90 CHARACTERISTIC AIR MASS PROPERTIES 
this air mass. At Ellendale the Pp 
air mass shows a very large tempera- 
ture inversion and constant value of 
w in the first km, instead of the 
instability found at Seattle. The 
temperature increase over the Seattle 
values shows a maximum at the 2 
km level. Probably, however, the 
marked increase in 7 and w found at 
1 km and above, in passing from 
Seattle to Ellendale, is considerably 
in excess of the change which actu- 
ally occurs in the air current itself, 
for the Pp current which reaches 
Ellendale comes from the west or 
west-southwest. It is not one of the 
direct northwest currents such as 
Seattle’s data represent but one: of 
the westerly currents which is doubt- 
less appreciably warmer and moister 
at upper levels than the fresh PP air 
from the northwest. However, the 
Ellendale data are absolutely typical 
for the Pp air masses which have 
crossed the mountains to the interior 
of the United States. The modifica- 
tion from the typical coastal PP 
properties is so pronounced that the 
symbol Npp instead of Pp is used for 
these air masses in the central United 
States. 
The weather conditions which pre- 
vail in the Npp air masses east of the 
Rockies are in general the best which 
are experienced in winter. Air move- 
ment is usually moderate, and convec- 
tive turbulence is experienced only 
in the afternoon when the large 
diurnal temperature range found 
under these conditions may be suffi- 
cient to overcome the normally pre- 
vailing surface temperature inver- 
sion. Temperatures are notably 
mild, skies clear except possibly for 
some scattered high clouds and visi- 
bilities generally very good except 
for the possible occurrence of morn- 
ing smoke haze in industrial regions. 
When, however, an advancing PP 
air current comes in contact with a 
strong outflow of cold Pe air on the 
eastern side of the Rockies, it is 
likely, as the warmer current, to 
ascend and advance over the Pe cur- 
rent along a typical warm front. 
Such a development may cause severe 
blizzard conditions in the Plains 
states, but this is, of course, a 
frontal phenomenon and in no sense 
a characteristic of the PP air mass. 
Polar Atlantic Air Masses in Winter 
[Owing to the prevailing west-east 
air movement the North Atlantic is 
not an important source region for 
air masses affecting North America, 
especially in winter when this move- 
ment is strongest. The source region 
for PA air affecting the U. S&S. is the 
part of the N. Atlantic adjacent to 
the continent and north of the warm 
Gulf Stream, from the Gulf of Maine 
north, where the waters are abnor- 
mally cold the year round; in winter 
these waters only effect a modifica- 
tion of Pc air masses which move off 
the continent and which may occa- 
sionally retrograde westward as PA 
air onto the eastern coast, where 
its properties are distinct from those 
of other air masses. As it moves 
off the coast the anticyclonic circula- 
tion of a large body of Pc air brings 
the old Pc, modified into PA, onshore 
again as a NE wind backing to SW, 
which return flow may be strength- 
ened by the approach of a disturb- 
ance (warm current) from the S or 
SW; it brings a sudden change of 
weather but seldom reaches beyond 
the Appalachians. 
The vertical structure of Pa on the 
north Atlantic coast of U. S. (Boston, 
e.g.,) is characteristically dry and 
stable aloft, with a fairly shallow, 
cold and moderately unstable moist 
layer at the surface; the lower levels 
