ISENTROPIC ANALYSIS 147 
which tends to develop cyclonic eddies. 
When cyclonic eddies are sharply de- 
fined they are generally associated 
with occluding or occluded cyclones. 
The dominating current in such cases 
is not the warm moist air coming 
from the south, but rather the cold 
dry air streaming into the eddy cyclo- 
nically from the north. The struc- 
ture of the cyclonic flow pattern nor- 
mally observed in occluding cyclones 
is shown diagrammatically in fig. 7. 
cyclonic eddy observed in tropical cur- 
rents moving northward. Where the 
polar air intrudes into the system 
from the north it develops cyclonic 
vorticity in order to counterbalance 
the decreasing cyclonic vorticity of 
the earth’s rotation. Each current 
attempts to impart the vorticity to 
its surroundings, and this is accom- 
plished through isentropic shearing 
stresses. Thus a branch of the moist 
flow is diverted from the mother cur- 
Fig. 7. FRONTS INDICATED AT THE SURFACE AND 
SCHEMATIC FLOW PATTERN AROUND AN OCCLUDED CYCLONE 
AS SHOWN BY THE MOISTURE LINES IN AN ISENTROPIC SUR- 
FACE IN M1p-Arr. 
The flow pattern is indicated by the 
moisture lines, and the arrows repre- 
sent the instantaneous flow of dry 
(D) and moist (M) currents relative 
to the movement of the cyclone. In 
the model it is observed that two 
systems are struggling for suprem- 
acy: an anticyclonic moist current, 
M, to the right, and a cyclonic dry 
eurrent, D, to the left. The moist 
eurrent, having come up from the 
south, tends to acquire anticyclonic 
curvature indicated by the directional 
flow arrow to the upper right. This 
part of the pattern may thus be con- 
sidered as the normal type of anti- 
rent into the cyclonic flow. Thus at 
some point there is branching of the 
moist flow, and this point appears 
to be situated in the vicinity of the 
occlusion point on the surface weather 
map. 
If this branching is due chiefly to 
lateral shearing stresses, we may 
arrive at some valuable rules by as- 
suming that in the region of branch- 
ing real horizontal divergence as well 
as divergence of the stream lines is 
occurring (Namias [15]). Thus, if the 
flow pattern prevails through a fairly 
deep layer of the atmosphere, the sur- 
face pressure falls in the region be- 
