204 Proceedings of the Ohio Academy of Science 
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, ete. 
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 
