Drainage of Cold Air 



49 



Location with reference to weather. — Atrriospheric drainage. 



The air is rarely, if eyer, perfectly still. This is well 

 illustrated in the vagaries of light' frosts, which touch here 

 and there where the air is the stillest or the radiation 

 most rapid. This is particularly true in the growing 

 months, when the earth becomes very warm' in the day 

 and loses the heat rapidly at nightfall, and when, also, the 



Fig. 1. Atmoapheric drainage. Peach buds on the trees in region of X are 

 frequently destroyed by late frosts, while those on trees in region of O are not 

 injured. 



sky is less overcast by clouds than it is in the winter 

 months. 



Much of this unrecognizable movement of the air is 

 due to the draining off or settling away of the cold air, 

 which is densest and therefore heaviest. It pours down 

 the valleys of hilly and mountainous. countries, and as its 

 vapor condenses it gives rise to the valley fogs and clouds 

 It lies in the low places, and there may cause frost. A 

 person riding across an undulating country on a still sum- 



