AMERICAN AIR-MASS PROPERTIES 
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date the designation S has been applied 
to all warm air masses that show 
relative humidities below 40%, which 
is taken as an indication of subsidence 
and horizontal divergence. The study 
indicates that most of the dryness 
results from the subsidence of high- 
level air from a polar source. Isen- 
tropic analyses have shown definitely 
that in winter a number of dry tongues 
move out from polar regions. 
Modified moist Polar air, MPw 
(NpM): The mean values for these 
air masses show a tendency for 
decreasing stability and increasing 
moisture content. Although there is 
evidence that considerable heat and 
moisture are added by surface effects, 
there must be an addition of moisture 
aloft by isentropic mixing. 
SUMMER SEASON 
Continental polar air, cA—becoming 
- CP (Pc - becoming - Nec): The rate 
of increase of moisture aloft seems 
slightly in excess of that possible 
through vertical convection and it is 
necessary to believe that horizontal 
mixing plays an important role. 
Maritime polar air, MA - becoming - 
MP’ (PP - becoming - Nepp) : The effects 
of subsidence and outflow from the 
upper portions of MP air seem to have 
about the same prominence as the 
effects of horizontal mixing so the 
mean values for MP air do not give 
positive evidence of the increase of 
moisture aloft by isentropic mixing. 
The notation cP or Np should be 
used in cases of doubtful history of the 
polar current, or in cases of overlap- 
ping layers of maritime and conti- 
nental air having been thoroughly 
mixed by vertical convection during 
the day or in some cases by mechanical 
turbulence. Since there is so little dif- 
ference in the properties of MP and cP 
after 1 day’s history over the continent 
during the summer months, it would 
be wise to label all polar air masses 
cP in the summer time, after they 
have moved east of Spokane and south 
of the Canadian border. 
Modified moist polar air, MP (NPM): 
The rapidly increasing moisture seems 
to be a combination of the effects of 
surface evaporation over water sur- 
faces or areas with abundant plant 
life, coupled with the effects of hori- 
zontal mixing. It seems possible there- 
fore for a polar air mass with a purely 
continental history to attain in summer 
quantities of heat and moisture com- 
parable to those obtained by an air 
mass moving over the Caribbean Sea. 
Maritime tropical ai, MT (Tm): It 
appears that in general the potential 
temperatures are higher in MT air 
than in continental air masses and that 
therefore the air would move upward 
along the isentropic surfaces as it came 
over the land. Some of the cases 
studied indicate that at times a given 
isentropic surface may slope down- 
ward from the Gulf to the continent. 
This means that portions of the MT 
column may at times move downward 
in approaching the continent. It will 
be noted that mean potential-tempera- 
ture surfaces are approximately hori- 
zontal between Miami and Pensacola. 
Taking the vertical distance in 
meters between the 303°A and the 
311°A surfaces as an inverse measure 
of stability, one finds the MT air to be 
more stable in the mean than MPx air 
but less stable than cPW air. This 
agrees with the statement made above 
to the effect that MP air was likely to 
cause increasing instability in air 
masses moving into its territory. The 
author is of the opinion that the 
greatest probability of rain occurs 
when MT air replaces MA or MP, or 
when MA or MP replace MT air. The 
latter sequence is more conducive to 
