764 The Outline of Science 



desiccation does seem to be increasing, yet in the Sahara vegeta- 

 tion is said to be encroaching again upon the desert. 



1 



The Causes of Weather 



Weather changes are the result of the general turbulence of 

 the atmosphere; the greater the turbulence the more profound 

 will be the changes. The turbulence is due in the first place to 

 the influence of the heat received from the sun, and it is increased 

 by the effects produced by the earth's rotation and revolution; 

 by the irregular distribution of land and water on the earth; by 

 the variation, according to latitude, of the intensity of the incident 

 solar heat ; and also by the fact that the atmosphere, being a gas, 

 is subject to changes of pressure and volume as well as of tem- 

 perature. 



Although the mathematical and theoretical side of meteor- 

 ology is now receiving close attention, it has been the observa- 

 tional side of the subject that has up to the present undergone 

 the greatest development, and indeed it is most essential that the 

 observation should form the basis of the theory, and that theory 

 should conform to the facts. Therefore we find that in nearly 

 every civilised country in the world there exists a meteorological 

 organisation, generally a government institution, whose duty it 

 is to obtain observational records and to employ them for the 

 preparation of forecasts and for statistical purposes. In Britain 

 the former Meteorological Office is now a special department of 

 the Air Ministry. 



The Two Regions of Atmosphere 



Our planet is surrounded by a great ocean of air. The air 

 itself is a mixture of gases, the chief of which, in the lower layers, 

 are nitrogen and oxygen; much smaller quantities of carbonic 

 acid gas and water-vapour are also present, together with the 

 rare gas argon. In the upper layers the composition may be 



