594 
Exploring the whole frontal zone for nongeostrophic 
isentropic motion, we find the conditions for down- 
gliding limited upwards by the zero isovel, while up- 
BIS OMA CBI BRR 
BISMARCK OMAHA COLUMBIA NASHVILLE 
MB 
200 
Lay 
NYS ia 
300 
Si 400 
SREY 
500 
inks SO Cf 
i 600 
See x 700 
ce —— EE Ls 
eS le Hor 
oa S\N ee 1000 
¢ 45° 40° 
NOV. 8,1948 15002 
MECHANICS OF PRESSURE SYSTEMS 
zone is about 149 in the upper portion, and it is quasi- 
vertical near the ground, while an intermediate portion 
around 700-800 mb tilts only by 430. The general 
4° Wie ee. 
Ut Uy ii 
NOV. 8, 1948 
500 MB. I1500Z 
Fic. 12.—Profile of the warm front of the incipient wave on November 8, 1948, 1500Z, and part of the 500-mb a for the same 
time. Sample evaluations of 20. — dv,/dn below diagram. Upper winds: Half barb 5 m sec" , full barb 10 m sec— 
gliding begins above that line. Moreover, we find that 
the dry-isentropes indicate a downgliding with a hori- 
zontal component across the front from cold to warm, 
while the upgliding along saturation isentropes remains 
almost parallel to the frontal slope. The frontal profile 
therefore must begin to bulge forward from cold to warm 
in the lower layers while remaining rather unchanged 
higher up. That is what happens when the apex of the 
frontal wave goes by, as will be discussed in connection 
with the maps in Fig. 14. 
The anticyclonic shear south of the westwind maxi- 
mum has grown to the state of dynamic instability as 
shown in Fig. 12 by means of the differentiation of v, 
along the 333° isentrope. Shears rather close to the in- 
stability limit also extend down to the 500-mb level 
and make possible the rather large and systematic wind 
component towards low pressure observed on the 500- 
mb map southeast of the jet stream. 
Figure 13 shows a profile across the cold front from 
Dodge City (Kansas) to Maxwell Field (Alabama) on 
November 9, 1500Z. The corresponding sea-level and 
850-mb maps are to be found in Fig. 10. The profile 
shows that strong horizontal temperature gradients 
have formed all the way to the top of the diagram. The 
frontogenetical process by horizontal advection has 
actually been operating through the whole troposphere 
(in Fig. 14 the resulting frontal zone shows up well even 
at the 300-mb level). The inclination of the frontal 
shape of the profile can be understood as the result of 
the bulging forward of the lower portion of the cold 
wedge after the passage of the frontal wave apex. The 
MXF 
MAXWELL 
01 1074 SEC"! 
33° 
NOV. 9,1948, 
1500Z 
Fia. 13.—Profile of the cold front on November 9, 1948, 1500Z. 
Sample evaluations of 20, — dv,/dn below diagram. 
nongeostrophic downgliding responsible for that process 
was found to be dynamically justified from the study 
of the frontal profile 24 hours earlier, vzdv,/dx + 
