650 
level showers or thunderstorms entirely within the 
polar air aloft. 
Somewhere within the mP air mass, usually between 
fifty and three hundred miles from its leading edge aloft, 
the potential density is equal to the potential density of 
themT air at theground over the Great Plainsregion. The 
mP air at that pomt begins to displace the m7 air at 
the ground. The line thus formed between surface mP 
and surface mT air has in general the characteristics of 
an occlusion of the warm-front type, though tempera- 
tures at the ground are usually the same immediately 
on either side of the line. There is usually a marked 
temperature gradient toward colder air on the mP 
side, and a sharp difference in moisture content between 
the moist m7 and the dryer mP air. This ‘front’ is 
not accompanied by precipitation but may mark the 
end of showers in the m7 air mass. As the system 
moves eastward or northeastward across the western 
Plains and Mississippi Valley a more important surface 
cold front normally becomes evident in the system 
between the mP air and a colder cP air mass to the 
north. 
The conditions thus produced as a result of the east- 
ward flow of cold air off the Plateau become somewhat 
similar to those pictured in Figs. 1 and 2, and a well- 
marked instability lime tends to develop. The height 
at which the cold air aloft advances farthest ahead is, 
however, considerably lower than is shown in Fig. 2. 
Above that level the boundary of cold air slopes up- 
ward to the left as with a conventional cold front. 
The existence in the United States of an extensive 
area of low elevation open on the south to a plentiful 
supply of warm moist air and bounded on the west by 
a plateau, combine to make the United States east of 
the Rockies a favorable region for the production of 
well-marked instability lines. This is not only because 
of the tendency for cold air off the Plateau to override 
mT air, but apparently also because the Rockies pro- 
vide a north-south barrier to lower-level air masses, 
favoring southward flow of cold air and northward 
flow of warm air, the zone of interaction between the 
pure mT air and the cold air being most often within 
the latitude of the United States. 
Unique geographical and topographical conditions 
are, however, by no means necessary to the formation 
of the instability line. It may be found anywhere in 
middle or subtropical latitudes and perhaps in other 
regions. But tornadoes in the United States appear to 
develop most often with the type of instability line that 
results when cold air from the Plateau overrides mT air 
over the Great Plains. 
Further Discussion of Physical Processes 
The pseudo-cold front, suggested by Harrison and 
Orendorff [5] as important in formation of the pre-cold- 
frontal squall line, is observed in nearly all cases. Un- 
questionably its relation to other factors involved must 
be considered in any complete explanation of the mech- 
anism. However, it seems equally certain that this 
factor must be secondary to other initial causative 
factors, because the existence of the shallow layer of 
MECHANICS OF PRESSURE SYSTEMS 
rain-cooled air in the proper location appears to be 
the result of showers already occurring. The same 
authors suggested that convergence within a potentially 
unstable air mass might act together with the formation 
of the pseudo-cold front as a pair of factors to produce 
the squall line. This might be taken as a suggestion that 
convergence first produces the line of squalls and that 
the resulting pseudo-cold front then assists in maintain- 
ing the squalls and perhaps in keeping them along a 
line rather than allowing them to disperse in a disor- 
ganized manner. Low-level convergence is of course a 
necessity along the instability line and needs, itself, 
to be explained in terms of other factors. Are there 
other factors which first act to produce low-level hori- 
zontal convergence and upper-level divergence, or is 
vertical motion merely the result of vertical imstability 
and thus the full cause of the convergence-divergence 
pattern? 
The Olivers [9], in discussing forecasting of frontal 
weather from winds aloft, stated that if the wind 
component perpendicular to a cold front increases with 
height through the frontal surface, no weather is pro- 
duced by the front, but that convergence in the frontal 
trough may produce weather in the warm sector which 
ceases abruptly on passage of the surface cold front. 
They extended this argument further as an explanation 
of the pre-coldfrontal squall line, that is, the air flowing 
downslope over the frontal surface may extend to near 
the ground for some distance ahead of the front and, 
beimg dry as a result of subsidence, would prevent 
shower conditions from forming along or immediately 
ahead of the cold front, while convergence in the 
trough would contmue to produce showers in the mT 
air. This explanation, as far as it goes, seems to agree 
well with observation, and if some of the rain falls 
through the dry subsiding air the condition would be 
favorable for formation of a pseudo-cold front in the 
warm sector as envisioned by Harrison and Orendorff. 
A difficulty is that air at the ground back of the squall 
line is indistinguishable from the mT’ air at the ground 
ahead of the line, except for cooling which can be 
accounted for by rain. However, this difficulty is not 
serious if the dry air does not extend all the way to 
the ground, or if its moisture content is increased suffi- 
ciently by the rain fallmg through it. 
It has also been suggested by various persons that, 
from vorticity considerations, the stronger flow of air 
above the frontal surface will tend to form a pressure 
trough as it flows into the warm sector, the argument 
being that subsidence will be greater at low than at 
high levels, resulting in vertical stretching and therefore 
increased cyclonic vorticity along and immediately 
ahead of the surface cold front. This argument is 
strengthened by its analogy to the explanation of the 
pressure trough which normally forms in the lee of 
extensive mountain ranges. 
H. B. Wobus (U. S. Weather Bureau, Washington, 
D. C.) suggested to the author some years ago that the 
existence of a warm tongue in the mean temperature 
through a deep layer is favorable for the production of 
instability along a line near the axis of the tongue, if 
