32 



THE VOYAGE OF H.M.S. CHALLENGER. 



recorded at Oxford during twenty-four years, for the seven months from April to 

 October, of which months August is represented on Plate II. fio-s. 26 and 27 : — 



During the other five months of the year electrical displays are infrequent. As 

 these figures for Oxford may be accepted as typical of the distribution of thunder- 

 storms during the day, and the times of the maxima and minima over the land surfaces 

 of the globe at some distance from the sea-coast, it is evident that the diurnal 

 maximum occurs in the afternoon, and is substantially coincident with the afternoon 

 minimum of atmospheric pressure ; whilst on the other hand, the maximum over the 

 open sea is closely coincident with the morning minimum of pressure. Over the land 

 the maximum of thunderstorms occurs during the hours of the day when temperature is 

 highest, but over the open sea during those hours when temperature is lowest. The 

 great majority of thunderstorms over the land thus occur during the part of the day 

 when the ascensional movement of the air from the heated surface of the ground takes 

 place, and they reach the maximum when the temperature and this upward movement 

 are also at the maximum. It thus appears that ascending currents and their necessary 

 accompaniment, descending currents in the atmosphere, play an important part in the 

 history of thunderstorms. 



In places where the climate is dry and rainless, like that of Jerusalem in the 



