ISENTROPIC ANALYSIS 
tropic surface itself is higher to the 
north and tilts southward, so that it 
may be 5000 m above sea level in the 
north and only 2000 m high in the 
south. Let us now superimpose upon 
this zonal state a velocity field as in- 
dicated by the lower half of fig. 2. 
Then air to the south is accelerated 
through lateral shear, air is piled up 
to the right of the accelerated stream, 
and the motion takes on a circulatory 
pattern indicated by the broken ar- 
row. Since the moisture is carried 
along by the air and the motion is 
adiabatic, successive patterns of mois- 
ture lines indicated in figs. 4b to d 
are soon developed. 
Once the moist tongue of an eddy 
is sketched in on an isentropic chart 
it is possible, with the help of the 
cross sections, to apply tests for the 
existence of the tongue in the specific 
area chosen. The basis for this test 
lies in the empirical fact that signifi- 
CLL 
Me, Mhhde 
Fic. 5. ILLUSTRATING THE CROSS SEC 
Moist ToNGuES. (a) Moist TONGUE PROBABLE; 
145 
sions of moisture above the chosen 
isentropic surface at either of the 
stations in the cross section, and if a 
moist tongue is assumed to lie be- 
tween stations, the tongue must be 
drawn as a narrow vertical filament 
of moist air—a highly improbable 
condition, and certainly one that is 
rarely observed. This test is perhaps 
more clearly illustrated by imagining 
an isentropic chart (say for 6 = 310°) 
where a moist tongue has been en- 
tered between two stations; in the 
axis of this moist tongue a mixing 
ratio of 7 g/kg has been indicated. 
Applying the cross section test we see 
that if the section is of the type 
shown in the fig. 5a, the moist tongue 
is real, while in fig. 5b it would be 
highly improbable, because here the 
moisture lines are quite arbitrarily 
drawn. 
If the eddies remained stationary 
and developed in a regular fashion, 
Mid TLTTESLEPES TE. 
TION TEST FOR 
(b) Moist 
TONGUE HIGH IMPROBABLE AND SOLUTION TO BE ABANDONED 
IN FAvor OF ANOTHER MorE LOGICAL ONE. 
cant moist tongues spread out aloft 
and if a moist tongue is present on 
an isentropic surface between aero- 
logical stations which are not more 
than 400 to 600 kilometers apart, it 
generally shows up in the cross sec- 
tion as an inversion of mixing ratio 
(or at least a minimum in the vertical 
gradient of moisture) at one or both 
of the stations. If there are no inver- 
it would be simple to follow the moist 
and dry tongues around them. Their 
life history, however, varies from 
eddy to eddy, and is closely allied 
with the energy of the current or cur- 
rents originally responsible for their 
development. Once the source of 
energy of the mother current dimin- 
ishes, the convergence necessary to 
maintain the eddy fails and the circu- 
