SURFACE ANALYSIS IN TERMS OF No 



201 



Figure 5.25. No chart for storm systems 0130E 21 Feb. 1952. 



By comparison, the cold front sweeping down across the Great Plains 

 (figs. 5.20 to 5.22) has a rather marked A^o gradient across the front, due 

 in large measure to the northward flow of warm, moist air that forms a 

 definite warm sector by 1330 EST on the 19th. It is perhaps significant 

 that the A^o contours indicate that the various frontal systems are transi- 

 tion zones rather than the sharply defined discontinuities of textbook 

 examples, a point that has been enlarged upon by Palmer [44]. 



Figures 5.22 to 5.25 trace the trajectory of this vigorous push of cold 

 air across the Gulf Coastal Plain and the southeastern states. The most 

 spectacular gradients on this map series are in the eastern half of Texas 

 where the marked contrast of cold, dry polar air of low A'^o and warm 

 moist Gulf air of very high A^o occurs. A prominent feature of meteor- 

 ological significance on figures 5.20 to 5.23 is the northward advection of 

 tropical maritime air in the warm sector ahead of the cold front. The 

 advection is evidenced by the northward bulge of high A^o over the 

 Mississippi Valley on both charts. Figure 5.25 shows the synoptic situa- 

 tion as the front moves off into the Atlantic Ocean and refractivity 

 gradients across the continent gradually weaken. 



The variation of A'^o due to the passage of the frontal system can be 

 seen on figures 5.26 to 5.30 where the 24-hour changes of A^o have been 

 contoured. The 24-hour change, designated AVo, is obtained by sub- 

 tracting from the current value the value of A''o observed 24 hours ago. 



