POLAR AIR MASSES—WINTER _ 85 
of a preceding outbreak which has as 
it were paved the way for it by dis- 
placing the warm maritime air masses, 
so that we find the fresh Pc mass dis- 
placing only moderately cold dry con- 
tinental Polar air masses (NPC) as it 
moves southward towards the Gulf. 
At this point it is necessary to inquire 
a little fuxther as to what happens to 
the original Pc, or CPW, air-mass prop- 
erties in the course of the later history 
of the mass. 
Modification of the Winter Source 
Properties of the Pc Air to Npc Air 
Following the first outflow of the 
Pc mass from the source region, 
where its properties are essentially 
those shown by the Ellendale data, 
there are three principal influences 
which may be at work to change these 
properties. Observed changes of the 
properties are to be explained then in 
terms of one or more of the following 
modifying influences: 
(1) Supply of heat by contact with 
and radiation from the surface be- 
neath, and supply of moisture by 
evaporation from this surface. This 
influence tends to make the lower 
strata of the Pc mass increasingly 
warm and moist, or to change its 
properties from CPW to cPK. 
(2) The effect of subsidence, which 
tends to intensify the temperature in- 
version at a low level and to effect a 
general warming of all the upper 
strata.* 
(3) The effect of turbulence occa- 
sioned by a rough undersurface in de- 
veloping a thoroughly mixed layer in 
hilly or mountainous regions. 
As the cold Pc air mass moves 
southeastward from the Canadian 
border west of the Great Lakes across 
the Plains states and down through 
the Mississippi Valley towards the 
*See J. Namias: Subsidence in the At- 
mosphere, Harvard Met. Studies No. 2, 1934. 
Also see figs 18, 19, on pp. 70-71 of this 
booklet. 
Gulf of Mexico, the modifications of 
its properties depend principally upon 
the extent of the snow cover, for on 
this depends the effectiveness of the 
first and most important of the above 
three influences. The path of the cold 
air advance is over flat country all 
the way to the Gulf of Mexico, so that 
turbulence effects become no more 
pronounced than they are at Ellendale. 
TABLE IV. 
Pc AIR—WINTER (CPW BEcoMING cPK) 
(Surface and first 3 km at) 
RoyAL CENTER, IND. BROKEN ARROW, OKLA. 
iY w RH fE ee 20 EE AE: 
°C g Yo WN “CE g % A 
0.95 86 260.5 
—22.5 0.45 91 251.5 —14.8 
—19.5 0.48 262.0 — 8.8 1.15 275.3 
—15.8 0.48 276.5 — 8.0 1.20 285.7 
—18.0 0.85 286.0 —10.3 1.03 294.3 
As long as the air mass continues 
moving over snow cover, there is only 
a very slight warming and increase 
of moisture to be observed. The air 
mass retains the characteristic cold- 
ness of the surface stratum, though 
the cold layer becomes shallower, and 
the temperature inversion sharper. 
This is doubtless due to subsidence, 
i.e., the sinking and spreading of the 
cold air mass. The Pc values of the 
air-mass properties given in Table 
IV for Royal Center, Ind., and Broken 
Arrow, Okla., are averages of ascents 
for the same period as those at Ellen- 
dale, January, 1930, in cases of mini- 
mum modification of the original Pc 
properties, i.e., in cases of the maxi- 
mum coldness and dryness of the orig- 
inal Pc air. Such cases are those where 
the trajectory of the air mass the en- 
tire distance to the point of observation 
has been over snow-covered ground. 
And this will be the case only for the 
rapid outbreaks of cold air where the 
freshly outflowing cold air is displac- 
ing only slightly less cold air of the 
same source. Otherwise the modifica- 
tion of the Pc properties takes place 
much more rapidly. 
