CONTINENTAL CROSS SECTIONS 



125 



FEBRUARY I 



325 \ V J ' 



\ 



/ •'530\ \ 



/ / ^ ^ 



AUGUST 



TTI 

 3 4 



DISTANCE (KmxIO' 



Figure 4.29. Wesi-eoM No cross section for the northern United States. 



lower density creates a refractive index low. Southward from the 

 Great Plains a considerable A^'o gradient is encountered as the Gulf source 

 region for tropical maritime air is approached. Latitudinal temperature 

 gradients produce a continued increase in A^'o to the southern extreme of 

 the chart at Balboa in the tropics. The A^o gradients of the summer cross 

 section are largely thermally controlled, ranging from cool temperatures 

 and low A'^o in the polar regions to a maximum between Brownsville and 

 Balboa near the warm-season heat equator. 



Three zonal cross sections have been prepared extending across the 

 northern, central, and southern portions of the United States. The 

 February chart for the northern cross section, Tatoosh to Portland, figure 

 4.29, exhibits lows at both coasts, with slightly higher values of A^o iu the 

 cold interior of the continent. The summertime chart clearly shows the 

 intrusion of tropical maritime air from the Gulf of Mexico pushing north- 

 ward over the Great Plains. The central U.S. cross section, figure 4.30, 

 shows, on the wintertime chart, slightly lower values of A^o on the west 

 coast at Oakland than at Hatteras. East of the Rockies, the chinook 

 winds of the prevailing westerlies assist in the formation of a relatively 

 warm, dry air mass of characteristically low A^o- On the summer map, 



