AEROGRAPHER'S MATE 3 & 2 



air particles, the forces that are in balance with 

 the pressure gradient when aloft are weakened 

 when introduced to the frictional layer over 

 the earth. This means that the pressure force 

 is the dominating force at the surface, and the 

 surface wind direction is pulled somewhat 

 toward the direction of the pressure force. 

 (See fig. 13-5.) 



Cyclostrophic Wind 



The Coriolis effect is at a minimum near 

 the Equator; and as a result, the pressure 

 gradient force is balanced primarily by the 

 centrifugal effect. The wind that results when 

 the pressure gradient force is balanced by the 

 centrifugal effect is called the cyclostrophic 

 wind. In the cyclostrophic wind, the pressure 

 gradient force is inward-directed and the 

 centrifugal effect is outward-directed. This is 

 the balance of forces for a low-pressure system 

 near the Equator. An example of cyclostrophic 

 winds are the winds found in a hurricane. 



Convergence and Divergence 



Convergence is the condition that exists when 

 the distribution of winds within a given area 



is such that there is a net horizontal inflow of 

 air into an area. The removal of the resulting 

 excess is accomplished by an upward move- 

 ment of air; consequently, areas of convergent 

 winds are regions favorable to the occurrence 

 of precipitation. 



Divergence is the condition that exists when 

 the distribution of winds within a given area 

 is such that there is a net outward horizontal 

 flow of air from the area. The resulting deficit 

 is compensated by a downward movement of 

 air from aloft; areas of divergent winds are 

 regions unfavorable for the occurrence of 

 precipitation. Since the wind flows from higher 

 to lower pressure areas, it can be seen that 

 convergence is associated with low-pressure 

 areas and divergence with high-pressure areas. 

 (See fig. 13-6.) 



WORLD CLIMATE 



Weather can be a valuable ally when you 

 understand the climatic patterns, the natural 

 factors which govern and regulate weather 

 changes, and the weather possibilities of 

 different or distant regions of the earth. A 

 study of climate and climatology of a region 

 can also help to avoid or minimize the hazards 

 it holds. 



(A) 



SURFACE 



_i o 

 < z 



> > 



SURFACE 



(B) 



HORIZONTAL 

 DIVERGENCE 



HORIZONTAL 

 CONVERGENCE 



Figure 13-6. — Convergence and divergence. (A) Vertical perspective; 



(Northern Hemisphere). 



209.39 

 (B) Horizontal perspective. 



298 



