101 
POLAR PACIFIC AIR—-SUMMER 
low over the continent, and relatively 
high over the ocean, especially over 
the northern area, so that in summer 
the middle Pacific anticyclone extends 
far northward into the region nor- 
mally occupied in winter by the so- 
called Aleutian Low, which tends to 
be displaced inland from its winter 
position with greatly diminished in- 
tensity. Consequently in summer 
there is normally prevalent a well- 
marked pressure gradient directed 
from ocean to continent along the 
entire Pacific coast from California 
northward, a condition which favors 
a steady transport of Polar maritime 
air southeastward along the entire 
Pacific coast. This condition is so 
persistent in summer that warm mari- 
time air from the south seldom if 
ever reaches the north Pacific coast 
at the surface directly, as it fre- 
quently does in winter. Occasionally 
there occurs a temporary cessation of 
the maritime Polar outflow on the 
north Pacific coast during the pas- 
sage further north of a disturbance 
following a more southerly course 
than is usual in summer. Further- 
more, the ocean surface temperature 
along the California coast is so low 
in summer, because of the upwelling 
of cold water, that Tropical maritime 
air masses must pass a great distance 
northward from their source region 
to reach the latitude of Seattle. 
It seems very probable that the 
change at San Diego from the Seattle 
values of the PP air mass properties 
would be in the direction of a marked 
increase in stability, especially at the 
surface. Probably even in the pre- 
vailing air flow from the northwest 
we would find a pronounced low tur- 
bulence inversion (the wind veloci- 
ties are usually too great to permit 
of the formation of a surface tem- 
perature inversion) with dense St or 
Steu clouds, which may at times 
approximate surface fog in their low 
elevation in the upper portion of the 
turbulence layer. The extreme local 
coldness of the ocean surface along 
the California coast is sufficient to 
cool the lower strata of even the 
coolest PP air masses from the north- 
west. 
It is quite impossible in summer to 
distinguish air of Polar Pacific origin 
from that of Polar continental origin 
after the former has reached the 
aerological stations in the interior of 
the U. S. In summer most outflows 
of Polar air over North America first 
become evident in a strengthening of 
the normal pressure gradient along 
the Pacific coast, and consequently 
in an intensification of the Pp air 
flow. This northerly current gradu- 
ally works inland, with an accom- 
panying general rise of pressure in 
the coastal region, and a gradual 
displacement eastward of the zone 
of strongest north or northwest 
winds. Consequently the Polar air 
current becomes increasingly conti- 
nental in its composition as the Polar 
source region from which the outflow 
takes place is displaced continually 
inland, or eastward. Under these 
conditions it becomes almost impos- 
sible to determine a boundary be- 
tween the air current of maritime 
and that of continental origin. In 
winter this distinction is easier to 
make, because of the very much 
greater coldness and dryness of the 
continental air. But in summer when 
the initial differences between the 
Pp and Pc air mass properties are 
so slight, by the time that the PP mass 
has crossed the mountains, usually 
rather slowly, and come probably 
into radiation equilibrium with the 
continental surface beneath, all dif- 
ferences between these air masses 
are so completely obliterated that it 
is neither possible nor of any advan- 
