

CLIMATE 315 



Climatic effect of forests. Forests have some influence on 

 continental climates. They lower the summer temperature by 

 increasing the radiating and evaporating surfaces, and by increasing 

 the cloudiness. They increase the relative humidity of the air, but 

 it seems to be uncertain whether they have much effect on pre- 

 cipitation. In any case, they tend to hold back the water after it 

 falls (that is, they retard the immediate run-off, p. 30), and to retard 

 the melting of snow, so that their general effect on the moisture of 

 the region is much the same as it would be if the precipitation were 

 increased. Forests also afford protection against winds. 



THE CLIMATES OF THE SEVERAL ZONES 

 The Tropical Zones 



The first characteristic of the climate of these zones is its high 

 temperature, with the oceanic climate more uniform and less warm 

 than the continental. The prevailing winds are easterly, north- 

 easterly in the northern tropical zone, and southeasterly in the 

 southern, with a zone of calms (the doldrums) between. Many 

 lands in the path of the trade-winds, as the Sahara and part 

 of Australia, are desert; but where these winds blow over moun- 

 tains or plateaus, they yield moisture to them, especially to their 

 windward sides or borders (p. 279). The abundant rainfall on the 

 east slope of the Andes, on the tableland of Brazil, and on the 

 higher parts of the Hawaiian Islands, are illustrations. Even in 

 the Sahara, there are mountains which occasion rain enough to sup- 

 port forests, but the streams from the mountains soon disappear in 

 the desert. 



Monsoon winds are strong in places in the tropical zone, and 

 locally give rain to regions which would otherwise be dry. Since 

 monsoons generally blow from sea to land during the warm season, 

 the monsoon rains generally fall at that time. 



The tropical zone does not depend entirely on winds for its rain- 

 fall. Rainfall and cloudiness increase toward the center of the 

 zone, while the strength of the winds decreases. In the doldrums, 

 the rising air of the convection currents carries up abundant mois- 

 ture, which, on cooling, is condensed and precipitated, giving daily 



