138 
AIR MASS ANALYSIS 
i eel 
chiefly along surfaces of constant 
equivalent-potential temperature. 
There is also some evidence in support 
of the view that the intensity of verti- 
cal mixing decreases and the intensity 
of lateral mixing increases as the ver- 
tical stability increases. This latter 
conclusion is known as Parr’s prin- 
ciple[6]. 
The importance of isentropic mix- 
ing in the atmosphere lies in the fact 
that if it is of appreciable magnitude, 
this mixing must lead to sizable shear- 
ing stresses operating across planes 
normal to the isentropic surfaces, 
whenever there are variations in wind 
velocity in a broad current flowing 
along the isentropic surface. If a 
westerly current flowing along an 
isentropic surface is of such character 
that the velocities are larger to the 
north and diminish to the south, 
shearing stresses will tend to speed 
up the westerlies to the south and 
retard the eastward flow along the 
northern edge of the current. In other 
words, the shearing stresses tend to 
distribute momentum uniformly over 
all filaments of the current. As a 
result of these stresses, frictional 
volume forces are set up which act in 
the direction of the axis of the cur- 
rent, retarding in the regions of 
velocity maxima, accelerating in the 
regions of velocity minima. Under 
steady state conditions these axial 
forces must be balanced by Coriolis 
forces associated with slight motions 
normal to the current axes. Thus, as 
pointed out by Ekman [7], in the 
Northern Hemisphere an accelerating 
force F, per unit mass, acting east- 
ward, produces a southward motion 
whose velocity is 
F 
2 sin d 
Fig. 2 shows diagrammatically the 
accelerating and retarding forces F, 
and F,, respectively, which operate on 
NORTE 
WEST 
Fic. 2.—Accelerating and retarding forces 
operating on an isentropic current profile in 
which velocity varies in neighboring fila- 
ments. 
account of lateral mixing within a 
broad westerly current whose velocity 
profile is indicated by the curve. The 
motions which result from the Cori- 
olis force are given by the arrows: 
marked v. 
It will be seen from Fig. 2 that air 
motions created by shearing stresses 
may result in regions of convergence 
and divergence. For example, at P 
air is being flung to the south, while 
at O it is being flung to the North, 
so that between these two filaments 
divergence must set in. Similarly, 
convergence must set in in the region 
between O and Q. At the ground in 
regions above which convergence of 
this nature is taking place at all lev- 
els the pressure rises, while below re- 
gions of divergence the surface pres- 
sure falls. Far to the right of the 
stream the lateral shearing stresses 
will also produce convergence, for to 
