ISENTROPIC ANALYSIS 
161 
[16] Simmers, R. G., Isentropic Analysis of a 
Case of Anticyclogenesis. Part I C of 
Fluid Mechanics Applied to the Study of 
Atmospheric Circulations, Papers im 
Physical Oceanography and Meteorology, 
Massachusetts Institute of Technology 
and Woods Hole Oceanographic Institu- 
tion, Vol. VII, No. 1, 1938. 
[17] Namias, J., Two Important Factors Con- 
trolling Winter-time Precipitation in the 
Southeastern United States. Transactions 
of the American Geophysical Union, 
1939, Pt. III, pp. 341-347. 
[18] Montgomery, R. B., A Suggested Method 
for Representing Gradient Flow in Isen- 
tropic Surfaces, Bulletin of the American 
Meteorological Society, Vol. 18, No. 6-7, 
June-July, 1937. 
[19] Namias, J., Thunderstorm Forecasting 
with the Aid of Isentropiec Charts. Bulle- 
tin of the American Meteorological Soci- 
ety, Vol. 19, No. 1, Jan. 1938. 
[20] Namias, J., Subsidence within the At- 
mosphere, Harvard Meteorological Stud- 
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Namias, J., Structure and Maintenance 
of Dry-type Moisture Discontinuities not 
Developed by Subsidence, Monthly Weather 
Review, Vol. 64, No. 11, 1936. 
[21] Méller, F., Die W&armequellen in der 
freien Atmosphare, Meteorologische Zeit- 
schrift, Vol. 52, p. 408, 1935. 
[22] Rossby, C.-G., On the Maintenance of 
the Westerlies South of the Polar Front, 
Part 1A of Fluid Mechanics Applied to 
the Study of Atmospheric Circulations, 
Papers in Physical Oceanography and 
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[23] Wexler, H., Observed Transverse Circula- 
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[24] Wexler, H. and Namias, J., Mean 
Monthly Isentropie Charts and their Re- 
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Transactions of the American Geophysical 
Union, Nineteenth Annual Meeting, 1938, 
Pt. I, pp. 164-170. 
Isentropic Analysis of a Thunderstorm Situation, June 22-27, 1937 
The following series of charts was 
used by Mr. Namias to illustrate his 
epoch-making paper on thunderstorm 
forecasting with the aid of isentropic 
analysis (Bull. Amer. Met. Soc., Jan. 
1938). 
The tephigram in Figure 1 failed to 
give evidence of lability to showers 
since there was a large negative area, 
but this was in the warm sector (Fig. 
38) and thunderstorms actually oc- 
curred in the vicinity that afternoon. 
Figure 2 gave every indication for 
showers but none occurred anywhere 
near the station that day. Therefore 
the assumption that any one particle 
or mass of air rises adiabatically 
through a resting medium and that 
airplane soundings taken in the early 
morning are representative of upper 
air conditions for the rest of the day 
are clearly invalid in some cases such 
as these. A study of these cases with 
the isentropic charts and cross-sections 
shown in Figures 4-7 permits these ex- 
ceptions to be foreseen. The isentropic 
charts (Figs. 4 and 6) show a large 
moist tongue advancing over Detroit 
on the 23rd-24th which rapidly in- 
creases the specific humidity by many 
grams (see Fig. 9, also Fig. 12 of Mr. 
Namias’ chapter on Isentropic Analy- 
sis) so that showers could occur. 
The advection of this moist air at 
intermediate levels destroys the pre- 
existing dry-type inversions (see Fig. 
9, below) and results in less stable 
lapse rates in these layers. Conse- 
quently parcels of air rising from the 
surface into this region will (by 
Parr’s principle) undergo less lateral 
(isentropic) mixing with the environ- 
ment. Moreover, since the convection 
now takes place in a generally moist 
stratum, the condensation levels of 
rising surface particles are low enough 
for upward pulses to reach, thus mak- 
ing available for convection the heat 
of condensation. In this way the ad- 
vance of a moist tongue aloft may be 
accompanied by a train of showers; 
this interpretation is given in Fig. 8 
for the period under discussion. The 
forecasted showers at San Antonio on 
the 27th failed to occur (Cf. Fig. 12, 
Namias’ Article X) because the center 
of the moist tongue passed somewhat 
to the north and the Gulf influence kept 
the lower levels too cool and stable 
(showers did occur farther west, how- 
ever). —R. G.S. 
