PRECIPITATION 19 



sufficiently- ventilated schoolroom seize such simple facts as 

 these as the preliminaries to the statement that warm air can hold 

 much more water vapour than cold air. Again, the heating of the 

 nozzle of the inflator as the air is forced into the bicycle tyre may 

 be used as an illustration of the fact that when air is compressed 

 it becomes warmer, while at the same time we explain that as it 

 expands it becomes cooler. 



If the neighbourhood afford no examples of hills high enough 

 to show the formation of cloud, pictures must be utilised to 

 explain that just as the moisture in our breath condenses into 

 vapour when we leave a warm room for the chill of a winter's 

 day, so the warm dense air of the lowlands as it rises up against the 

 cold mountain-side grows chill, and forms a cloud of vapour. If we 

 hold a bright object in our breath it becomes clouded with vapour, 

 which form minute drops. In the same way the light vapour which 

 we see against the mountain is mist or cloud to those who are within 

 it. In some such ways try to explain the formation of rain-clouds, 

 try to give an idea of the invisible load of moisture everywhere 

 present in the air, try to show that the air only requires to be 

 cooled down enough, and however dry it may appear it will throw 

 down some moisture. 



All air then contains moisture, and air only requires to be 

 :ooled for this moisture to appear as mist or rain. What cools 

 the air in natural conditions ? There are two main causes : air 

 becomes cooled as it expands in ascending, and it also becomes 

 cooled by passing from a region of higher temperature to one of 

 lower. Take the first point. We have noticed that in a room 

 the air is hotter near the ceiling than on the floor. But then the 

 upper layers are retained by the ceiling. If they had been free to 

 move they would have risen higher and higher, cooling as they rose, 

 like the hot air which cools as it rises up a mountain-side. When, 

 then, in summer we enjoy the pleasant warmth of the turf on 

 which we are lying, we have to remember that the pleasant warmth 

 would soon become suffocating heat were it not that the air as it 

 is warmed by the hot soil is constantly rising, cooling as it rises, 

 and that it is always new layers which are becoming warm. As 

 the warmed air rises and cools its moisture may form fleecy clouds 

 such as those we see in the sky. 



