102 
tage to distinguish between them. 
Consequently the designation Pc can 
be used indifferently, in summer, for 
Polar continental air masses or Polar 
air masses of Pacific origin after they 
have reached the interior of the U.S. 
—Ea«cerpts. 
The PA Air Masses 
The Polar Atlantic air masses are 
more important in the late spring 
and early summer than at any other 
time of the year. At this season the 
ocean surface in their source region, 
from’ Cape Cod northeastward to 
Newfoundland, is at its maximum of 
abnormal coldness for the latitude 
and at its coldest in comparison with 
the continental region to the west. 
This relative coldness is a conse- 
quence of the slowness of the cold 
continental ocean current from the 
north in warming up in the spring, 
and possibly in part to the drift of 
ice into this region with the Labrador 
current. It follows that this ocean 
region in late spring and early sum- 
mer becomes a real cold air source. 
Whenever the normal eastward move- 
ment of air over this region ceases, 
as it frequently does in the spring 
and summer, the tendency is towards 
the immediate development of a sta- 
tionary anticyclone thermally main- 
tained by the cooling of the stagnant 
air mass present in the region. Such 
developments frequently manifest 
surprising persistence over the cold 
water, and usually lead eventually to 
an overrunning of the north Atlantic 
coastal region by the cold PA air of 
the anticyclonic circulation. Occa- 
sionally it happens that a general 
southward movement of the cold air 
follows down the entire Atlantic 
coast as far south as northern Flor- 
ida, bringing with it a decided drop 
in temperature. The difference in 
temperature between this cold mari- 
time air and the hot continental air 
CHARACTERISTIC AIR MASS PROPERTIES 
may amount to as much as 20° or 
25°C. Usually, however, the PA air 
masses in summer do not make their 
influence felt south of Cape Hatteras, 
and the greater part of the time only 
on the north Atlantic and especially 
the New England coast. 
The term PA, in summer as in win- 
ter, is applied to those air masses 
which were originally Pc, but which 
have remained long enough over the 
cold waters of the north Atlantic to 
have become appreciably modified. 
We have seen that in winter very 
little time is required to effect such 
a modification because of the marked 
initial coldness of the air mass. In 
late spring and early summer, how- 
ever, the water surface is colder 
than the surface strata of the Pc air, 
so that the modification takes place 
slowly. On the other hand, the gen- 
eral stagnation of the air movement 
over this north Atlantic area is fre- 
quently so persistent at this time of 
year that the air mass may have days 
in which to reach a condition of equi- 
librium with respect to the surface 
beneath. As the stationary maritime 
anticyclone develops under these con- 
ditions, the cold air usually reaches 
the coastal stations at first as not 
much more than a sea breeze, but on 
the following days the cold air mass 
usually invades the whole coastal area 
east of the Appalachian Mountains, 
and occasionally advances far to the 
south. The properties of the cold air 
mass are shown best by the New Eng- 
land coast stations, although in the 
case of a southward displacement of 
the air mass the characteristic cold- 
ness is retained to a surprising de- 
gree. The surface air temperature of 
the mass, as indicated by an outlying 
station like Nantucket, is probably 
very close to that of the cold ocean 
surface from which it is moving. This 
temperature is likely to be about 5°C 
