An Introduction to the Study of South African Rainfall. 1 



of days of rain, and the average number per annum, in twenty- 

 six years ; the total fall in twenty-six years, and the annual average ; 

 the average per day, and the average per rainy day ; the greatest, 

 fall on record, in one day, and one month ; also the chance of a 

 rainy day in any coming month. It seems that a fall exceeding an 

 inch in twenty-four hours is not as a rule to be anticipated in June, 

 July, or August, nor a month's rain exceeding 2 inches. The 

 heaviest falls have occurred in the spring months, a single day's 

 rain not infrequently making the month's total equal to that of the 

 summer months. It is curious that the rate of fall should be 

 greatest in September. There seems to be some indication of 

 maximum rates at the equinoxes and solstices. 



Table 16 gives the number of daily rainfalls of given quantity in 

 the twenty- six years — the daily falls in 16a, and the monthly in 16b. 

 It appears from this that upon nearly one-half the total of rainy 

 days the fall is less than one-tenth of an inch. It must be remem- 

 bered, however, that many of these small rains are associated in 

 some way with the heavier falls, i.e., they precede or follow wetter 

 days. October is remarkable for the relatively great number of 

 days upon which the fall is between -5 inch and 1 inch. In 

 September, on the other hand, it seems to be the rule for twice as 

 many heavy falls above 1 inch as below. In fact, of the whole 

 September fall of something less than 24 inches in twenty-six years, 

 nearly 14 inches, or 58 per cent., fell in the seven days upon which 

 the fall exceeded 1 inch per diem ; whereas only 8*5 per cent, 

 fell in the three days upon which the fall was between *5 inch and 

 1 inch. In October more than 40 per cent, of the rain comes in 

 showers of "6 inch (more or less) each. 



Some individual tendency to periodicity is indicated by rains of 

 different intensity : good rains, say exceeding -5 inch, have a. 

 maximum frequency in March and a minimum in July ; which 

 happens to be the case also for very light showers. But rains of 

 between *1 inch and -5 inch have a maximum frequency in 

 February and a minimum in August. 



In Table 16b the most interesting month is October. In this. 

 month it is more than an even chance that the rain will be between 

 1 and 2 inches in amount. There is only one instance on 

 record in which the rain for the month exceeded 2 inches, the 

 excess falling in one day. The last column shows that in 

 March and October the monthly rain is as often above the average 

 as below. 



There is not yet sufficient material available here to enable us 

 to single out the rains of the different types from the ombrometrio 



