METEOROLOGICAL ANALYSIS IN MIDDLE LATITUDES 023 
advection of warm air is indicated only in eastern 
Montana. The sea-level pressure tendencies are rising 
throughout most of the area of cold-air advection and 
falling over the area of warm-air advection, as is usual 
[1]. Thus the distribution of advection may be appraised 
by only a cursory investigation of the combined sea- 
level and 700-mb charts. 
This combination likewise promotes the ease with 
which the principle of steermg may be applied to a 
specific meteorological situation. Examining Fig. 2 from 
the standpoint of steering, we can infer swiftly and 
with moderate assurance that the northern portion of 
the high-pressure area located in the northern Great 
Plains States should move southeastward and then 
eastward to near New York, while the portion of the 
cold-air dome which has moved south of the strong 
belt of westerly winds, into Texas and Oklahoma, will 
have to remain trapped in that region. The isolated low 
aloft over Texas is moving slowly eastward as is indi- 
cated by the sea-level pressure falls east of it and sea- 
level pressure rises west of it, but it seems clear that 
since it is now out of the westerlies it should not move 
far in the next 24 hr, and that any wave activity it 
might induce on the cold front could not move along 
the front to the northeast until after the passage of the 
rapidly moving high-pressure center in the northern 
Great Plains States. 
The intimate connection between the upper-level 
circulation and the attendant weather is again dra- 
matically illustrated in Fig. 3, the combined analysis 
for 24 hr later. Here in the South Central States we see 
that the precipitation and cloudiness from the weak 
wave in the Gulf of Mexico extends only as far north 
and west as central Missouri, while northern Missouri 
remains perfectly clear. Likewise the circulation around 
the upper-level center over Oklahoma shows that the 
moisture from the south is carried only to the same 
region in Missouri but no farther. The reason for the 
distribution of cloudiness and precipitation is not at all 
apparent from the surface map alone, while from looking 
at the upper-level map alone one would remain unaware 
of the actual distribution of weather. It is the combina- 
tion of the two sets of data that presents the complete, 
pellucid picture of the synoptic situation. 
If we examine the combined analysis over the south- 
eastern states to elicit the explanation of the widespread 
cloudiness to the north of the stationary front, we find 
that the orientation and curvature of the upper-level 
contours with respect to the surface pattern is again 
rewarding. Considering first the orientation of the 
upper-level contours in this region, we note that the 
upper-level flow is essentially parallel to the surface 
front. This indicates that there is no downslope flow 
over the frontal surface, a circumstance which greatly 
enhances the likelihood of postfrontal middle clouds. In 
this particular instance, the existence of clouds in the 
area to the rear of the front becomes a certainty, since 
the upper-level flow emanates from a region of ample 
moisture supply. 
Let us consider next the curvature of the upper-level 
contours over the South Atlantic coast. Directly under 
ae 
a portion of the anticyclonic flow we find on the surface 
map a wave on the front in South Carolina. Since the 
formation or perpetuance of a wave is unlikely beneath 
such a flow aloft, this wave should not be of much 
synoptic significance unless the upper-level contours 
above it change their characteristics of curvature, 
orientation, or both. 
Over West Virginia the weak cyclonic curvature of 
the upper contours in conjunction with a certain amount 
of warm-air advection, seems adequate to have caused 
precipitation in that region. Again the surface and the 
upper-air analyses considered separately fail to give a 
lucid explanation for the distribution of cloudiness and 
precipitation; with both charts examined in combina- 
tion, the causative factors stand out. 
Let us continue our examination of the combined 
analysis in Fig. 3 with a view toward determining areas 
of temperature advection in the layer between the 
surface and 700 mb. The salient features of the field of 
advection are the strong warm-air advection from Iowa 
northeastward, the strong cold-air advection in North 
Dakota and Montana, and the cold-air advection in 
southeastern Texas, all so plainly revealed as to obviate 
further comment. Similarly the significant features of 
the steering pattern are apparent. The contours at the 
upper level indicate that the frontal system in the north- 
ern Great Plains States will continue to move rapidly 
eastward and that the trough associated with this sys- 
tem will in all probability overtake the slowly moving 
trough in Oklahoma. The wave located south of Louisi- 
ana is being steered to the northeast by the increasingly 
strong southwesterly winds over it. At the same time 
the combined analysis reveals the interesting proba- 
bility of the development of this wave. Several circum- 
stances favor this development: the wave on the surface 
is close to the upper-level center, with the flow above it 
cyclonically curved; there is air-mass contrast across 
the front; and the solenoidal field is being intensified by 
increasing cold-air advection to the rear of the wave 
with pronounced warm-air advection in advance of it. 
The combined analysis illustrated here successfully 
expedites the analysis by making it easy to evaluate all 
these factors with the use of but one chart. 
Figure 4 affords an even more vivid illustration of the 
use of combined analysis for bringing out the well- 
established but all too frequently unnoticed relation- 
ships between the upper-level flow and the concomitant 
sea-level patterns and weather phenomena. In Fig. 4 
the difference between the two cold fronts in the central 
and southeastern portion of the country with respect to 
the extent of postfrontal middle cloudiness again cor- 
relates nicely with the characteristics of the upper flow 
over the frontal surface: Where the upper flow is parallel 
to the front and has cyclonic vorticity, there is rain; 
where the upper flow, increasing as if is with height, is 
nearly perpendicular to the surface, front the skies are 
clear. Along the Divide in Montana where the front is 
being overrun by westerly and northwesterly winds 
aloft, there is no precipitation. In this area the sharp 
anticyclonic turning of the upper-level flow suggests 
