562 Principles of Weather Forecasting. 



that which is lighter ; then because of the resulting unbal- 

 anced pressure the air above flows over outward, as repre- 

 sented by the upper arrows. But as soon as some air has 

 left the expanding area the whole column is made lighter, 

 while the shaded areas become heavier from the added 

 amount, and there is an unbalanced condition through the 

 whole hight. At the center there is an area of low pres- 

 sure and around it one of high, hence the winds set inward 

 from all sides at the surface and outward above, as shown 

 by the arrows in the diagram, and we have what is called 

 a cyclonic system of winds, where the currents are mov- 

 ing inward toward a low pressure area below and outward 

 above toward one that is higher. 



If the central area is one where the air is contracting and 

 becoming denser then air will flow in upon it from above, 

 as shown in the upper part of the diagram of Fig. 261. But 

 as soon as air moves from the surrounding area upon the 

 central one the inner region becomes a high area, where 

 the greater pressure forces the air outward below and in- 

 ward above. Such a wind system as this has been named 

 an anticyclone. 



GENERAL CIRCULATION OF THE ATMOSPHERE. 



715. The World System of Winds. In the region of the 

 equator, where the heat is greatest, the air is continually 

 expanding, and flowing toward the poles above ; this makes 

 the pressure greater on either side, resulting in surface 

 winds setting toward the equator, as represented in ver- 

 tical section in Fig. 262, which it will be seen is essentially 

 the cyclonic system of Fig. 261. Farther toward the poles 

 on either side, where the overflowing air from the equator 

 accumulates, a high pressure belt is developed, from under 

 which part of the air flows toward the equator below and 

 another toward the poles ; these are the tropical high pres- 

 sure belts. 



At the poles, where the air is continually cooling, it is 



