THE SKY 59 



Much more profitable than the naming of the cloud types is the 

 investigation of the invisible phenomena which are accompanying 

 the visible one of cloud formation. In the early morning when the 

 sky is clear, the ground will be found to be soaked with dew, all the 

 plants and shrubs being dripping with it. As the sun's rays grow 

 stronger the dew disappears. But we have already noticed that 

 damp clothes, for instance, even in a warm room, do not dry much 

 unless the window is opened to let the damp air escape. Therefore 

 we conclude that the layer of air close to the ground must be moving, 

 even if there seems to be little wind, because otherwise the drying 

 process could not be continuous. We know that as the sun beats 

 down it warms this air, which then rises, to be continually replaced by 

 fresh air. As it rises up into the clear sky with its load of moisture 

 it cools, until finally, when it reaches a height of somewhere about 

 4500 to 6000 feet, it has become too cold to hold its moisture any 

 longer, and this condenses to form a cloud. The fact that on any 

 given day the level at which this condensation occurs is more or 

 less constant, explains the flat lower edge of the clouds. When 

 cloud-formation has reached considerable dimensions, as happens 

 usually about midday, then the clouds form a partial screen across 

 the sky, and so diminish evaporation. This checks the rapidity 

 of the upward movement of the heated air, and thus a condition 

 of balance may be more or less obtained, so that the clouds largely 

 dissolve away at sunset. 



The actual conditions are of course much more complex than 

 this would suggest, but this is the essence of the matter. If, 

 instead of melting away and flattening out towards sunset, the 

 cumulus clouds become denser and more massed towards evening, 

 then the atmosphere has received a greater load of moisture than 

 it can hold, and rain is likely to follow. Over most of Great Britain 

 it is unlikely that the teacher will have an opportunity of demon- 

 strating the formation of cumulus round elevated headlands, lofty 

 hills, rocky islands, and so on, as an ordinary summer phenomenon, 

 but if exceptionally such local conditions present themselves, 

 they should be taken full advantage of. If they are not available, 

 pictures of Alpine summits, or of the "tablecloth" over Table 

 Mountain at the Cape of Good Hope, will be found useful. 



Very different from the low-lying cumulus and stratus clouds 



