CLOUDS AND CLOUD FORMS 



39 



the winds. It is visible only where it clings 

 to the lee-side of the peak, and it stretches 

 out into the air as far as shelter is afforded it 

 in the shape of a long, thin flag. At a distance 

 it looks as though it were something perma- 

 nent, whereas it is only a continuous-forming 

 cloud cut sharp on its sides by the keen edges 

 of the wind. 



But these illustrations are of exceptional 

 clouds, and even with them the rising currents 

 alone are hardly sufficient to account for their 

 being sustained in air. The majority of clouds 

 are formed in open space and their air-currents 

 have no mountain-sides to protect them. Nor j 

 are the common clouds subject to such violent 

 destruction as the banner clouds. Moist currents 

 are rising, clouds are forming and reforming, 

 changing, sinking, disappearing ; but they are 

 not often slashed into strips by the winds. We 

 must seek a third cause for their being sus- 

 tained in air, and it has been suggested already 

 by the word " renewal." Clouds after they are 

 formed are practically self-renewing. When 

 the ascending air-current condenses into cloud 

 the heat of the air-current goes upward with a 

 tendency to form newer and higher clouds as it 

 rises ; but the moisture of the current, robbed 



formed in 

 open space. 



Self-re- 

 newal of 



