100 
is characterized by greater stability 
between 1 and 8 km than is summer 
Pc air at the inland stations, never- 
theless, no condition approaching iso- 
thermalecy is found at any level in 
the Pp air masses, but rather quite 
an appreciable lapse-rate at all levels. 
Furthermore, we find that from 1 
km upwards the PrP air mass is 
markedly colder than the Pc mass at 
any of the inland stations. The tem- 
perature found at 3% km in fresh Pc 
air at Seattle is the same as that ob- 
served at 4 km in Pc air at Ellendale, 
where this air mass is colder aloft 
than at any of the other inland 
stations. Furthermore, the values of 
w at Seattle are slightly less than 
those in the Pc air masses at Ellen- 
dale and Royal Center. Rather note- 
worthy is the marked constancy of 
equivalent-potential temperature in 
the Pp air at Seattle. This constancy 
of Oa indicates the absence of poten- 
tial instability at any level of the Pp 
air mass, but on the other hand it 
definitely does not represent the 
stable structure of an air mass cooled 
from below. 
The coldness and dryness of the 
summer Pp air masses at Seattle in- 
dicate almost conclusively the cor- 
rectness of the assumption, based on 
the study of the winter properties of 
this air mass type, that the source of 
the air mass is the same as that of 
the Pc mass, or at least that it is as 
truly Polar or Arctic in character. In 
winter we found that although the 
lower strata of the PP air mass were 
greatly warmed and moistened by the 
warm ocean, the upper strata were 
quite as dry and only a little warmer 
than the same strata of the Pc air 
mass at Ellendale. In fact, recent 
observations extending to higher 
levels indicate that above 4 km the 
Pp air masses are frequently, and 
even in winter, colder than the 
CHARACTERISTIC AIR MASS PROPERTIES 
Pc air. In summer we find the Pp 
air mass cooler than the Pc mass 
at all levels, and above the first km 
just as dry. The relative coolness 
of the maritime Polar air mass in 
summer depends primarily upon the 
coolness of the ocean surface relative 
to the land surface. Not only are 
the lower strata of the maritime air 
mass less heated by their contact 
with the earth’s surface than are 
the same strata of the Pc mass, but 
there is also less terrestrial radia- 
tion from the cool water surface to 
the upper layers of the maritime air 
mass. The absence of the marked 
heating by contact which occurs in 
winter at the warm ocean surface is 
reflected also in the low elevation to 
which the dampness of the summer 
Pp air mass extends, i.e., the small 
vertical extent of the penetration of 
mechanical or convective turbulence. 
Nevertheless, the lapse-rate in the 
summer Pp air masses at Seattle is 
steep enough to indicate that the air 
flow has been from colder to warmer 
surfaces. Probably the heating is 
rather gradual during the progress 
of the air mass from the cold Polar 
seas southward. This heating prob- 
ably becomes effective at upper levels 
only by means of direct radiation 
from the surface beneath without 
mechanical or convective turbulence 
having played any role above an ele- 
vation of about 1 km, which we found 
to be a normal depth of the tur- 
bulence layer at Seattle. 
But there is besides the relative 
coolness of the water surface in sum- 
mer another fact which should not 
be overlooked in the explanation of 
the coolness of the PP air masses at 
this season. This fact has to do with 
the change in the normal atmospheric 
pressure distribution along the north 
Pacific coast from winter to summer. 
In summer the pressure is relatively 
