144 
field within and surrounding the cur- 
rent must take place more or less as 
they do in the case of the wake 
stream, investigated by Tollmien [11]. 
Thus if momentum is injected into 
a region where no horizontal pressure 
gradients previously exist, Rossby [12] 
has shown that there will be a bank- 
ing of the current so that pressure 
rises along the right edge of the 
stream and falls along the left edge. 
This effect is due to the action of an 
initially unbalanced Coriolis force 
which attempts to create a new pres- 
sure distribution in order to balance 
the motion. There is thus a tendency 
for a mutual adjustment of pressure 
and velocity distributions. These ad- 
justments are affected whenever at- 
mospheric flow becomes out of balance 
with its pressure gradient. 
The results of Rossby’s theory which 
are of immediate practical value in 
isentropic analysis are: 
1. Fast-moving streams tend to 
suffer a “banking” process so that a 
ridge of pressure tends to build up to 
the right (in the Northern Hemi- 
AIR MASS ANALYSIS 
sphere) and a trough to the left of 
the stream. 
2. The action of lateral shearing 
stresses operating on a current in 
which the velocity varies (as in Fig. 
2) produces super-gradient winds 
along the boundaries, and thereby 
causes air to be flung across the iso- 
bars from lower to higher pressure, 
and this tends to build up higher 
pressure along the right edge of the 
stream. 
3. The shear zones on either side 
of fast-moving currents are dynami- 
cally unstable (as shown by Pekeris 
[13]) and tend to break the current 
into eddies having the vorticity of the 
original current profile. Thus, anti- 
cyclonic eddies are formed to the right 
of fast-moving streams, cyclonic ed- 
dies to the left. 
The development of an eddy affects 
the moisture distribution and creates 
‘a distinct pattern of moisture lines 
along an isentropic surface. In fig. 
4a, for example, we have a zonal dis- 
tribution of moisture in an isentropic 
surface in middle latitudes. The isen- 
iw) 
@ 
Fic. 4..—SUCCESSIVE STAGES, (a), (b), (c), (d), IN THE DEVELOPMENT OF 
AN ANTICYCLONIC EDDY AS REVEALED RY THE MOISTURE LINES. 
