REPORT ON ATMOSPHERIC CIRCULATION. 



31 



while the curve for the observations made near land partakes of the character of 

 both curves. Much yet requires to be done in collecting the suitable data of 

 observation for a proper treatment of the question of the diurnal curves of the rainfall 

 of different climates. Such data, however, so far as collected, show the general occur- 

 rence of a maximum from about 11 a.m. to 6 p.m., and this peculiarity of the curve is a 

 particularly outstanding feature of the curves of continental climates during the summer 

 months of the year, when thunderstorms have their maximum annual occurrence. A 

 marked diminution of the rainfall is generally observed from about sunset to midnight, 

 or during the hours when, in many climates, the amount of cloud falls to the 

 minimum, and the evening maximum of pressure takes place. The time of the 

 morning minimum of pressure, from about 2 to 6 a.m., is, curiously, in many places 

 strongly marked as a maximum, whereas in others it is equally strongly marked as 

 a minimum, of which the Challenger and Batavia curves may be taken as typical 

 examples. 



Variation of Thunderstorms. — The following table shows the distribution 

 through the hours of the day of the cases of occurrence during the cruise — (l) of 

 thunderstorms or thunder with lightning, and (2) of lightning alone : — 



Of the 45 thunderstorms recorded, 26 occurred over the open sea, and 19 near 

 land. Of those recorded over the open sea 22 occurred during the ten hours from 

 10 p.m. to 8 A.M., whereas during the other fourteen hours of the day only 4 

 occurred (Plate II. fig. 25). Hence the important conclusion that over the open sea 

 thunderstorms are essentially phenomena of the night, and occur chiefly during the 

 morning minimum of pressure. On the other hand, as regards the thunderstorms 

 which occurred near land, they are pretty evenly distributed during the twenty-four 

 hours. 



Over land, but especially where the climate is more or less continental in its 

 character, the distribution of thunderstorms during the day is the reverse of the above. 

 The following table shows the number of (l) thunderstorms, and (2) lightning only, 



