Thirty-first Annual Meeting 201 
sole cause of thunderstorms on the ocean. Number three is 
by far less frequently the cause of thunderstorms than the 
other two, for while the actual uwnder-running is of rather fre- 
quent occurrence, it seems probable that only occasionally is 
the uplift of sufficient magnitude to cause a thunderstorm. 
6. Periodic Recurrence of Thunderstorms. 
Keeping in mind the conditions or factors absolutely essen- 
tial to the formation of a thunderstorm, we are well prepared 
to consider, perhaps in a measure to anticipate, the periodic 
recurrence and distribution of thunderstorms, for while it is 
possible, of course, for a thunderstorm to occur on any day at 
any hour, yet the-fact is, and for obvious reasons, the day has 
its period of maximum thunderstorm activity, the year its max- 
imum period, and there is some evidence of irregular cyclic 
periods of maximum activity, each maximum depending upon 
the simple facts that the more humid the air and the more 
rapid the local vertical convections the more frequent and also 
the more intense the thunderstorms. 
Taking the day as our unit, we find the period of maximum 
thunderstorm activity is not the same over the land as over 
the ocean. Vertical convection of the atmosphere over the 
land is most pronounced, of course, when the surfaces are most 
heated, namely, in the afternoons; hence the inland or con- 
tinental thunderstorm occurs most frequently, in most places, 
between 2 and 4 P. M. Over the ocean, however, the temper 
ature gradients that are most favorable for rapid vertical 
convection are most frequent during the early morning hours, 
and therefore thunderstorms usually occur on the ocean between 
midnight and 4 A. M. If we take the year as our unit, we find, 
for reasons that will readily occur to all, that thunderstorms 
are most frequent, over the land, when the surface heating is 
at a maximum, in middle latitudes in June and in the higher 
latitudes in July or August. Over the ocean, however, the 
thunderstorm is most frequent in the winter months. 
Furthermore, since the thunderstorm is vitally associated 
with rainfall and high temperature, it must follow that a cycle 
of warm, wet years would give a maximum of thunderstorms 
and a cycle of cold, dry years a minimum. 
We have the key to the geographical distribution of thun- 
derstorms in the conditions essential to their production, and 
