ISENTROPIC ANALYSIS 
143 
§ 4. ISENTROPIC FLOW PATTERNS 
An inspection of the distribution of 
moisture along an isentropic surface 
covering a sufficiently large area im- 
mediately brings to light the fact that 
there are regions of high and of low 
moisture concentration. If the net- 
work of aerological stations were suf- 
ficiently dense, it would be possible 
to draw mechanically a series of lines, 
each denoting a given mixing ratio. In 
this manner one could locate sources 
of moisture, or regions of injection of 
moist tongues, and likewise the re- 
gions from which dry air is supplied. 
Moreover, these currents could be fol- 
lowed in a continuous fashion from 
Gay to day as they travel along the 
isentropic surfaces. Unfortunately 
the distribution of aerological stations 
in any part of the world is much too 
sparse to permit such a mechanical 
delineation of moist and dry currents, 
and for this reason it becomes neces- 
sary to develop models and indirect 
clues by means of which characteris- 
tic “flow patterns” may be drawn 
which approach the real solution. 
The topic of the source of our moist 
and dry currents is reserved for a 
later section of this chapter. For the 
present we shall treat the funda- 
mental flow patterns which thus far 
have established themselves in the 
daily isentropic analysis. Once these 
models are recognized on the daily 
isentropic chart, the analysis of the 
moisture lines becomes appreciably 
simplified. 
The patterns of the large-scale mo- 
tions in the atmosphere appear to be 
controlled in large part by the rota- 
tion of the earth. If we consider a 
fluid chain of particles located in and 
moving with an isentropic surface, it 
follows from Bjerknes’ circulation 
theorem that the total absolute circu- 
lation of this chain remains constant 
as it moves from latitude to latitude. 
The absolute circulation (C.) is equal 
to the sum of the circulation of the 
fluid chain relative to the earth (C-;) 
and the absolute circulation obtained 
by the chain if it momentarily were 
fixed to the earth (Cw), or: 
C. = Cr + Co . 
Positive values of C. and C; indicate 
circulation with cyclonic sense, nega- 
tive values anticyclonic sense. It can 
be shown that Cw is given by 
G; o — 2 w z 9 
where = is the area enclosed by the 
projection of the fluid chain on the 
equatorial plane. Thus, if an origin- 
ally stationary isentropic fluid chain 
moves northwards without change of 
the horizontal area it encloses, its 
equatorial projection increases, and 
since (Cr; + 2q 5) shall remain 
constant, C, must decrease; hence, 
the chain will gain an increasing 
amount of anticyclonic circulation 
which adds to the circulation orig- 
inally possessed by the chain. Simil- 
arly, a southward moving system 
tends to develop a cyclonic circulation. 
Cyclonic circulation normally ex- 
presses itself as a cyclonic curvature 
of flow, anticyclonic circulation as an 
anticyclonic curvature of flow. Thus 
the polar currents of cold dry air 
coming out of the north are generally 
cyclonically curved, while the warm, 
moist flows from a southerly direction 
have anticyclonic curvatures. These 
large-scale flows are the fast-moving 
streams and generally dominate the 
flow patterns observed on the daily 
isentropic charts. The axes of such 
streams may be delineated on the 
isentropic charts as curved lines along 
which the wind velocities are a max- 
imum. Once these current axes are 
determined, they serve as a frame- 
work for isentropic flow patterns. 
Once an air mass is set in motion, 
certain adjustments of the pressure 
