66 
NATURE FOR ITS OWN SAKE 
Cloud . 
forms, 
Why clouds 
Jwat. 
cooling and becoming moister as it mects 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 aérial 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. 
