30 



THE VOYAGE OF H.M.S. CHALLENGER. 



maximum period, and also its diurnal maximum period, which is from six to eight hours 

 later than that of thunderstorms. 



Variation in the Amount of the Rainfall. — During the cruise every instance 

 of precipitation, — rain specified as passing showers or continued rain, drizzle, 

 sleet, or snow, — were recorded in their place of occurrence among the two-hourly 

 observations. These have been tabulated and summed up, with the following 

 result : — 



Total, 1341 



952 



2293 



These figures show (Plate II. fig. 24) that, as regards the occurrence of rain over 

 the open sea during the day, there is one maximum of 130 instances at 2 A.M., and one 

 minimum of 95 instances at 4 p.m.; and that, while for the twelve hours ending 8 a.m. 

 the number of cases was 706, for the twelve hours ending 8 P.M. the number was 635. 

 Hence the frequency of occurrence of rain over the open sea is simply inversely as the 

 temperature. Near land the distribution of rain during the twenty-four hours is 

 different, the results showing two maxima and two minima, the secondary maximum 

 occurring from 10 a.m. to 2 p.m., the two maximum periods being the times of 

 maximum and minimum temperature, and the two minima the early morning and early 

 evening respectively. 



Dr. Bergsma has shown, from sixteen years' observation made at Batavia, the 

 diurnal variation at that place, of which the following are the percentages of the daily 

 amount which fell every two hours : — 



Midnight to 2 A.M.. 

 2 a.m. „ 4 „ 

 4 6 



6 » » 8 >, 



8 „ „ 10 „ 



10 ,, ,, noon, 



It will be observed that this curve is the reverse of the curve for the open sea, 



