POLAR CANADIAN AIR—WINTER 81 
reaches the interior of the continent, 
the characteristic NPP air mass type. 
Finally, for the Te air mass, Groes- 
beck indicates the characteristic prop- 
erties as the air mass first advances 
northward from the Gulf of Mexico, 
while Boston indicates the modified 
form which the air mass assumes as 
it moves northeastward, approximat- 
ing the characteristic Ntm (Nté@) 
type. 
IV. WINTER AIR 
The Pc Air Masses. Let us con- 
sider now the significance of the char- 
acteristic properties of each of the 
air mass types tabulated in TABLES 
III-V, bearing in mind that the entire 
discussion applies essentially only to 
the winter months. The Pc air masses 
are those which originate over the 
snow and ice covered regions extend- 
ing from the interior of Canada 
northwestward over Alaska and 
northward into the Arctic. The pass- 
age of an air mass of this type over 
Ellendale marks the advance of the 
typical cold wave from the Canadian 
northwest, the characteristic proper- 
ties of the air mass observed at Ellen- 
dale being practically those acquired 
in the source region. We note at once 
from Table III the extreme coldness 
of this air mass at Ellendale but also 
the fact that the temperature in- 
creases above the ground, reaching a 
maximum at about the 2% km level. 
Actually we find that in the early 
stages of the cold air outbreak when 
the wind is still moderate to fresh 
northwest or north from the ground 
up to high levels, the temperature 
decreases through the first few hun- 
dred meters above the ground. This 
is a result of the mixing affected by 
[For a critique indicating some 
limitations involved in analyzing the 
properties and movements of air 
masses solely by comparison of the T, 
q, and @z values at standard elevations 
in the free air as done here, see the 
paper by Prof. Rossby in Bull. Amer. 
Met. Soc., June-July, 1937, pp. 201-9. 
Isentropic analysis was developed to 
avoid these limitations (see article X 
in this booklet) .—#Hd. ] 
MASS PROPERTIES 
mechanical turbulence. Consequently 
a minimum temperature is often 
found just below the 1 km level, with 
a marked inversion above. In the 
later stages of the cold air outbreak, 
as the wind diminishes and becomes 
very light at the ground, radiational 
cooling effects a lowering of the air 
temperature at the ground, while sub- 
sidence effects a raising of the tem- 
perature at upper levels.* Thus, with 
the possible exception of a shallow 
turbulence layer near the ground, the 
Pc air mass shows very marked stab- 
ility through the lower 2 km, usually 
even a rather large temperature in- 
version, which tends to increase with 
the aging of the air mass over the 
Plains states and the Mississippi Val- 
ley, at least as long as the mass 
remains over a snow or ice-covered 
surface. Passage of the Pc air mass 
during its progress southward over 
bare ground or especially over open 
water, leads to a rapid heating and 
moistening of the lower strata. Con- 
sequently the typical Nec air mass in 
more southerly latitudes is likely to 
be characterized by rapidly diminish- 
ing stability, but during the winter 
*See J. Namias: Subsidence in the At- 
mosphere, Harvard Met. Studies No. 2, 1934. 
