86 CHARACTERISTIC AIR MASS PROPERTIES 
We notice in passing southward 
from Ellendale that the surface tem- 
perature of the Pc air becomes some- 
what warmer, as would be expected, 
but remains extremely cold. At Royal 
Center and Broken Arrow the surface 
relative humidity remains very high, 
and the temperature inversion in- 
creases slightly and becomes lower in 
elevation. w still increases with eleva- 
tion at all stations. These changes 
are just what we should expect from 
a moderate degree of subsidence and 
a slight increase of heating by ter- 
restrial radiation, and as a result of 
evaporation from a snow surface 
which is not here so cold as it is 
farther north in the source region. 
Especially during the daytime is the 
sun much more effective at the lower 
latitudes in raising the temperature 
of the snow surface, and doubtless 
more heat is transmitted from the 
ground beneath, which is warmer, 
through a snow cover which is thinner. 
The characteristic curves for these 
stations representing their average 
Pc properties on the Rossby diagram 
are strikingly similar. And equally 
uniform is the clear, dry, cloudless 
weather attending the passage of the 
Pc air mass, as long as it retains this 
characteristic temperature and mois- 
ture distribution. In the later stages 
of the Pc outbreak, if the cold air out- 
flow weakens and the conditions be- 
come stagnant, the gradually lowering 
and intensifying subsidence inversion 
is naturally attended by markedly de- 
creased visibilities below the inver- 
sion, particularly in industrial regions 
where smoke pollution of the atmos- 
phere is pronounced. It is, however, 
the exception rather than the rule for 
such stagnation to follow in the case 
of a strong outbreak of Pc air. The 
stagnant anticyclonic condition with 
low subsidence inversion and poor 
visibility usually occurs in the U. S. in 
the southeastern states, and only with 
mild outbreaks of Pc air. And it sel- 
dom leads to fog formation, because 
the cold air, being continental, is usu- 
ally too dry. In Europe, on the other 
hand, where the stagnation of the 
cold air masses is more frequent and 
prolonged, and where the Polar and 
Arctic air masses are frequently of 
maritime origin and comparatively 
moist, this condition often leads to the 
widespread formation of fog or low 
stratus cloud, with practically zero 
visibility below the temperature in- 
version. Above the inversion visibili- 
ties are always excellent. Of course, 
active overrunning of the Pc mass by 
a warmer moister air current com- 
pletely changes the type of weather. 
Such overrunning by warm Pacific air 
in the Rocky Mountain region or by 
warm Gulf air in the Plains states. 
produces the typical blizzard condi- 
tions for these respective regions. 
As soon as a typical Pe air mass 
leaves snow-covered territory, the 
modification of its characteristic 
properties becomes very rapid. Heat- 
ing of the lower strata by contact 
with and radiation from the ground, 
and increase of moisture by evapora- 
tion from the ground surface, effect. 
a rapid transformation of its prop- 
erties, most marked nzar the ground 
and diminishing aloft. At the same 
time that the lower strata of the air 
mass are being rapidly warmed and 
moistened from the ground, there is 
doubtless also some direct radiational 
heating of the upper strata in re- 
sponse to the increased terrestrial 
radiation from the warmer ground 
surface. The initial heating of the 
upper strata does not have to wait on 
the establishment of thermal convec- 
tion. The ground heating of the air 
mass is especially rapid during the 
daytime in southerly latitudes, where 
