POLAR ATLANTIC AIR—SUMMER 
at the beginning of May, about 10°C 
at the beginning of June and 12° to 
15°C later in the summer. There is 
almost no daily period in these tem- 
peratures. In spite of the fact that 
these temperatures indicate a cooling 
of the air mass from the temperature 
which it originally possessed over the 
continent as Pc air, the PA air mass 
is found to have just as in winter 
a rather unstable structure up to 
about 1 km. Since this lapse-rate 
cannot be explained in this case as 
eaused by heating from below, me- 
chanical turbulence remains as the 
only obvious explanation of the in- 
stability of the lower km of the PA 
air mass. Stratiform cloud forms 
indicate a marked inversion at the 
top of the turbulence layer. It is 
very probable, in view of the pre- 
vailingly stagnant anticyclonic con- 
dition associated with this air mass, 
that the inversion is intensified by 
continual subsidence of the upper 
strata of the mass. The wind velo- 
city is usually strong enough in the 
cool maritime anticyclone to justify 
the turbulence explanation of the un- 
stable ground layer of the PA air 
mass, and the long exposure of the 
mass over the cold water could ac- 
count for the loss of much heat from 
this stratum by turbulent transfer 
downward to the cold surface. Two 
April ascents at Boston in this type 
of air mass showed a _lapse-rate 
nearly nine-tenths of the dry adia- 
batic rate up to 1 km, where there 
was a thin Stcu cloud layer, with a 
temperature inversion immediately 
above of 5°C. We find typically in 
the PA air mass in summer some St or 
Steu or Frst clouds at the top of 
the instability layer, though they are 
usually much thinner than in the 
same mass in winter, seldom covering 
the entire sky, and frequently ap- 
pearing as only a few scattered Freu 
103 
or completely disappearing. Precipi- 
tation never falls from these clouds 
in summer. This follows from the 
initial dryness of the Pc air mass, 
and the very small amount of evapor- 
ation which takes place from the cold 
water in summer. There is very 
little difference between the specific 
humidity of the Pc and that of the Pa 
air mass at this season. The cooling 
of the air mass at the cold water 
surface, an effect which we assume 
to be carried upward by turbulence, 
produces usually a thin saturated 
stratum at the top of the turbulence 
layer (as indicated by the cloud form- 
ations mentioned above), but seldom 
more than about 70% relative hu- 
midity at the surface. Hence the real 
PA air mass does not become foggy 
over the water, but is on the contrary 
usually characterized by excellent 
horizontal visibility, apart from the 
occasional thin cloud layer men- 
tioned above. There is, however, 
occasionally visible over the ocean 
on a clear afternoon in this air mass 
a very noticeable whitish haze, even 
when the relative humidity is appre- 
ciably below saturation, and the con- 
dition is far from being one of real 
fog. This may be due to the presence 
of tiny water droplets on salt nuclei. 
It is obvious that the general effect 
of the underlying cold water surface 
in the source region of the PA mass 
in summer is to effect a cooling of 
the air mass from beneath making 
it essentially stable, though this is 
apparently counteracted close to the 
ground by turbulence. Once the PA 
air comes over the warmer land it 
rapidly becomes warmed from below 
and hence more unstable. 
It should be mentioned at this 
point that warm, moist, continental 
air in summer, and even more Tropi- 
cal maritime air masses from the 
Gulf Stream, which move into the 
