POLAR PACIFIC AIR—-SUMMER 99 
reservoir in the north as they do in 
winter, they usually appear more in 
the nature of almost stationary deep 
northerly currents (more NW-ly in 
winter) that may remain in the same 
longitudinal zone for several days. 
Thus Royal Center in summer does 
not, as in winter, get Pc air that has 
passed first over Ellendale, but rather 
directly out of the north, with the re- 
sulting differences mentioned above. 
At Pensacola, Fla., the maximum 
modification (to Nec) of the original 
Pc properties may be seen; note the 
high w and some decrease in @ e with 
elevation, which indicate conditional 
instability, especially in the after- 
noon, so near to actual instability 
that local thundershowers may de- 
velop. The surface temperature is 
not far from the lower limit for Tm 
air in summer (340°).—R. G. S.] 
The Pp Air Masses 
Generally speaking, we expect to 
find in summer all maritime air 
masses relatively stable, and conti- 
nental masses relatively unstable, 
compared with their winter vertical 
structure, because of the seasonal 
reversal of the normal temperature 
differences between land and water 
surfaces. We have seen that for the 
Pc air masses this difference is very 
pronounced, that the condition of ex- 
treme stability of the winter conti- 
nental Polar air is changed in summer 
to a condition of moderate instability, 
especially marked during the daytime. 
But a glance at the summer proper- 
ties of the PP air mass at Seattle at 
upper levels (Table VII) shows the 
presence of a surprisingly good lapse- 
rate, especially through the first km. 
Since all the Seattle ascents were 
made during the early morning hours, 
this surface instability cannot be ex- 
plained as the result of insolational 
heating during the short interval that 
the air has been moving inland from 
the sea. It must be explained rather 
as the result of turbulent mixing 
effected in an air mass initially mod- 
erately stable by its passage over the 
mountainous promontories or along 
the devious water route to the head 
of the fjord where Seattle is located. 
The constancy of w, and the increase 
of the relative humidity from an 
average surface value of 62% to an 
average of 91% at 1 km is further 
evidence of the correctness of this 
assumption. Stcu clouds are nearly 
always present with a base elevation 
between 8 and 14 hundred meters 
under these conditions. But the Cu- 
nb clouds and showers characteristic 
of the Pp air mass in winter are 
definitely absent. The reason is 
obvious when we note a lapse-rate 
between the 1 and 3 km levels of 
only four-tenths of the dry adiabatic 
rate, whereas in winter between these 
levels we found a lapse-rate of more 
than seven-tenths of the dry adia- 
batic rate. The large decrease of w 
to be noted in the Pr air mass in 
summer between the 1 and 2 km 
levels is noteworthy as indicating defi- 
nitely that between these levels must 
lie the upper limit of the turbulence 
layer and doubtless therefore of the 
Steu cloud layer also. 
TABLE VII. SEATTLE PP—SUMMER 
Elevation 
Above 
Sea Level Th w RA a - 
(km) XG g Yon oak 
Surface 16.5 7.1 62 308 
1 8.5 6.3 91 308 
2, 4.5 3.9 307: 
3 0.5 2.3 308.5 
3% — 25) Ue 310 
In general, however, in spite of 
the fact that fresh Pp air at Seattle 
