66 



NATURE FOR ITS OWN SAKE 



cooling and becoming moister as it meets with 

 cold currents, until at last it attains a height 

 where the dew-point * is reached. Then begins 

 the change into cloud. 



The hot air of summer rising upward reaches 

 its dew-point very soon, and the usual result is 

 the formation of the large cloud which we call 

 the cumulus. When there is little heat or 

 moisture in the rising air, and no pronounced 

 cold in the aerial regions through which it 

 passes, as is often the case in the spring of the 

 year, the air-current may rise to a greater height, 

 and when finally the dew-point is reached the 

 condensation appears in the form of the stratus 

 or cumulo-stratus cloud. The dryer and colder 

 the ascending current, the higher it must rise 

 before it condenses ; and so at times it rises to 

 the region of frost, then freezes into the thin 

 clouds of the upper cirrus, which are made up 

 of tiny ice-needles floating in curls and wisps 

 against the blue sky. 



When once formed, the clouds are heavier 

 than the air in which they float, and their nat- 

 ural tendency from the moment of their forma- 

 tion is downward and earthward. Knowing 

 this fact, we are often led to wonder why they 

 * See Chapter V. for explanation of the dew-point. 



