20 THE BOOK OF NATURE STUDY 



Again, we have noticed that in spring, when the weather is 

 often dry, east winds are common, and are often cold. We have 

 seen that they are cold because they come from those plains of 

 Europe which are not, like our islands, warmed by nearness to 

 the sea. We now ask ourselves why they are dry. The answer 

 is of course that, coming from a colder region to a warmer one, 

 they become capable of taking up more moisture, and have no 

 tendency to throw dow r n their existing load. 



In such ways we suggest that cooling air seems to make it 

 damper, that heating it seems to make it drier, and that therefore 

 rain, no less than the appearance of a cloud of vapour when we 

 breathe, means that warmed air is being chilled down. If the 

 air be chilled down to a very great extent the moisture may pass 

 direct from the state of vapour to that of snowflakes, and fall as 

 snow. On the other hand, if the raindrops in descending fall 

 through a very cold zone of air, as happens not infrequently in 

 thunderstorms, freezing takes place, and the drops fall as hail- 

 stones. In its simplest form this is the explanation of rain, hail, 

 and snow ; the first and last especially, however, require much 

 more detailed study. 



We have striven to give a picture of the ever-changing mantle 

 of air which lies over the whole of the surface of the globe. We 

 have seen that in different parts it is unequally heated, and this 

 unequal heating starts movements which present themselves to 

 our senses as wind, and which are determined by those differences 

 of pressure which the barometer registers. We have learnt also 

 that all air can take up a certain amount of water, the amount 

 varying with its temperature, warm air always taking up more 

 than cold air. Therefore we know that cooling air has always a 

 tendency to cause the invisible moisture to appear as mist, rain, 

 snow, or hail, according to the conditions. 



We have noticed also, without troubling about Boyle's law, 

 that air cools when it expands ; that it cools as it rises, while it 

 becomes warmer as it is compressed, that is, it becomes capable 

 of holding more vapour as it descends. We have repeated in 

 imagination or in reality our mountain ascent, and have added to 

 our first note that it is not only colder on the hill but rain is more 

 frequent there, because the cold mountain cools the air and makes 



