ISENTROPIC 
lower layers are becoming richer in 
moisture while the upper layers are 
becoming drier, the energy due to con- 
vective instability is increasing. This 
combination of effects makes it easier 
for frontal activity to produce pre- 
cipitation, and in general adds to the 
supply of energy available for cyclo- 
genesis. It also facilitates the out- 
break of local showers due to diurnal 
heating if the moist layer is thick. 
A quick test for the conservatism 
of any particular isentropic flow pat- 
tern with height is afforded by the 
300° 305° 
ANALYSIS 149 
cross sections. Thus in fig. 9A we 
have the conservative case where it 
makes little difference in the flow pat- 
tern whatever isentropic surface is 
chosen for analysis, while fig. 9B 
shows the case in which the flow pat- 
tern at a surface @ = 305° would 
differ appreciably from that on the 
surface @ = 295°. In the latter case 
it is necessary to construct isentropic 
charts for both these surfaces to ob- 
tain a more complete picture of the 
atmospheric flow patterns. 
Fic. 9A. A conservative cross-section indi- 
eating that choice of isentropic surface will 
not materially affect location of major flow 
patterns. 
Fic. 9B.—Cross section indicating that flow 
patterns change appreciably from one isen- 
tropic surface to another. 
§ 6. THE REPRESENTATION OF GRADIENT FLOW IN ISENTROPIC SURFACES 
The principal reason for the use 
of isentropic charts is that they afford 
a means of determining atmospheric 
motion independent of assumptions 
regarding the existence of gradient 
flow. Nevertheless it is frequently 
desirable to know the gradient flow, 
and it appears that the mutual ad- 
justment of velocity and pressure 
distribution takes place in such a 
fashion that most of the time cross- 
isobar components of the wind are 
small compared with the gradient 
flow. 
A highly satisfactory method of 
representing gradient flow in isen- 
tropic surfaces has been suggested by 
Montgomery [18]. His development 
leads to the expression for the stream 
function of the gradient wind in an 
isentropic surface: 
y — > £ + © 
where C, is the specific heat of air 
at constant pressure, T is the absolute 
air temperature at the isentropic sur- 
face, and @ the geopotential. Using 
meter-ton-second units ® is expressed 
in dynamic decimeters, and the value 
of the constant C, is approximately 
1000. 
If lines for equal values of his 
function y are drawn on an isentropic 
chart, they form streamlines and 
serve a purpose similar to isobars on 
