426 
Pressure Jumps. The envelope of the characteristics 
illustrated in Fig. 2 has.a definite physical meaning in 
the case of flow under an inversion. It means that the 
Fre. 5.—Motion of a compression wave shown in successive 
cross sections. The slope of the wave increases until it ‘‘breaks”’ 
and a pressure jump is formed. 
higher values of inversion height have overtaken the 
lower and are covering the same values of x. This 
hypothetical situation is illustrated in Fig, 7. This is a 
th CHARACTERISTICS 
PRESSURE JUMP 
= 
A B x 
_ Fic. 6.—The characteristics of a compression wave in an 
inversion. The pressure jump is represented as a line in the 
x, t plane. 
very unstable situation from a mechanical standpoint. 
The density p is greater than the density p’ so that the 
overhanging air with density p will fall down into the 
rest of the fluid. This will usually occur before such an 
advanced unstable state as the one illustrated in Fig. 7 
will occur. The zone in which this air is fallmg down is 
usually a rather narrow zone of the flow and is marked 
by chaotic motion of the air. Such a zone is illustrated 
in the cross section for f; in Fig. 5, and the a, ¢ diagram 
is shown in Fig. 6. The heavy line in Fig. 6 is called a 
DYNAMICS OF THE ATMOSPHERE 
pressure jump. It is assumed to be a very narrow region 
in the x, ¢ plane in which the transition from a low 
inversion height with still air (in this case) to a high 
_ inversion height and moving air is accomplished. The 
INVERSION 
e! 
TYTAULLUVA UU VUUTIAUVTA VATA IE 9 
Fic. 7.—The shape an inversion would have after an en- 
velope of the characteristics had formed if the pressure jump 
did not occur first. 
best indication that this is a narrow region is given by 
synoptic data which will be discussed later. The fact 
that the hydraulic jump is confined to a narrow zone in 
the flow in a channel is an indication that this intimately 
related phenomenon will also be narrow. A barograph 
placed at station A (Fig. 5) will have a trace similar to 
trace A in Fig. 8, but an observer at station B will see 
a trace with a marked jump in the pressure (see trace 
B in Fig. 8). This effect of the jump im the imversion 
Pp 
b 
a 
Nn 
fe) 
+ 
B 4 2 OTK 
Fic. 8.—Barograph traces recorded during the history of a 
compression wave. 
height on the pressure record at the station led Tepper 
[19] to adopt the name “pressure jump” for this phe- 
nomenon. The pressure surge (after it breaks) of Ab- 
dullah [1], the squall line (not the pressure pulse) of 
Brunk [4], and the jump of Freeman [7] are all pressure 
jumps. If a flow under an inversion is essentially one- 
dimensional and if for any reason there is acceleration 
of the air into a region covered by the inversion at a 
constant height, a pressure jump will form (if the 
Coriolis force is neglected). This is just a general de- 
scription in words of the phenomenon illustrated in 
Fig. 5. The jump forms because if there is a region of 
constant inversion height, the flow is a simple wave. If 
the air is accelerating into this region, it is a compression 
wave and the straight characteristics converge to form 
the pressure jump. 
