Wind System over the North Atlantic 



77 



most southerly position. During the spring the center of the high gradient 

 moves due north, and from July on, due east, until in November the 

 center lies almost exactly at the center of the yearly average chart (Chase, 

 1951). Chase described the changes in pressure distribution as follows 

 (his p. 4) : 



The general appearance of the mean monthly pressure distri- 

 butions is similar to that of the annual average. Although the 



Fig. 48. Map showing mean sea-level pressure for July (Chase, 1951, MS). 

 Pressures are given in the excess of millibars over 1000 mb. 



Westerhes are normally stronger in winter than in summer, the 

 circulation around the Bermuda- Azores High, when taken as a 

 whole, is greater in summer. The mean maps for January and 

 July. . .illustrate this fact. In July the Trades are stronger in 

 proportion to the increased pressure gradient and the east and 

 west ends of the high are much better developed. The Westerhes 

 show a decrease in width of band and in pressure gradient. The 

 decrease is greatest in the area north of 50° N. South of that 

 parallel, the average Westerlies are nearly as strong as in winter. 

 This is the result of more steadiness of direction in summer rather 

 than of high velocities. 



