THE INSTABILITY LINE 
in many cases at least, there is no true discontinuity 
of density but rather a discontinuity in the gradient of 
density. In synoptic practice it is customary, however, 
to continue to carry and designate this line as a cold 
front, because at a later stage in the history of the low 
the instability line tends to dissipate and the “front” 
again takes on the characteristics of a true cold front. 
Along the instability line, when it is well developed, 
there is marked cyclonic wind shear at low levels, asso- 
ciated with a pressure trough. At higher levels, as in 
extratropical cyclones generally, there is a pressure 
trough which roughly parallels the surface cold front. 
In many cases of well-marked instability lines, the pres- 
sure trough aloft coincides with the instability line. In 
some cases it is back of the surface cold front, as is nor- 
mal in lows which do not have an instability line. 
As with shower conditions generally, surface tempera- 
tures usually fall with the arrival of the instability line, 
and this fact, combined with the cyclonic wind shear, 
suggests a surface frontal structure. However, the tem- 
perature often rises after passage of the instability line 
and remains high until after passage of the cold front; 
moisture content also remains high until after passage 
of the cold front. 
The fact that cyclonic wind shear is observed at the 
surface along a well-developed instability line provides 
one clue to its location on the surface synoptic chart. 
Or, when a line of squalls is not yet in existence, the 
appearance of a line of cyclonic wind shear within a 
warm moist air mass, particularly m the warm sector 
of a low, may indicate the existence of an incipient 
instability line. The instability line is either in a pres- 
sure trough, or if parallel to the isobars, is along a dis- 
continuity in pressure gradient. The wind shear in the 
horizontal is similar to that along a front, but unlike a 
front, the wind discontinuity is more nearly vertical 
except along the outrush of cooler air at very low levels 
ahead of individual squalls. 
With the density of reporting stations in the United 
States, the position of the line of shear can ordinarily 
be located on the synoptic chart only within a range of 
fifty or one hundred miles, using only reported pressures 
and winds. A more exact location of the instability line 
requires detailed examination of individual reports, 
particularly for the time of onset of squalls associated 
with the line and for the time of the wind shift. 
Since the instability line is associated with a hori- 
zontal discontinuity in pressure gradient moving with 
the line, it might be expected to exhibit a tendency 
field similar to any pressure trough, that is, falling 
pressure ahead and rising pressure behind, or falling 
ahead and falling less rapidly behind, etc. When there 
is a sharp pressure trough, a tendency field of this type 
can be detected, but in general the three-hour tenden- 
cies represent a confused pattern that may be mislead- 
ing if not considered with caution. When the line is ac- 
companied by severe squalls, there may be rapidly fall- 
ing pressure ahead of the line, a sharp rise with the 
onset of squalls, then a rapid fall within an hour or so, 
all of which may take place within the three-hour period 
for which the tendencies are reported. Also, the severity 
649 
of the weather along the line tends to vary from station 
to station, and there are corresponding irregularities 
in the reported tendencies. The tendencies are, how- 
ever, a useful guide if viewed with caution and if 
allowances are made for known squall activity at in- 
dividual stations. Their use as a guide to location of the 
instability line should be looked upon as subordinate 
to the more direct location of the pressure trough or line 
of wind shear, or the observed onset of squalls at in- 
dividual stations. 
Upper winds, when available, are useful in locating 
the line of wind shear at the gradient level if reports are 
not too far apart. Surface winds are similarly helpful 
if allowance is made for local topographical effects and 
for the fact that isolated squalls may occur ahead of or 
behind the instability line. 
There is no clear guide to the speed of movement of 
the instability line other than its past history and the 
known speed of movement of a low with which it may 
be associated. The pre-coldfrontal squall line moves 
slightly faster than the cold front which it precedes. 
The movement of the instability line is at about the 
same speed as the axis of the warm tongue aloft, which 
is normally along or ahead of the instability lime and 
moves with a speed somewhat less than that of the 
wind speed aloft normal to the axis. The movement 
of the axis of warm air aloft is not particularly useful 
for forecasting the movement of the instability line once 
the line is in existence, but for longer-period fore- 
casting its movement is some guide to the time and 
location of possible future squall-line developments. 
Over the western Great Plains and the Mississippi 
Valley, a cold front aloft or the advection of colder 
air aloft often has its inception as a surface cold front 
over the western mountain region. The surface front, 
marking the arrival of fresh maritime polar (mP) air 
from the Pacific, moves across the Plateau region. The, 
shallower portion of the cold air, near the leading edge, 
becomes heated by the warmer ground over which it 
moves, and also by compression as it flows downward 
along the east slope. The heating effect is less within 
the deeper portion of the cold air so that a horizontal 
temperature gradient is established within the cold 
air. The leading edge of the cold air while over the 
Plateau region remains colder than the surface air 
ahead of the front, but as the front moves down the 
east slope of the Plateau it encounters maritime tropical 
(mT) air which overlies the western plains region and 
which may intersect the east slope at an elevation of 
from three to five thousand feet above sea level. Typi- 
cally, under these conditions, there is a stable layer near 
the top of the mT air, and the mT air below this stable 
layer has a potential temperature lower than that of 
the leading edge of the cold air, so that the leading 
edge of the cold air seeks out a level within the moist 
mT air or above it where the potential density is the 
same as at the base of the cold air. Thus the leading 
edge of the mP air overrides the surface mT air. Oc- 
casionally there is sufficient instability and moisture 
in the overriding mP air for the development of high- 
