88 CHARACTERISTIC AIR MASS PROPERTIES 
extremely heavy convective precipita- 
tion. This change which takes place 
in only 36 to 48 hours over the warm 
waters of the Gulf amounts often to a 
warming of the surface stratum of 
the Pc mass by 25° or even 30C’, 
and an increase of w from perhaps 1 
to about 15 g. Unfortunately few 
soundings from these regions are 
available, but they indicate a rapid 
approach to if not actual convective 
equilibrium under these conditions. 
For the north-central and north- 
eastern U.S. the modification of the Pc 
air mass properties by the Great 
Lakes is very important. During the 
late fall and early winter, while the 
Lakes are still free of ice, this effect 
is particularly well marked. On the 
coldest Pc outflows with a northwest 
wind blowing across Lake Michigan, 
for example, which is less than a hun- 
dred miles wide, the temperature may 
be 10°C, or even more, higher at 
Ludington on the east shore than at 
Green Bay on the west shore. And 
whereas the sky will be cloudless at 
Green Bay, at Ludington there will 
be low eeiling and constant snow 
flurries. And this difference is char- 
acteristic of all the Lake stations; 
passage of the cold air across the 
Lakes produces always a low Nbst or 
Cunb cloud with almost continuous 
snow flurries. Apparently the in- 
stability layer produced over the 
Lakes is rather shallow, as evidenced 
by the characteristic lightness of the 
precipitation and the amount of the 
temperature increase in the ground 
stratum of the air mass. However, 
jit is marked enough so that consider- 
able cloudiness with snow flurries 
characterizes the Pc current from the 
Lake region eastward up the moderate 
slopes to the Appalachian Divide. 
The surface values of w are found to 
have increased in passing over the 
Lakes from a normal value of about 
0.5 g¢ to perhaps 2.0 g, and the tem- 
perature to have been raised by some 
10C°. This modification is a maxi- 
mum at the ground, disappearing at 
some 24 km elevation. In fact, the 
thermal influence of the Great Lakes 
favors very markedly the formation 
of slowly moving shallow cold fronts 
in this region, which are indicated 
at the ground by a _ well-marked 
wind-snift from southwest to north- 
west and a corresponding trough 
in the isobars. But observations of 
winds aloft show always that at as 
low as 2 km elevation this discon- 
tinuity has completely disappeared, 
the wind at that level being uniformly 
northwest. This means that from the 
Lake Region eastward to the Appala- 
chians the tendency is to a continuous 
overrunning of the somewhat modified 
Pc air at the ground by colder Pc air 
aloft coming more directly from the 
source. This doubtless favors the ap- 
pearance of the instability snow flur- 
ries which characterize the Pc weather 
in this entire region. 
The Pp Air Masses. The PP air 
masses are those originating in the 
Arctic or sub-Arctic, which reach the 
Pacific coast of the United States or 
Canada after a more or less pro- 
longed sojourn over the waters of 
the north Pacific ocean. Initially 
these air masses come from the Arctic 
source regions with properties which 
probably approximate those which we 
have found to be characteristic of Pc 
air, but prolonged heating and moist- 
ening over the warm waters of the 
north Pacific produce, to an extreme 
degree, the type of modification which 
we found to be effected in the Pc 
air masses in winter by the Great 
Lakes. They are changed from a 
cold, dry, extremely stable condi- 
tion to one of marked conditional 
instability with a comparatively high 
moisture content in the lower strata. 
