722 
moving northward and then northwestward over the 
advancing and deepening cold air mass. The explanation 
of the precipitation then, clearly brought out by the two 
combined charts, must be the enforced elevation of the 
tropical air, rather than orographic lifting of the cold 
air mass. 
Other than that in the southern Great Plains, the only 
area of precipitation of any large extent is that indicated 
in the vicinity of Lake Superior. Here the obvious 
causative factor is the well-known heating effect of the 
2% e31026 10000 98 9 &20 
= 5 1020 
Soho 
ye EAS 
0 1014 
O0630Z OCTOBER 25,1949 
AAAACOLD FRONT 
@aaaWARM FRONT 
OM y STATIONARY FRONT 
@AadA OCCLUDED FRONT 
QAAAGOLD FRONT ALOFT 1014 1017 
4Bdee 14 
acs I 
bs oe u : : “lees Low 
OBSERVATIONS AND ANALYSIS 
portion of the surface front and therefore little or no 
frontal weather; (2) that east of the Appalachian Moun- 
tains, the portion of the front which is moving slowly 
southward is parallel to the upper winds and should 
therefore produce lifting of the air above the frontal 
surface. In the area to the rear of this portion of the 
front some cloudiness and precipitation are in evidence. 
This is the only area where the combined analysis sug- 
gests the possibility of frontal precipitation directly 
behind the front. 
1005 
Fie. 3—Combined sea-level and 700-mb analysis for the United States at 0630Z, October 25, 1949. 
Lakes. The important point here is the concurrence of 
cyclonic curvature of the sea-level isobars and the 
700-mb contours with the area of shower activity. The 
intimate connection between the curvature of the flow 
pattern and the occurrence of such instability phe- 
nomena as the ‘Lake effect”’ stands out pellucidly by 
virtue of this particular method of superimposition of 
charts. 
Turning next to the portion of the cold front extend- 
ing from Tennessee northeastward to Massachusetts, 
we note two things: (1) that the flow from the west is 
stronger aloft than it is at the surface, indicating no 
lifting of the upper air mass by the eastward-advancing 
The front in the southeastern United States, with a 
wave along the coast of South Carolina, is in a position 
which often causes the weather along the coast to 
deteriorate suddenly. In this example, however, the 
presence of a closed high-pressure cell aloft near South 
Carolina clearly indicates that the front is lifeless and 
that the forecaster need have no worry lest it develop 
in the future. 
Looking at the analysis again for the purpose of 
detecting the areas of warm- or cold-air advection, we 
come upon striking evidence that strong cold-air ad- 
vection is occurring through Iowa, Illinois, Indiana, 
Ohio, and most of the area north of these states. Strong 
a 
