AMERICAN AIR-MASS PROPERTIES 
For the winter season the moist 
transitional stage (figure 8, original 
article) from MP as represented at 
Dayton, to TM at Pensacola and St. 
Thomas, Virgin Islands, shows con- 
tinual subsidence and increasing mois- 
ture. The persistent stability through- 
out the transitional process, the 
difference in potential temperature 
between 500 m and 5,000 m remaining 
practically constant from the MA to 
the Tm stage, indicates that the prin- 
cipal addition of moisture must occur 
by means of mixing along isentropic 
surfaces, with some probability of con- 
vection in the final MT stage. 
The dry winter cycle (fig. 9 of orig- 
inal article) shows rapid subsidence 
with slight increase of moisture by 
isentropic mixing from the MA to the 
S stage. When this air mass moves to 
lower latitudes the subsidence de- 
creases, rapid surface heating develops 
and the S air begins to mix both verti- 
eally and horizontally with MT air. 
The vertical mixing is probably con- 
fined to the lowest layers affected by 
daytime convection and most of the 
increase in moisture aloft appears to 
be due to isentropic mixing. 
The identification of the moist and 
dry cycles is more difficult in the sum- 
mer season because of the greater 
tendency for vertical convection during 
the day, with convection occurring 
under saturated conditions which can- 
not be analyzed by charts using poten- 
tial temperature surfaces. However, 
the mean values, indicating unsat- 
urated conditions, suggest for the 
moist cycle conditions similar to those 
observed in the winter season, namely, 
113 
subsidence and surface heating with 
rapid increase of moisture by isen- 
tropic mixing. Appreciable quantities 
of heat and moisture may be added to 
MT air over the continental United 
States. Thus the highest value of 
equivalent potential temperature, 366° 
A, observed during this study was 
found at 1,000 m at Dayton on August 
22, 1936! 
The dry cycle in summer represents 
rapid modification of the polar air 
masses with subsidence aloft and slow 
addition of moisture by isentropic 
mixing over the continent, then con- 
tinued heating accompanied by vertical 
convection and convergence over the 
Gulf and Caribbean, followed by a 
slow spreading out and subsidence as 
the air assumes an anticyclonic trajec- 
tory on its return from the lower 
Caribbean to El] Paso. From El Paso 
to the Mississippi Valley there is ap- 
parently a continued addition of heat 
and moisture and the MT air again 
becomes convectively unstable over the 
Mississippi and Ohio Valleys. 
In view of recent discoveries of the 
meteorological significance of isen- 
tropic charts it is further recommended 
that more attention be given to the 
slope of potential temperature sur- 
faces in situations free from condensa- 
tion. Allowance should be made for the 
effects of horizontal mixing along 
isentropic surfaces and unless the isen- 
tropic surfaces in one air mass actually 
intersect the ground or at least show 
a sudden increase in slope, the synoptic 
analyst should label the air masses 
differently with caution.— (EH xcerpts.) 
Illustrations 
The following collection of weather maps and cross sections selected from 
articles published in the BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 
is reprinted to offer examples of frontal and air-mass-analyzed weather situa- 
tions, though the technique of presentation used is not altogether conventional. 
But they will give those who have no access to other publications nor to MS 
analyzed maps of a meteorological service or institute, some idea of interpre- 
tations of atmospheric structure and movements that can be or are made by 
experienced map analysts.—Ed. 
