CH. II 



CLIMATE 19 



America the rainfall is heavy on the sierras on the east 

 coast, while the rainfall decreases on their western 

 slopes, and only increases again when the still higher 

 ranges of the Andes cause fresh precipitation of rain. 

 And so on in other continents. 



The saturation of the atmosphere or its relative 

 humidity is greatly dependent on the amount of rain- 

 fall, although, as will be seen later, it is also increased 

 by the neighbourhood of large sheets of water, such as 

 the sea, lakes, or rivers. Thus the relative humidity in 

 the Nile basin increases from a mean of 34 per cent in 

 the morning and 20 per cent in the afternoon, for the 

 year, at Assouan, which is just outside of the Tropics, 

 to 80 per cent in the morning and 68 per cent in the 

 afternoon at Bakoba on Lake Victoria. 1 



Yet another factor having- a bearing- on forest 

 vegetation is the range of temperature ; yet, within 

 the Tropics, it is not of the same importance as in other 

 zones, as the extremes are not as great. According to 

 Schimper, 2 the differences between the mean tempera- 

 tures of the hottest and coldest month near the equator 

 lies between 1 and 5 centigrade (1*8 and 9 Fahr.), 

 and does not exceed these figures even in the interior 

 of continents, and, according to him, even towards the 

 limits of the Tropics, and in the extremest climates met 

 within the Tropics, the annual variation hardly exceeds 

 13 C. (23-4 Fahr.). The latter statement is not 

 entirely borne out by observations made in the Nile 

 basin, 3 for in the northern part of the Sudan, at Wadi 

 Haifa, the difference between the hottest and coldest 

 month is over 18 C. (32 '4 Fahr.), while at Assouan 

 it is 19 C. (34*2 Fahr.). It must also be borne in 

 mind that although close to the equator the mean 

 diurnal range may not exceed 13 C. (23 '4 Fahr.), i.e. 

 from 17 to 30 C. (62'6 to 86 Fahr.), this range may 

 vary, e.g. at Berber, from 9 C. (48*2 Fahr.) in January 



1 Captain H. G. Lyons, The Physiography or the River Nile and its Basin, 

 1906. 



2 Op. cit., transl. p. 213. 3 Captain H. G. Lyons, op. cit. 



