POLAR CANADIAN AIR —SUMMER 97 
the air mass at Boston, where ob- 
viously the 1 km level is just above 
the top of the surface turbulence 
stratum. But equally striking in the 
comparison of the data at these two 
stations is the disappearance of the 
dry Ts strata between 2 and 4 km 
at Boston. For some reason the moist 
TG air mass averages much deeper 
at Boston than in the southwest. This 
seems to be a result of the strong 
flow of the Tc currents in this region 
and the horizontal convergence which 
accompanies active cyclogenesis. The 
dry Ts air mass is usually found 
here above the 4 km level. 
The weather types which we have 
discussed above in connection with the 
principal air masses, are the predom- 
inant types which we meet with in the 
United States, in winter, in the ab- 
sence of marked frontal activity. We 
have seen how the study of the char- 
acteristic air mass properties as in- 
dicated by the aerological observa- 
tions facilitates the understanding 
and explanation of these weather 
types and consequently favors especi- 
ally the forecasting of those, elements 
which are important in aviation. 
Moreover, recent aerological investi- 
gation of frontal activity and of the 
development of cyclonic disturbances 
here and abroad show that the air 
mass properties are also of much 
importance for the explanation and 
forecasting of all those phenomena 
which depend upon interaction at the 
boundary between air masses of 
markedly different properties (see 
article by Haurwitz and notes by 
Bergeron, elsewhere in this booklet). 
V. SUMMER AIR MASS PROPERTIES! 
The Pc and Npc Air Masses 
During the warm season the source 
properties of the American Pc air 
masses are very different from those 
characteristic of the cold season. The 
snow cover which is always present 
over this source region during the 
colder half of the year completely 
disappears, and the bare ground sur- 
face becomes much warmed during 
the long summer days. Consequently 
instead of being cooled from beneath 
as in winter, the Pc air masses are 
heated from the ground. Even though 
the air mass may originate over the 
cold waters of the Arctic, it quickly 
loses the coldness and stability of its 
lower strata over the warm land. It 
is usually impossible to tell from its 
properties whether the air mass came 
initially from the ocean or not. This 
holds for the Pacific as well as for 
the Arctic Ocean, for in any case by 
the time the air mass has crossed the 
- western mountain ranges or moved 
southward from the Arctic Ocean to 
the field of observation it has ac- 
quired about the same characteris- 
tics. These characteristics are typi- 
cally as follows (Cf. Figs. 16-18 of 
Art. VIII-IX by Namias) :— 
(1) A fairly low moisture content 
(low compared with that of summer- 
time air masses of more southerly 
origin). The values of w at the 
ground usually are near 5 or 6 grams, 
and decrease steadily with elevation. 
Relative humidity is also low, especi- 
ally during the warmer time of day, 
so that conditions are favorable for 
the steady supply of moisture by 
evaporation from the warm ground 
and its transport upwards by convec- 
tion. It is probably in this way that 
the observed moisture distribution 
has been established. With increas- 
ing age and displacement southward 
of the air mass the w values increase 
steadily at all levels from the ground 
up. 
(2) A moderately low tempera- 
1Excerpts and abstracts by R. G. Stone from 
“American Air Mass Properties,” 1933. 
