200 Proceedings of the Ohio Academy of Science 
This, however, is not sufficient to account for all the observed 
facts, since it is not clear just how either the condensation or 
the congelation could suddenly take place throughout a cloud 
volume great enough to produce the observed effects. We 
must, therefore, look for some other explanation, and this we 
shall probably find, 7 the difference between the actual tem- 
perature gradient of the surrounding atmosphere and the adiabatic 
temperature gradient of the saturated air within the cloud itself; 
or, in other words, the cause of the violent up-rush and tur- 
bulent condition within large cumulus clouds is, presumably, 
the difference between the temperature of the inner or warmer 
portions of the cloud itself and that of the surrounding atmos- 
phere at the same level. 
5. Causes of Convectional Instability. 
As we have just tried to show the sine qui non of the thun- 
derstorm is the rapid vertical corivection of moist air; the up-rush 
must be rapid and the air must be mozst; one without the 
other is not sufficient. We may have, for example, a very rapid 
convection over a desert region but there being no moisture 
there will be no cloud-formation and therefore no thunderstorm. 
On the other hand we may have air ever so humid but if the 
movement upward is too gentle not even a cloud may result, 
but if a cloud, certainly no thunderstorm. It is obvious, 
therefore, that we must have both ‘‘rapid convection’’ and 
[eiaOish aad! 
This leads us to a consideration of the conditions under 
which the vertical temperature gradients necessary to this 
convection can be established. These conditions are, according 
to Prof. Humphreys, three in number, namely: 
(1) A strong surface heating, expecially in regions of light 
winds. 
(2) The over-running of one layer of air by another at a 
temperature sufficiently lower to induce convection. 
(3) The under-running and consequent uplift of a saturated 
layer of air by a denser layer. 
Of these three conditions, the first mentioned—‘‘strong 
heating surfaces’’—is, for obvious reasons, of most frequent 
occurrence over the land surfaces of the earth; number two is 
also of frequent occurrence on land and is, perhaps, well nigh the 
