THE METHOD OF CHARACTERISTICS 
Weather Associated with a Pressure Jump. Tepper 
[19] has described the surface weather phenomena that 
accompany the passage of a pressure jump. He refers 
to a larger-scale pressure jump than the one described 
between the mountains. The origin of the jump will be 
discussed in the next section, but the scale is one of a 
change from an inversion height of 2000-8000 ft to one 
of 5000-10,000 ft with temperature differences across 
the inversion of 1-5C. The jump im height is moving 
faster than the conditionally unstable air above. If this 
air were dry, the phenomena associated with precipi- 
tation would not occur. The data over a network of 55 
stations in a rectangle 8 mi by 20 mi were averaged. 
Several outstanding phenomena accompanied the pres- 
sure jump of May 16-17, 1948: the wind shift, the 
temperature break, the rain gush, the wind-speed max1- 
mum, and the maximum pressure difference.? These 
terms are almost self-explanatory and a complete de- 
seription may be obtained from Tepper’s original paper. 
The magnitudes of some of these quantities are given 
below. 
Average total rise in pressure.............. 2.3 mb 
Average maximum precipitation intensity .. 0.02 in. min™ 
Average maximum surface wind speed...... 27 mph 
Average maximum rate of temperature fall. 1C min 
Average total wind shift................... 75° 
The foregoing tabulation gives the conditions averaged 
over all 55 stations, but the pressure jumps are better 
known and most respected for the more extreme weather 
phenomena. These are described in some detail by 
D. T. Williams [23] and are summarized here. 
Pressure: The pressure would rise 2-6 mb during 
time intervals of the order of 5 min. Gradients were as 
steep as 1-2 mb mi. Following the abrupt rise in 
pressure there was a leveling off or a slow fall. 
Wind: Winds ahead of the line were usually southerly 
and an almost instantaneous shift to westerly or north- 
westerly accompanied its passage. The peak gust over 
the whole network was of the order of 50 mph. In many 
cases the winds blew at right angles to the isobars. 
Precipitation: Rain in the first mile behind the pres- 
sure Jump was usually light, becoming heavy farther 
back. 
Temperature: Temperatures fell sharply upon the 
passage of a pressure jump. Maximum drops occurred 
while heavy rain was falling and usually the decrease 
was proportional to the rate of rainfall. With the ending 
of the period of heavy rain, temperatures rose slowly, 
frequently returning to near their original levels. 
Such phenomena are naturally of mterest to meter- 
ologists who have expressed their interest throughout 
the years by marking such pressure jumps and their 
accompanying precipitation as prefrontal squall lines 
on weather maps. Harrison and Orendorff [11] describe 
the synoptic aspects of the squall line in great detail. 
They show that it is certainly not the result of an upper 
2. H. R. Byers maintains that these properties (the pres- 
sure trace in particular) are common in varying degrees to 
any line of thunderstorms. (Statement from the floor of the 
108th meeting of the American Meteorological Society, at 
Tallahassee, Florida, December 1950.) 
427 
front. However, the importance of the barograph trace 
and the pressure pattern in locating squall lines has not 
been fully recognized. The careful and accurate analysis 
of the Daily Weather Map of the U.S. Weather Bureau 
since 1946 has made it increasingly evident that the 
squall line is a real phenomenon. In fact the squall lines 
and fronts on these maps move in a manner so similar 
to that of a shock wave and piston in theoretical dis- 
cussions of gas flow in a tube that a connection only 
waited for someone who had sufficient knowledge of 
both motions. This connection was established by Tep- 
per [19] in his detailed synoptic study of the squall 
line of May 16-17, 1949. He established that this 
particular squall line was a pressure jump and proposed 
that most persistent, well-developed, squall lines are 
pressure Jumps. 
The Prefrontal Squall Line. Tepper [19] has proposed 
that the prefrontal squall line is a line of showers and 
thunderstorms that is started by a pressure-jump line. 
A study of Fig. 5 shows that if the pressure Jump repre- 
sents a large jump in the height of the inversion, it will 
have the same effect as a rapidly moving cold front on 
the air above the inversion. In fact, since the slope of the 
advancing surface is usually much greater in a pressure 
jump, the release of whatever moisture is in the upper 
air should be more rapid and therefore more spectacular. 
It is believed that the lifting of the air above the inver- 
sion by the advancing surface of the inversion causes 
most of the precipitation in squall lines. (It is also 
possible that the sudden lifting of the air under the 
inversion will cause saturation and that the resultant 
change in the lapse rate will allow the air to break 
through the inversion and thus the origin of the storms 
will be in the lower air mass. This possibility is disre- 
garded in this discussion because such a break-through 
would violate the theoretical model for which equations 
(14) and (15) are true. Of course, this does not mean 
that it cannot or does not occur.) Tepper [19] empha- 
sizes that the storms that make up the squall line are 
caused by the mechanical lifting along a pressure Jump 
and that the jump can occur with no resulting storms or 
precipitation of any type if the air above the inversion 
is dry enough. Tepper’s study of prefrontal pressure 
jumps developed from an attempt to discover some 
theoretical explanation for the intense pressure gradi- 
ents that occurred during the seven squall lines that 
were studied by Williams [23]. Williams’ work was a 
synoptic study of the dense network of stations that the 
Cloud Physics Project maintained near Wilmington, 
Ohio. This study made it clear that there was a very 
narrow region in which the pressure changed very 
rapidly. This region also moved rapidly and was asso- 
ciated with the thunderstorms that made up the squall 
lines which he was studying. An idea of the width of the 
zone of rising pressure can be obtained from the ac- 
companying map (Fig. 9). Tepper established that the 
surface phenomena discussed by Williams occurred in 
a sequence and moved in such a way that a jump in 
the height of an inversion (or a stable layer) was the 
most likely cause of the phenomena. He proposed a 
model of a squall line based on the work of Freeman 
