AMERICAN AIR-MASS PROPERTIES 
109 
Further Studies of American Air-Mass Properties 
(Excerpts from Mon. Wea. Rev., July, 1939, pp. 204-217) 
ALBERT K. SHOWALTER 
U. S. Weather Bureau, Washington, D.C. 
WINTER SEASON 
Continental Arctic Air: As shown 
by Willett and Wexler cA air is prob- 
ably formerly maritime polar air 
(MPK) which is cooled by surface 
radiation forming cAw. A study of 
this air mass by Wexler has shown 
that there is a very sharp inversion 
near the surface, and above, a lapse 
rate approaching the isothermal. 
Since the effect of surface cooling 
rarely extends above 3 km above sea 
level, the uncooled air above would 
still be MP and very few observations 
of cA air are available above that 
level. Occasionally cA air is mechan- 
ically lifted to 4 km at Cheyenne and 
in such a case it will be cooled adia- 
batically and a temperature very low 
for that height will result. Unstable 
Arctic air, cAK, sometimes occurs in 
the Hudson Bay and the Great Lakes 
regions but insufficient data are avail- 
able to include cAK air in this study. 
The change in designation from 
cAw to cPw has been more or less 
arbitrary but usually one or two days 
elapse before cAw air is considered 
cPw. A more definite criterion for the 
change in notation is the formation of 
a new Arctic front. Eventually the 
original polar air may become tropical 
air, so CPw merely marks the transi- 
tional stage. 
The striking thing in the movement 
of cAw - becoming - cPW air into the 
southern United States is that the 
steepening of the lapse rate which 
would be expected from the addition 
of heat from below does not occur in 
the mean, except in the lowest few 
hundred meters. Apparently subsi- 
dence proceeds so rapidly at all levels 
above the shallow turbulent-convective 
layer that the great stability charac- 
teristic of cAW air near its source is 
still preserved and cAw - becoming - 
cPk is rare, except behind deepening 
cyclones. As the polar air feeds into 
low latitudes it spreads out to occupy 
several times its original area and thus 
the compensating subsidence shown by 
the various mean cross-sections and 
tables of average properties [see 
original article] is accounted for. 
There seems to be considerable mcis- 
ture added in the lower levels by sur- 
face evaporation but because of the 
extreme stability of cAw and cPw air 
it does not seem likely that the effects 
of surface addition of moisture extend 
to any appreciable elevation. Consider 
for example, the mean value of 2.2 
g/kg for specific humidity with a po- 
tential temperature of 288° A at 2 km 
at Oklahoma City in cPw air. To 
establish an adiabatic lapse rate to 
carry such a quantity of moisture up 
to 2 km by vertical convection, a po- 
tential temperature of 288° A is re- 
quired at the surface. Since a surface 
potential temperature of 288° A is 
found only in air of very nearly trop- 
ical properties it is evident that the 
observed amount of moisture could not 
have been carried up to that elevation 
by simple vertical convection. The 
addition of moisture at higher levels in 
modified polar air is probably best ex- 
plained by the principle of horizontal 
mixing along isentropic surfaces as 
discussed by Rossby (see following 
art. on Isentropic Analysis). 
Maritime Arctic Air, MA :—When an 
outbreak of~polar air moves over only 
a very small part of the Pacific Ocean 
before reaching the United States it is 
usually designated as MAK. If its 
trajectory has been far to the south, 
