84 CHARACTERISTIC AIR MASS PROPERTIES 
of the instability snow flurries, cloudi- 
ness, and gustiness. 
There is nothing very distinctive 
about the synoptic situation which 
leads to the outflow southward of Pc 
air from the source region in North 
America. Usually the outbreak is pre- 
ceeded by a gradual building up of 
high pressure and low temperatures 
in the source region, often without 
any very obvious atmospheric move- 
ments. Sometimes the cold air out- 
break follows quickly on the first ap- 
pearance of the cold anticyclonic cir- 
culation in the north, but often there 
is an extended delay, during which 
cold air surges in the north seem on 
the point of breaking out, only to 
recede again. Sometimes the final out- 
break follows on the passage inland 
from the Pacific Ocean of a disturb- 
ance usually already occluded in the 
Aleutian Island region. Other times 
the cold air suddenly advances with- 
out any very obvious disturbance to 
start it moving. Usually, however, 
when this happens, one or more dis- 
turbances are likely to form along the 
cold front as it advances southward. 
This is especially likely to happen as 
the cold air pushes southward to a 
point where it begins to come in con- 
tact with warm moist air from the 
south, particularly Tm air from the 
Gulf (Te). The disturbances usually 
appear first as flat wave formations 
on the cold front, but are likely to de- 
velop rapidly into intense occluding cy- 
clones which greatly accelerate the 
movement of the Pc air behind them to 
more southerly latitudes. In such a 
development the advance of the Pc 
air mass and cold front is likely to be 
checked at first by the warm air cur- 
rent. Then very quickly an extensive 
overrunning of the Pec air by the TG 
current takes place. The overrunning 
circulation tends to develop cyclonically 
or counter clockwise, with heaviest 
precipitation extending far back of 
the cold front. As J. Bjerknes® has 
pointed out, this type of overrunning 
is to be expected only when the warm 
current is unstable; but as we shail 
see presently, marked conditional in- 
stability is always characteristic of 
our Tropical air from the Gulf. Refs- 
dal’ was the first to call attention to 
the importance of the réle which con- 
ditional instability in the atmosphere 
may play in the development of frontal 
disturbances and in the regeneration 
of old occluded disturbances. The 
writer is inclined to the belief that the 
moist-conditional instability (Feucht- 
labilitat) of our TG air masses is 2 
very important factor in the control 
of the interaction between the warm 
and cold air mass and the stability or 
the tendency to occlusion shown by 
wave disturbances on the cold front 
between such air masses. An excel- 
lent instance of the weather sequence 
with the gradual advance of a cold 
front followed by an extensive Pe air 
mass may be found on the weather 
maps and upper-air data for Jan. 13- 
16, 1930 in the U.S. Following the first 
sweep of the cold air southeastward 
to the Gulf of Mexico, the whole 
frontal system followed by the Pc 
outflow advances steadily eastward un- 
til the whole eastern and central U. S. 
is covered by the cold air. On the 
other hand, it sometimes happens that ~ 
the cold air outflow from the source 
region takes place in one sudden ab- 
rupt outbreak, without the develop- 
ment of any well-marked disturbance. 
In this case the cold air sweeps every- 
thing before it, apparently too rapidly 
for any localized disturbance to de- 
velop on the cold front. Furthermore, 
these very fast moving cold air out- 
flows usually occur only on the heels 
6Hxploration de quelques perturbations» at- 
mosphériques. -..  Geofys. Publ., Vol. IX, 
No. 9 
