CHAPTER XVIII. 



ATMOSPHERIC DRAINAGE. 



In the flat coast country atmospheric drainage is 

 of very little importance. Generally its effect is 

 two-fold: cold currents are deflected from higher 

 ground, but, being held in the locality of the ob- 

 stacles placed for their hindrance create a lower 

 temperature around them. Where there is suffi- 

 cient slope to allow currents, which hug the earth, 

 to drain off on lower lands, a white frost is often 

 entirely avoided; but wind-breaks, though mitigat- 

 ing the seriousness of freezes, so hinder surface 

 air during still nights that the lowest temperatures 

 are found in their lee just where we ordinarily ex- 

 pect the most protection. Night currents keep close 

 to the ground, moving independent of wind pres- 

 sure, and, being cold, seek the lowest places. Their 

 movement is often two to six miles per hour, not- 

 withstanding the air appears practically still. In- 

 struments designed for measuring this motion indi- 

 cate its existence practically all the time. No ade- 

 quate idea of the importance of these movements 

 can be realized without keeping in mind that at- 

 mosphere is a very poor conductor of heat ; were it 

 not that heat sets air rapidly in motion it would be 

 the only substance used for insulating all cold 

 storages. Discolor a current and watch while its 



