CONSTANT PRESSURE CHART ANALYSIS 205 



5.6. Constant Pressure Chart Analysis 



The same frontal system as above was analyzed for selected constant 

 pressure levels. The 850 mbar charts (about 5,000 ft above mean sea 

 level) and the 700 mbar charts (about 10,000 ft above mean sea level) 

 were prepared for the times of radiosonde ascent (10 A.M. and 10 P.M. 

 EST) throughout the synoptic sequence from the radiosonde reports of 

 43 U.S. sounding stations. It is not necessary to reduce the 850 mbar 

 or 700 mbar level data, since it is already referenced to the indicated 

 constant pressure level. Contours for the charts aloft are shown on 

 figures 5.31 to 5.36 while their respective 24-hour changes, Nsbo and A^too, 

 are given on figures 5.37 to 5.40. 



The Nsbo charts show that the northerly flow of warm humid air within 

 the w^arm sector that was so prominent on the A^o maps is also clearly in 

 evidence at the 850-mbar level. Further, a change pattern similar to 

 that on the A'o maps is also observed at the 850-mbar level, particularly 

 on figure 5.34. That is, a rise in Agso values in the w^arm sector and a 

 decrease behind the cold front is apparent. Surprisingly enough, by the 

 time the frontal system is well-developed, at 1000 EST on the 20th, the 

 Agso values are nearly as large as those on the surface. 



The A'voo charts are more difficult to interpret than those of A^o or 

 A'sso- It appears that at this altitude the wet term is usually negligible 

 and N will normally vary inversely as temperature since the pressure is, 

 of course, constant at the 700-mbar level. By 1000 EST on the 19th (fig. 

 5.34) an intrusion of low A" values is observed in the 700-mbar warm sector 

 due to the advection of warm, low-density air northwards. The chart 

 for 24 hours later (fig. 5.42) displays two prominent highs in which 

 A700 = 225. One of these highs lies between the squall line and the cold 

 front and the other just south of the apex of the 700-mbar wave. Inter- 

 estingly enough, these two highs are due to quite different causes. The 

 high centered over Atlanta appears to have arisen from the unusually high 

 transport of moisture to the 10,000-ft level, since the 700-mbar wet term 

 at Atlanta increases from 4.5 to 25 N units in the 24-hour period ending 

 with 1000 EST on February 20. The second high, centered over Omaha, 

 appears to be due to an intense dome of cold air, as indicated by the drop 

 of the 700-mbar temperature from -7.3 °C to -21.4 °C in 12 hours 

 preceding map time. When temperatures are below °C, the wet term 

 contribution to A^ is quite small and density changes become significant 

 in producing changes in A. Falling temperatures produce higher density 

 air and, consequently, a region of high A^ values around Omaha as de- 

 picted on the A^voo chart of figure 5.31 and AA700 chart of figure 5.40 which 

 shows this change more clearly. 



