70 BOTANICAL FEATURES OF THE ALGERIAN SAHARA* 



in spring 27 per cent, in autumn 27 per cent, and in summer only 4 per cent. 

 On the High Plateau the percentages are 30, 20, and 34 for winter, spring, 

 and autumn, and 16 for summer. In the desert the percentages for winter, 

 spring, and autumn are 37, 39, and 20 respectively, while in summer 4 per 

 cent of the entire rainfall occurs. On the High Plateau, however, and in 

 the Saharan Atlas the distribution of rain is much more equable, since 16 

 per cent falls on the High Plateau in summer and 13 per cent in the Saharan 

 Atlas the same season. We have, therefore, the interesting result that both 

 in the Tell and on the desert there is a long, dry summer season, but in 

 the intervening country more or less rain falls at this time of year. It 

 seems very probable that a careful study of the plants of these regions 

 would show reflected in the vegetation this peculiar character of climate.* 



The mean relative humidity changes in a marked manner as one passes 

 from the Tell, across the High Plateau, and enters the desert. For ex- 

 ample, at Fort National the mean relative humidity is 85 per cent. On 

 the desert it varies from 54.6 per cent at Ouargla to 42.6 per cent at In 

 Salah. At times in midsummer the humidity in the desert is too low to 

 measure with instruments ; it is often 7 or 9 per cent. On the other hand, 

 the humidity in autumn is surprisingly high, ov/ing in part to the lower 

 temperature and in part to the northerly winds. However, no dew is 

 reported and probably its occurrence is rare. 



With so great difference between the Tell and the desert in relative 

 humidity is associated marked variation in the rate of evaporation. For 

 example, at Algiers the total annual evaporation is 1,654 mm., while at 

 Ghardaia it is 5,309 mm., which is possibly the greatest amount of evapo- 

 ration thus far reported. Thus, the difference in evaporation between the 

 Tell and the desert is nearly as the ratio 4 to i. 



The evaporation-rainfall ratios for the Tell, High Plateau, and the desert 

 are of great interest. The seasonal evaporation-rainfall ratio for the littoral 

 is 2.5 to I ; that of the Tell is 3.5 to i ; that of the High Plateau is 9.4 to i ; 

 and that of the desert is 46.5 to i. If we represent the evaporation-rainfall 

 ratio as unity, the ratio for the Tell becomes 1.4 and the ratio of the High 

 Plateau becomes 9 to 3.7, while the desert ratio is 18.6. 



The annual and daily variations of temperatures in the desert are nat- 

 urally relatively great. At Algiers the annual variation is approximately 

 40,7° C, while at Ghardaia it is 47.9° C. As great an annual variation as 

 57° C. has been observed at Ghardaia. The daily variation of temperature 

 is especially marked on the High Plateau and the desert, ranging 17° C. 

 more or less on the High Plateau, and 20° C. more or less on the desert. 

 These figures are occasionally overstepped: for example, at Batna in July, 

 1904, when the maximum daily range was 21.8° C; and an observation 



* Plants with subterranean water-storage organs — bulbous plants — are said to be 

 a feature of the High Plateau. 



