$2 CHARACTERISTIC AIR MASS PROPERTIES 
season it never becomes really un- 
stable in the Mississippi Valley before 
it reaches the Gulf of Mexico. 
We notice from Table III that the 
specific humidity at Ellendale is very 
low at all levels but with a pro- 
nounced maximum at the 2 km level, 
whereas the relative humidity is uni- 
formly rather high at lower levels. 
The indication is that the radiational 
cooling of the surface air strata to 
very low temperatures has led to 
condensation and deposit of moisture 
from these strata. Generally speak- 
ing, values of specific humidity less 
than 1 g and a rapid increase of D5 
with elevation, are characteristic of 
the Pc air masses. 
A Rossby diagram for Pc air (cPW) 
at Ellendale shows the typical extrem- 
ely characteristic form of the 6-w 
curve for this type of air at its source. 
And this type form is characteristi- 
cally conservative at least as long as 
the air mass remains over a snow- 
covered surface. The curve is nearly 
vertical, showing a small increase of 
w at intermediate levels, but with w 
values less than a gram at all levels. 
The potential and equivalent-potential 
temperatures increase some 40C° in 
4 km; consequently the existence of 
any mechanically produced turbulence 
effects rapid transfer of heat down- 
ward in the turbulent layer. 
The normal flying conditions pre- 
vailing in the Pc air mass in winter 
in the northwestern and central 
United States may be readily sur- 
mised from the air mass properties. 
The condition will be one of extreme 
cold at the ground, but the tempera- 
ture will be much higher at inter- 
mediate levels (1 to 2 km above 
ground). Bumpiness will be mark- 
edly absent, flying being unusually 
smooth, except perhaps for the shal- 
low turbulence layer near the ground 
when the winds are fresh. Condensa- 
tion forms are characteristically 
absent, though at the beginning of 
a Pc outbreak a low stratus or strato- 
cumulus cloud deck may mark the top 
of the turbulence layer, but is likely 
to dissolve rapidly. Otherwise the 
air mass should be cloudless, high 
clouds indicating the presence of 
another air mass aloft. Occasionally 
the radiational cooling of the lower 
strata of the air mass leads to the 
local formation of frost smoke or ice 
erystal fog at very low temperatures. 
The intensification and lowering of 
the subsidence inversion and the dis- 
appearance of all surface wind in 
the stagnant and aging Pc air mass 
favors the accumulation of smoke and 
dust pollution in the lower levels of 
the atmosphere. Consequently the 
Pc air mass during its later history, 
and especially in industrial regions, 
is likely to be marked by unusually 
dense smoke haze below the inver- 
sion and by sharply marked haze 
layers. But the air mass in the 
central United States is usually too 
dry for the formation of any ex- 
tensive dense fog below the sub- 
sidence inversion. Fog is likely to 
appear in the old Pc air mass first 
upon the approach of a warm front 
with a lowering warm front cloud 
deck and precipitation. This is a very 
dangerous condition for flying, as it 
favors not only fog formation but 
also heavy ice formation on the plane. 
However, this condition pertains 
essentially to the warm front zone 
and not to the Pc air mass as such. 
In the northeastern United States, 
and generally along the Atlantic sea- 
board, the Pc air mass properties 
vary significantly from the -charac- 
teristics which they show in the Mis- 
sissippi Valley and the northwest. 
This is probably to be attributed to 
the following facts: 
(1) The existence of considerable 
