770 The Outline of Science 



And if the pressure of one portion of a gas increases, there will 

 be a flow of gas towards the other portion which is at lower 

 pressure, and in the case of our atmosphere this transference of 

 air is made very evident to us as wind. 



Now if the air over one part of the earth's surface becomes 

 heated to a higher degree than that over surrounding parts, the 

 heated air will expand, and, in doing so, it causes the air layers 

 above it to become compressed. These layers will then have a 

 greater pressure than the surrounding air at the same levels. A 

 flow of air, therefore, takes place towards the region where 

 pressure is lower, and this additional air will, in turn, compress 

 the air in the layers below it, so that now there will be greater 

 pressure in the surface air surrounding the heated area, and the 

 air at the surface will commence to move towards the heated area 

 from all sides, and a "circulation" will thus be established, with 

 the result that a wind will blow. 



A rough illustration of a circulation is found in what hap- 

 pens when a window in a heated room is opened top and bottom ; 

 the warm air will flow out at the top, and cooler air will flow in 

 at the bottom to replace it. 



Small local atmospheric circulations are found everywhere, 

 and every day, all over the world; but, because the sun's heating 

 power is so very much greater in Equatorial regions than else- 

 where, and because in higher latitudes the sun's altitude in the 

 heavens varies considerably during the course of a year, it fol- 

 lows that great seasonal changes of temperature must occur, and 

 also that great seasonal circulations are set up in the atmosphere 

 like great currents in the ocean, and these great seasonal circula- 

 tions dominate the weather and its changes. 



M 

 The Effects of Turbulence in the Atmosphere 



The turbulence that takes place within the lower layer, or 

 troposphere, is much too complicated to describe in detail, but its 



