POLAR AIR MASSES—WINTER 8 
a) 
the clear skies characteristic of Pc 
weather favor marked insolational 
heating of the ground, and in particu- 
lar when the Pc mass is displacing a 
Tropical maritime air current, in 
which case the ground will be par- 
ticularly warm and moist and the cold 
air mixes with remnants of the warm 
moist air. In this way under ordinary 
conditions the southern portion of an 
advancing Pc mass becomes rapidly 
transformed, as it moves southeast- 
ward across the U.S., from the typical 
source CPW properties of the air mass 
to a CPK, or unstable air mass, in 
which the water vapor falls off rap- 
idly with elevation to the low values 
aloft initially characteristic of the Pc 
mass. This modified cPK form of the 
Pc mass is designated as Nec on the 
TW, Ile “Wo THES. a 
In contrast to the increasing instab- 
ility of the Pc air mass as it is trans- 
formed to Nec, should be noted the 
increasing stability of the Pp air mass 
as it is transformed to Npp. In the 
. extreme case the Npc mass, or part 
of it, may be turned northward again, 
become a return flow over a colder 
surface, and tend to a reéstablishment 
of cPW stability. In general, this 
rapid modification of the properties 
of the southern portion of the Pc air 
mass relative to the northern portion 
means that the initial horizontal homo- 
geneity of the air mass is destroyed, 
or, in other words, the isentropic sur- 
faces do not remain in their initial 
horizontal position. This fact has long 
since been remarked by Bergeron® as 
characterizing all Polar air masses, 
and offered as the explanation of the 
readiness of the Polar air masses 
to form secondary fronts within 
themselves. Any wind discontinuity 
5’T. Bergeron und G. Swoboda: Wellen und 
Wirbel an einer quasi-stationarer Grenzflache 
liber Europa, Veréff Geophys. Inst. Univ. 
Leipzig, Ser. 2; Bd. III, no. 1, 1924. 
quickly produces a discontinuity of the 
other elements under these conditions. 
Generally speaking, the transforma- 
tion over land of the CPW to CPK air 
mass has little significance for the 
general weather conditions. Visibility 
at the ground is improved, and apart 
from the possibility of the format:on 
of a few cumulus clouds during the 
day, the skies remain clear. But the 
effect of open water on the CPW air 
mass in winter is immediate and pro- 
nounced, the mass becomes CPK forth- 
with. The addition of moisture and 
heat to the cold dry air mass takes 
place very rapidly from any water 
surface. In surprisingly quick time 
the lower strata of the air mass are 
rendered convectively unstable, and 
heat and moisture are carried to suc- 
cessively higher levels by active con- 
vection. The quantitative effect of 
this influence on the vertical distribu- 
tion of temperature and specific hu- 
midity will of course depend upon the 
Pc air mass properties, the warmth 
of the water surface, and the length 
of the sojourn of the cold air over the 
open water. The extreme degree to 
which such a modification may go is 
indicated by a comparison of the prop- 
erties of our PM air masses at Seattle 
with our Pc masses at Ellendale. Ori- 
ginally these two air masses have 
about the same properties, but the 
PP is modified during its passage over 
the ocean. A still more extreme case 
is that of our Pc air which may reach 
the Gulf of Mexico, under favorable 
winter conditions, still as a very cold, 
dry and essentially stable mass. But 
the winter northers in the Central 
American countries, which are invari- 
ably continuations of our Pc out- 
breaks across the Gulf of Mexico 
southward into the sub-Tropical cir- 
culation, are typically only moderately 
cool at the surface and attended by 
