Chapter 13 — CIRCULATION OF THE ATMOSPHERE 



factor that produces movement of air, or wind. 

 Assume that at three stations the pressure is 

 lower at each successive point. This means 

 that there is a horizontal pressure gradient — 

 a decrease in pressure in this case — for each 

 unit distance. With this situation, the air moves 

 from the area of greater pressure to the area 

 of lesser pressure. 



If the force of the pressure were the only 

 factor acting on the wind, the wind would flow 

 from high to low pressure, perpendicular to the 

 isobars. Since experience shows the wind does 

 not flow perpendicular to isobars, but at a slight 

 angle across them and towards the lower 

 pressure, it is evident that other factors are 

 involved. These other factors are the Coriolis 

 effect, caused by the rotation of the earth; 

 frictional force, caused by the wind coming in 

 contact with the surface over which it is passing; 

 and centrifugal effect, due to the curvature of 

 the isobars. If a unit of air moves with no 

 friction force involved, the movement of air 

 would be parallel to the isobars; this wind is 

 termed a gradient wind if the isobars are 

 curved so that centrifugal force as well as 

 Coriolis and pressure gradient forces are 

 involved, and a geostrophic wind if the isobars 

 are straight so that only Coriolis and pressure 

 gradient forces are involved. 



In figure 13-5 you can see that the flow of 

 air is from the area of high pressure to the 

 area of low pressure, but as we mentioned 

 previously, it does not flow straight across the 

 isobars (or isoheights). Instead the flow is 

 circular around the pressure systems. Recall 

 that as previously stated, when the pressure 

 gradient force (PGF) causes the air to begin 

 moving from the high-pressure to the low- 

 pressure system, the Coriolis (deflective) force 

 and centrifugal force begin acting on the flow 

 in varying degrees. 



The Coriolis force commences deflecting 

 the path of movement to the right (Northern 

 Hemisphere) or left (Southern Hemisphere) until 

 it reaches a point where a balance exists between 

 the Coriolis and the pressure gradient force. 

 At this point the air is no longer deflected and 

 moves forward around the systems. 



Once circular motion around the systems 

 is established, then centrifugal force must be 

 considered. 



Centrifugal force acts outward from the 

 center of both the highs and the lows with a 

 force dependent upon the velocity of the wind 



• corner 



1020 



GRADIENT WIND 

 FLOW 



209.38 

 Figure l3-5„— Examples of circulation about 

 high and low pressure systems. 



and the degree of curvature of the isobars. 

 However, the pressure gradient force is acting 

 towards the low; therefore, the flow in that 

 direction will persist. If while moving toward 

 the low the flow is moving parallel to the 

 curved portion of the analysis (fig. 13-5), it 

 is termed a GRADIENT WIND. If it is moving 

 parallel to that portion of the analysis showing 

 straight flow, it is referred to as GEO- 

 STROPHIC WIND. 



We defined pressure gradient as being a 

 change of pressure with distance. This means 

 that is our isobars are closely spaced, then the 

 pressure change is greater over a given dis- 

 tance; it is smaller if they are widely spaced. 

 Therefore, the closer the isobars, the faster 

 the flow. 



Frictional Force 



Friction tends to retard air movement. 

 Friction depends on the nature of the surface 

 over which the air is moving. It is least over 

 water surfaces and greatest over mountainous 

 terrain. The effect of surface friction extends 

 from the surface to approximately 3,000 feet. 

 It is usually safe to say that the wind above 

 3,000 feet is the same as the gradient or 

 geostrophic wind in direction and speed. Since 

 the frictional effect decreases the speed of the 



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