POLAR PACIFIC AIR—-WINTER 83 
areas of open water in the Great 
Lakes in winter. The rapid heating 
and moistening which this effects in 
the lower strata of the cold, dry, 
stable Pc air masses are evidenced by 
the persistent low cloudiness, con- 
vectional snow flurries and relative 
warmth of the onshore Pc current 
which has crossed any of the lakes. 
Presumably this instability becomes 
rapidly less at upper levels. 
(2) The presence of the highlands 
and numerous ridges of the Appa- 
lachians which must be crossed in the 
eastward advance of the Pc current. 
On the western slopes of the Ap- 
palachians the effect of the forced 
ascent of the Pc current which has 
already been rendered conditionally 
unstable over the Great Lakes is to 
cause general cloudiness and more 
or less continuous snow flurries as 
long as the cold air flow continues 
unabated. On the coastal slopes the 
fohn action causes a partial or total 
dissipation of the cloud deck and 
snow flurries. However, the rough 
terrain maintains a steep lapse-rate 
through the lower km or more, with 
low relative humidity and relatively 
high temperature at the ground. 
_ (8) The increasing depth and 
strength of the Pc outflow in the 
eastern United States. The rapid in- 
tensification and increasing vertical 
depth characteristic of cyclonic dis- 
turbances moving northeastward over 
the United States causes an accelera- 
tion of the Pc outflow over the east- 
ern United States especially at higher 
levels. This helps to increase the 
turbulence in this air flow at low 
levels, while the rapid transport 
southward of air from Arctic sources 
at upper levels maintains a good 
lapse-rate aloft. 
These effects are to be seen clearly 
in the data for Boston in Table III. 
We note the high surface tempera- 
ture in contrast to that at Ellendale, 
and the steep turbulence lapse-rate in 
the first km and the moderate rate 
above, such that at the 3 and 4 km 
levels the characteristic Pc tempera- 
tures are notably colder at Boston 
than at Ellendale. We observe also 
the low surface relative humidity 
increasing with elevation and the 
almost constant value of @E in the 
first km, both of which indicate 
thorough turbulent mixing, while 
even at higher levels #m increases 
much more slowly than in the Pc 
air at Ellendale. 
These characteristics of the Pc air 
mass in the northeastern United 
States and along the Atlantic coast 
explain the typical flying conditions 
which are met with during its preva- 
lence in this region. As long as the 
cold air flow continues actively, the 
roughest and gustiest flying condi- 
tions which may be experienced in 
winter are to be expected up to 
considerable elevations. Along the 
Atlantic seaboard the skies are likely 
to be almost clear, with excellent 
visibility and, normally, only scat- 
tered fracto-cumulus or cumulus 
clouds, though scattered light snow 
flurries sometimes occur in the after- 
noon. In the Appalachians and on 
their western slopes one may expect 
a rather low broken cloud deck, often 
continuous over considerable areas, 
with frequent, extensive and often 
heavy snow squalls. After the force 
of the Pc outbreak has been spent, 
so that the air mass becomes rela- 
tively stagnant, then subsidence, radi- 
ation and the cessation of the tur- 
bulent mixing all act to establish 
quickly in the Pc air mass the low 
inversion, thermal stability and dense 
stratified smoke haze so characteristic 
of this air mass in the middle west. 
Of course, this change is accom- 
panied by the complete disappearance 
