POLAR AIR MASSES—WINTER 89 
The data from Seattle in Table III 
give the average conditions in the PP 
air mass as it arrives on the north 
Pacific coast direct from more north- 
erly latitudes. When these air masses 
have made a swing southward and 
reach the Pacific coast from the west 
or west-southwest they are still 
warmer and moister than indicated 
by the data from Seattle in Table III 
but especially so in the upper strata. 
Consequently, the convective instab- 
ility is less pronounced and is dis- 
placed upward to the higher strata of 
the air mass. 
If we compare the properties of the 
Pp air mass at Seattle with those of 
the Pc at Ellendale, we note that, 
whereas at the ground the former 
is 84°C warmer than the latter and 
has a specific humidity nearly 15 
times as great, at the 3 and 4 km 
levels the temperature difference is 
only 6 or 7°C, while the difference in 
the specific humidity almost vanishes. 
Probably 3 km is about the upper 
limit to which convection has pene- 
trated in the Pp air mass. The equiv- 
alent-potential temperature at this 
level is somewhat lower than at the 
ground but it begins to increase 
rapidly above. 
As would be expected from the 
conditional instability and high moist- 
ure content of the lower strata of the 
Pp air mass on the Pacific coast, fly- 
ing weather conditions are not favor- 
able in the Pp current. Visibility is 
good in the absence of condensation 
forms but we find normally a rather 
low broken cloud deck, which may be 
solid over large areas, with extensive 
cumulo-nimbus formations and con- 
vective showers which are frequently 
rather heavy and may follow in quick 
succession. Winds are likely to be 
gusty, and flying extremely bumpy. 
On the western slopes of the coastal 
mountain ranges the clouds are likely 
to be low and thick and the precipita- 
tion (snow in the mountains) to be 
heavy and almost continuous. Such 
conditions will persist as long as the 
steady onshore flow of the Pp air 
continues. 
It happens quite frequently in win- 
ter that a westerly flow of air from 
the Pacific (usually PP air masses) 
continues for considerable periods of 
time, so that the weather conditions 
prevailing over the greater part of 
the United States may be dominated 
by these air masses. We have just 
seen the unfavorable weather condi- 
tions which prevail in the PP air cur- 
rent in the Pacific coast region but 
this air mass is importantly modified 
in crossing the western mountain 
ranges to the interior of the country 
in the following ways: 
(1) Much of the moisture con- 
tained in the lower strata of the PP 
air mass is condensed in heavy con- 
vective showers and the heat of con- 
densation is supplied to the air mass 
strata at intermediate levels. 
(2) The descent of the eastern 
slope of the Rockies dissipates the 
cloud deck and convective showers in 
the Pp current, while the heat of con- 
densation already supplied to the 
intermediate strata of the air mass 
and adiabatic compression (f6hn 
effect) cause a marked warming of 
the air mass at the 1 km level and 
above. 
(3) The warming of the surface 
strata is checked by radiational and 
contact cooling over the cold con- 
tinental surface and perhaps also by 
mixing with remnants of the cold Pc 
air. 
The effect of these various influ- 
ences acting on the PP air mass is 
readily seen by comparing the PP 
data in Table III from Seattle with 
those from Ellendale, which may be 
taken to typify inland conditions in 
