EFFECTS OF FORESTS UPON LOCALITY 3 



country consist of pasture land, or if it be under cultivated 

 crops, though the absolute humidity would be increased, if 

 the neighbouring country were a dry, more or less barren, 

 waste. Now, inasmuch as the average temperature of the 

 air is less in woodlands than in the open country, it follows 

 that the relative humidity of the atmosphere is much 

 increased by the presence of woodlands ; especially does 

 this increase take place in the summer and early autumn, 

 when the difference in the air temperatures is at its 

 greatest. 



Hence, on account of this increase in the relative humidity, 

 there is always, cceteris paribus, a greater likelihood of rain 

 or mists occurring in a well-wooded area than in a treeless 

 country. This tendency to an increased rainfall is usually 

 only noticed at very high altitudes, or in dry, barren countries ; 

 for in the former case the extremes of temperature by day 

 and night are nearly always far greater than at low latitudes, 1 

 and in the latter case the absolute humidity in the summer 

 months, as well as the relative humidity, is generally much 

 increased. Furthermore, when the surrounding country is 

 dry and somewhat barren, the summer rainfall is more 

 regular. So also, an increased dewfall may always be 

 expected on grass-land or on crops in the neighbourhood 

 of woodlands. Then again, as air rises and gets more 

 rarified, it becomes cooled, and, if its relative humidity were 

 already near the saturation point, the moisture contained 

 therein will consequently be precipitated either as mist, dew, 

 or rain. On the other hand, at low altitudes in fertile districts 

 large unbroken stretches of woodland may actually tend to 

 lessen the rainfall, for although the average relative humidity 

 is generally greater, yet the relative humidity at night-time, 

 in the summer months, is usually less in woodlands than in 



1 However, at the lowest altitudes, or those relatively lowest in a 

 locality, there are often greater extremes of temperature by day and 

 by night, than are experienced at medium altitudes. Especially is this 

 the case in the spring and autumn, when, in the lowlands and coombes, 

 frosts are often experienced at night ; whereas, at somewhat higher 

 altitudes, these frosts do not occur. 



