorm in actual progress 

 and note its mechanism and some of its more important phe- 

 nomena. Just here the slide would be very helpful but we shall 

 content ourselves just now with the bare mention of some of 

 the things that one may look for in the well-defined thunder- 

 storm. Among these may be mentioned the winds, the squall 

 cloud, the pressure, the temperature, the humidity, the rain, 

 the hail, the so-called "rain-gush/' the rate of advance of the 

 storm, the lightning, and the thunder. 



First, the thunderstorm winds must be carefully considered 

 if one is to understand fully the mechanism of the thunderstorm 

 itself. As every one knows, as a thunderstorm approaches a 

 given place the wind at that place is generally light and from 

 a direction that carries it across the path of the approaching 

 storm, that just before the rain begins the wind begins to die 

 down, almost to a calm, and to change its direction. When 

 this change is complete it blows for a few moments, rather 

 gently, directly toward the nearest portion of the storm front, 

 and finally as the rain is almost at hand, there is a sudden 

 change of direction and the wind now comes, often in violent 

 gusts, directly away from the storm and in the direction the 

 storm is moving, a direction quite different from the original 

 direction of the wind. As a rule these strong gusts of wind last 

 through the early part of the storm only and then follow gentle 

 winds again, at first following the storm but after an hour or 

 so they blow from the same general direction as the original 

 surface winds. Now, as we have tried to show, the thunder- 

 storm is the child of a cumulus cloud and the cumulus cloud 

 is the child of a vertical convection which results from a more 

 or less super-adiabatic temperature gradient. This gradient 



