8 



BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



while the rainfall is go. 2 mm.; yet at El Golea, with a rainfall of 47 mm., 

 there are 23.4 rainy days each year.* 



On the desert the rains are often of a torrential nature, as facts presented 

 above would indicate, and sometimes as much rain falls within a few hours, 

 or even a few minutes, as usually occurs in an entire year. How much of 

 the annual precipitation is of this character and how much is of the non- 

 torrential kind the usual summaries leave entirely out of the account. It 

 is well known that the former type of storm is more destructive and less 

 useful to plants than the latter type. To illustrate the irregularity of the 

 rainfall in the northern Sahara the monthly precipitation at Ouargla for 

 several years is presented in table 2. 



Table 2. — Rainfall at Ouargla, in millimeters. 



The seasonal distribution of rainfall at four desert stations shows also 

 marked irregularities. The distribution (in percentages) is given in table 3 . 



Table 3. — Seasonal distribution, in percentages, of rain. 



The relative humidity at the desert stations is often very low, sometimes 

 running in midsummer between 7 and 9 per cent for 6 days, and occasion- 

 ally being too low to measure with the instruments employed. The mean 

 relative humidity (table 4) shows the general very dry condition of the air 

 of the desert as contrasted with a station in the Tell, and also indicates 

 something of the monthly variations in this factor experienced among the 

 desert stations themselves. The averages given are from 4 to 8 years, 

 except the mean annual for In Salah, which is for 2 years only. 



Table 4. — Mean relative humidity, in percentages. 



* The most recent records available, 1897-1908, do not give a satisfactory account of 

 the precipitation on the desert. For instance, meteorological records covering eight 

 years for El Golea do not report on the rainfall. Records of five years at In Salah take 

 the rainfall into account on one year only, and on that year no precipitation occurred. 



