THE INSTABILITY LINE 
By J. R. FULKS 
U. S. Weather Bureau, Washington, D. C. 
Introduction and General Features 
The synoptic meteorologist frequently encounters 
nonfrontal squall lmes, particularly m the warm sectors 
of certain types of extratropical cyclones. These non- 
frontal squall lines, now designated by the International 
Meteorological Organization as znstability lines, are rela- 
tively frequent in the United States east of the Rockies 
and may be severe in character. Some are accompanied 
by tornadoes and presumably extreme vertical insta- 
bility through a relatively deep layer of the atmosphere. 
A well-developed instability line is marked by squalls 
or thunderstorms along a line that is usually several 
hundred miles in length, and in the typical case fifty to 
three hundred miles ahead of a surface cold front. Or, 
where scattered showers or general rains are already 
occurring, the instability line represents a sharp inten- 
sification of squall conditions and rainfall, lasting usu- 
ally for about an hour at any one location. Since these 
conditions may occur, for the most part, between regu- 
lar synoptic reports, the best evidence is often in the 
reported time, character, and amount of rainfall, or in 
frequent intermediate reports. The pattern of reported 
thunderstorms is usually also an indication of the exist- 
ence and location of an imstability lme, but in some 
situations scattered thunderstorms occur before and 
after its passage. By convention the line is taken to be 
the leading edge of the band, usually thirty to fifty 
miles in width, of greatest convective activity. In more 
complicated cases, there may be several bands of 
squalls. Aircraft encounter marked and sometimes dan- 
gerous turbulence in flying through an instability line. 
The term istability line is also applied to the in- 
cipient condition, when synoptic factors indicate that a 
nonfrontal squall line is forming, and may be applied 
to the line of instability in the dissipating stage when 
squalls have ceased. 
Alternately, the term instability line is defined as a 
pseudo-cold front for which the cold air is presumably 
produced by precipitation fallmg from a line of ac- 
companying showers. While the pseudo-cold front mark- 
ing the leading edge of the outflowing raim-cooled air 
from instability showers is observed in most nonfrontal 
squall lines, it appears to be normally a secondary 
effect, other factors probably being more important 
in forming and maintaining the line of squalls. However, 
there are some cases where thundershowers develop over 
a localized region of, say, 10,000 or 20,000 square miles 
forming a surface layer of cool air which flows outward 
in all directions, new thundershowers then tending to 
develop along the resulting pseudo-cold front where the 
wind direction in the warm air is favorable to lifting over 
the cooler surface air. 
Unlike a true front, the instability line is transitory 
in character, usually developing to maximum intensity 
within a period of twelve hours or less and then dissi- 
pating within about the same period of time. Although 
by definition it is nonfrontal in character, 1t may cross 
a front. Often the line extends from the warm sector 
of a low northward across the warm front and is asso- 
ciated with a line of squalls in the northeastern quadrant 
of the low. 
Harrison and Orendorff [5] in 1941 studied the ‘‘pre- 
coldfrontal squall line,” especially from the standpoint 
of airline operations. In addition to describing the squall 
line and associated conditions, they examined physical 
factors possibly contributing toward its development. 
Without attempting to reach definite conclusions they 
presented data which suggested a pseudo-cold front of 
rain-cooled air moving against a rainless zone in the 
same air mass, and convergence within a potentially 
unstable air mass, as being at least possible causative 
factors in most of the cases studied. One of the signifi- 
cant facts they reported was that activity along the 
cold front decreases during the active stage of the squall 
line but regenerates as the squall line dissipates. Their 
study was confined to the Cheyenne—New York airway 
(between April 1939 and May 1940) and probably was 
not fully representative of squall-line conditions else- 
where; for example, they reported no sustained tem- 
perature change through the squall line aloft. The 
heights of upper levels considered were not fully speci- 
fied; presumably they were flight levels and thus often 
below heights at which cooling is frequently observed 
in the general region of the instability line. 
Lloyd [6], in his studies of tornadoes, attributed at 
least the tornado-producing squall-line condition to an 
upper cold front. Advection of colder air aloft and the 
resulting decrease of temperature, as would be required 
by existence of an upper cold front, is observed in many 
cases. But cooling aloft also takes place, in some in- 
stances, ahead of the squall condition, indicating that 
the degree of instability near the leading edge of the 
cooling aloft is not always sufficient for convective 
activity. Back from the leading edge, temperatures aloft 
may be lower, and surface temperatures higher, so 
that at some point the critical lapse rate necessary for 
convection may be realized. The exact distance between 
the squall condition and the leading edge of the colder 
air aloft is difficult to determine by observation, but 
appears to vary from a few miles (possibly less) to hun- 
dreds of miles. Where the leading edge of advection of 
colder air aloft is very close to the squall condition, 
and there is evidence of a discontinuity in density as 
required for a front aloft, the condition is not properly 
classified as “‘nonfrontal.” In many instances, however, 
the existence of a discontinuity aloft along or near the 
647 
