648 
squall condition is difficult or impossible to determine 
from synoptic data and the only practical recourse for 
the synoptic meteorologist is to follow the instability 
line. 
Synoptic Aspects and Some Associated Physical Factors 
Figure 1 represents diagrammatically a typical well- 
marked instability line in the warm sector of an extra- 
Co) ES —————S 
Fig. 1.—Model of cyclone with instability line in warm 
sector. Shading indicates areas of active squalls, and hatching 
shows warm-front precipitation. 
tropical cyclone. Squalls and thunderstorms (shaded 
area) are shown as a nearly solid band along the insta- 
bility line and at scattered points elsewhere in the warm 
sector and within the area of warm-front precipitation 
(hatched). In this type of low, and at this stage of 
development, precipitation is rarely observed behind 
the portion of the cold front following the instability 
line. 
Figure 2 is a vertical cross section through the line 
AB of Fig. 1. Thin solid lines represent isotherms of 
320° 16000 FT 
315° 
i. << |2000FT 
8000 FT 
4000 FT 
Fig. 2.—Vertical cross section through cyclone with warm 
sector instability line. The section is taken along the line 
AB of Fig. 1 and is on the same horizontal scale; the vertical 
scale is indicated roughly in feet on the right-hand side. 
Thin quasi-horizontal lines are potential temperature, labeled 
in °K on the left. The dashed line CD is the axis of warm air, 
temperatures decreasing or changing but little horizontally 
to the right, and decreasing to the left of CD (except near the 
ground where temperature decreases sharply with onset of 
instability line squalls). Shading indicates cloud masses, and 
hatched lines show active precipitation; the double horizontal 
line indicates a stable layer. 
MECHANICS OF PRESSURE SYSTEMS 
potential temperature. The line CD is the center of a 
warm tongue which extends roughly at right angles to 
the given vertical section. Thus, any constant level or 
constant pressure surface cutting the vertical section 
will have a warm tongue, the axis of which intersects 
the line CD. Generally the horizontal temperature gradi- 
ent to the right of CD is less than to the left of CD. As 
the system moves from left to right (normally eastward) 
cooling aloft over any fixed point will begin as the line 
CD passes, and since the cooling takes place first at 
the higher levels, there will be a progressive decrease 
in vertical stability until the arrival of the instability 
line. Often, at least in the central and eastern United 
States, there is a stable layer or inversion at approxi- 
mately 114 km above the surface and to the east of the 
instability line; this inversion disappears with the ar- 
rival of the instability line. It is probable that this 
stable layer, when it exists, plays an important part in 
the mechanism of the instability line, because it pro- 
vides a ‘“‘cap” under which the temperature and mois- 
ture content may increase until, combined with any 
cooling aloft, the vertical instability is sufficient for 
vertical convection. This effect is made possible by 
slower eastward movement of the surface air as com- 
pared to the system as a whole, and is enhanced by 
conditions usually favorable for low-level advection 
northward of moisture and warmer air, as pointed out 
by Means [7]. Apparent warm advection is in some 
cases, however, indicative of lifting rather than low- 
level warming. 
The cooling aloft, as illustrated in Fig. 2, is in part 
the result of horizontal advection, as may readily be 
verified by inspection of constant-level or constant- 
pressure charts. The typical condition is one of cooling 
by advection aloft over a region where there is warming 
by advection at lower levels. The warming by advection 
at low levels is generally greater in magnitude than the 
cooling aloft, but cooling aloft is significant in that it 
permits vertical convection to extend to higher levels 
than would otherwise be possible, thus becoming an 
important factor in the intensity of resulting convective 
activity. It seems certain, however, that factors other 
than advection are also important in the cooling which 
takes place aloft over the warm sectors of many lows. 
An early example of cooling aloft ahead of a surface 
cold front, accompanied by warm-sector precipitation 
was presented by J. Bjerknes [1] in 1930. This study 
was based on a detailed examination of frequent sound- 
ings at Uccle, Belgium, during the period December 
26-28, 1928. 
The cold front as shown in Fig. 2 does not extend far 
above the surface. When the instability line is well de- 
veloped, there is usually little or no evidence of the 
existence of the associated cold front at more than two 
or three thousand feet above the ground. The position 
of the cold front at the ground is usually quite well 
marked by a wind shift, pressure trough, and change 
of moisture content (dryer on the cold air side). Usually 
there is little, if any, temperature change immediately 
across the front, but there is a horizontal temperature 
gradient toward colder air to the rear, suggesting that, 
