ISENTROPIC ANALYSIS 
159 
$9. THE MEAN STATE OF THE ATMOSPHERE AS REVEALED BY ISENTROPIC CHARTS 
If the daily aerological soundings 
during a given month are averaged 
for various stations, we may construct 
isentropic charts and cross sections 
representing the mean state of the 
atmosphere for that month. The mean 
air flow may be obtained by com- 
puting the resultant winds from pilot- 
balloon observations. (See the mean 
charts published monthly in the Mo. 
Wea Rev.). Similarly, it is possible 
to construct mean seasonal isen- 
tropic charts, and, if data were avail- 
able, normal charts. An example is 
shown in fig. 14. 
The outstanding feature of the 
normal summer pattern is the ex- 
istence of two well-defined anticyclonic 
cells —one centered over western 
Texas, the other somewhere off the 
southeastern coastal states. Some 
light on the question of the formation 
of these eddies is furnished by north- 
south atmospheric cross sections, a 
typical one for the summer season be- 
ing reproduced in fig. 15. This sec- 
SAULT STE MARIE DETROIT DAYTON 
the United States is south of this 
front and is within what appears as 
a thermally homogeneous air mass. 
In spite of the zonal homogeneity of 
temperature, and the consequent lack 
of solenoids to generate kinetic en- 
ergy, there is observed a prevailing 
eastward flow of the tropical air. Ac- 
cording to Rossby [22] the eastward 
current in the homogeneous air is 
maintained by frictional stresses from 
the much stronger westerly current 
to the north, and this energy is con- 
tinually being dissipated in the form 
ot eddies further to the south. If 
such eddies are maintained in more 
or less fixed locations over a _ suffi- 
ciently long period of time, the mean 
chart will display an eddy pattern in 
the moisture lines and in upper-air 
winds. The mean isentropic charts 
for individual summer months invar- 
iably reveal such eddies, and most of 
the time there are observed two anti- 
cyclonic cells placed about as they are 
in fig. 14. 
NASHVILLE MONTGOMERY PENSACOLA 
Fig. 15.—North-south vertical cross section 
for August, 1936, showing the distribution of 
potential temperature 
(broken lines) and 
specific humidity (full lines). 
tion brings out the well-known fact 
that over North America the prin- 
cipal summertime Polar front is gen- 
erally found in the vicinity of the 
Canadian border. Most of the time 
In all the mean summer isentropic 
charts studied there has been a moist 
tongue projecting from northern Mex- 
ico recurving towards the northeast 
and east. The region covered by this 
