The Atmosphere 



winds from the south are set up, and these, deflected by 

 the Earth's rotation, become the South-West Monsoon 

 which carries the summer rains to India. 



In the northern winter the Continent cools the more 

 rapidly, and a north-east wind results. 



Here, then, is a part of the world in which the 

 climatic conditions normal to the latitude are completely 

 overridden by others set up by the configuration of the 

 land, and as we know that the distribution of land and 

 water has been very different in the past, we may have 

 in this an explanation of some at least of the climatic 

 changes which parts of the Earth's surface have under- 

 gone during the geological periods. 



We have seen that the amount of moisture which 

 the air is capable of holding depends upon its tempera- 

 ture, and anything! which brings about a lowering of this 

 temperature will therefore tend to cause the deposition 

 of water in the form of rain or dew. 



One of the principal causes of rainfall is the cooling 

 which takes place when air is caused to expand. 



When a gas expands a part of its heat is used up 

 in bringing about the expansion, and the temperature 

 consequently falls, unless a further supply of heat is 

 provided from outside. Similarly, when a gas is com- 

 pressed it immediately becomes warmer. 



Let us now see how this will affect the distribution of 

 rainfall over the Earth's surface, and for this purpose we 

 will again refer to the diagram Fig. i. Over the equator 

 it will be noticed that the air is rising, that is, it is passing 

 from a region of high pressure to one of lower pressure. 

 This being the case, it will, of course, expand and conse- 

 quently become cooled and deposit its excess of moisture. 



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