relation was incorporated into the distance 

 separating the points such that 



Ip] hs(naut. miles) = ^.^^. 

 »*^ ' sin (p 



Using this convention, LAS varies from 

 350 nautical miles at 60° latitude to 600 

 nautical miles at 30° latitude. Pressure 

 gradients at latitudes below 30° were ap- 

 proximated by taking pressure differences 

 across distances equal to one-half those 

 specified by equation (2) and doubling the 

 values so obtained. 



Substituting equation (2) into equa- 

 tion (1) and setting Q = 7.29 x 10-5 per 

 sec and P= 1.25 x 10" -^ ton/m3, we have 



V (m/sec) = 0.99 Ap ("*) 



Thus, pressure difference values read from 

 the charts in millibars caji be conveniently 

 interpreted as wind components in m/sec. 



System of Indices 



The network of points used for comput- 

 ing pressure gradients is illustrated in 

 figure 1. The points of each pair are shown 

 joined by a heavy solid line zmd are labeled 

 with an identifying location number. The 

 positive direction assigned to the geo- 

 strophic wind component at each location is 

 indicated by a short arrow. This network 

 was inscribed on a transparent overlay for 

 use in reading data from the pressure 

 charts. Linear interpolation was employed. 



A similar system of geostrophic indices 

 involving pressure differences between pairs 

 of points, each separated by a distance 

 inversely proportional to the sine of their 

 mean latitude, was used by Chase (1954) to 

 describe mean seasonal cycles and fluctua- 

 tions over a 3-year period in the North 

 Atlantic Trades and Westerlies. 



Figure 2 (page 4) shows the relation 

 of the pressure gradient network to the mean 



Figure 1. --Location charts showing points, marked by open circles, between which pressure differences 

 tabulated in table 1 were read. Arrows indicate positive direction of the geostrophic wind 

 component associated with each point pair. 



