Nov., 1921 THUNDERSTORMS 27 



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 under-rtinning 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, w^e 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 



