98 CHARACTERISTIC AIR MASS PROPERTIES 
TABLE VI. THE Pc AIR MASSES—SUMMER 
== STATION x 
Elevation Ellendale Royal Center Pensacola (NPC) 
Above 
Sea Level ie w oe qt w Ce ah w 6, 
(km) XG g oA MG g cA aC; g cA 
Surface NOG) Ga Bil 70 88 Bila DO Ua BR 
1 162 - BG . Bile 128 Bg Bill 19.5 > 928; ae30 
2 I8 BOQ 9 Bly HI 4th BO) WA BRS 
3 aL) “Boll BIA BO BG  Bilil WO 60 Be 
4 — 95 BO Biles —= i118 iU4 Bild 
5 == 72 jl Bile 
6 15,5 We Bs20) 
ture. The Pc air mass in summer is. ation of moisture from the ground 
characteristically cool, compared with 
masses of southern origin, but 
hardly to be distinguished in tem- 
perature from the MP (PP) masses 
of the Pacific. The dryness of the 
Pc mass favors a large daily tem- 
perature fluctuation in its lower 
strata near the ground, for this con- 
dition is conducive to radiational 
cooling by night and _ insolational 
heating by day. In general, as it 
proceeds southward, the air mass 
tends to become increasingly un- 
stable. However, since nearly all the 
upper air data available are for the 
early morning hours, this instability 
is not apparent in the data shown in 
Table VI. In the middle west a 
diurnal temperature rang: at the 
ground of approximately 15°C is 
typical for this cool dry air mass, 
a range which is usually sufficient to 
establish a dry adiabatic or super- 
adiabatic lapse-rate up to 2 or 2% 
km by early afternoon. 
(3) A lack of condensation forms. 
Due to the dryness of the summer 
Pe air mass, it is typically cloudless. 
In spite of the marked daytime in- 
stability of the mass, the condensa- 
tion level is so high that only a few 
scattered high Cu clouds at most are 
likely to be observed. In the later 
portion of its life history, however, 
the conditions favoring rapid evapor- 
to the Pc air mass may so increase 
its moisture that eventually even 
thundershowers may develop locally. 
This is, however, unusual, occurring 
only in cases of marked stagnation of 
the air mass movement.—H xcerpts. 
[The Ellendale data show the prop- 
erties of the air that has come SH 
over western Canada; there is a 
moderate lapse-rate with distinct 
stability near the ground which later 
in the day (data are early a.m. as- 
cents) will probably approximate 
the dry adiabatic up to 2 or 2% km 
and often exceeding it in the first 
km. Up to 3 km w decreases steadily 
and a), is constant (but increases 
above that). At Royal Center the 
general trend of the vertical distribu- 
tion of these elements is similar; 
however, lower levels are distinctly 
colder and moister here in spite of 
the fact that the latitude is 6° farther 
south. At higher levels it is warmer 
and dryer at Royal Center than at 
Ellendale. These differences result 
from the fact that the Pc air at Royal 
Center has passed over the cold Iud- 
son Bay waters while the Ellendale 
Pc air has come over the warm north- 
western Canadian interior. It is char- 
acteristic of summer Pc outflows, that 
instead of showing properties of a 
discrete mass of cold air rapidly 
breaking out from some cold air 
