30 AIR MASS ANALYSIS 
to the cold air. The cold front soon 
retrogrades, and thus, by definition, 
becomes a warm front. 
Normally, ciearing takes place 
rapidly behind the cold front. The 
continuous cloud deck associated with 
the front passage soon appears to 
break up into Cu or Stcu clouds. 
Actually, however, these lower clouds 
are not a result of the discontinuity 
surface, but rather are associated 
with the cold air mass which is dis- 
placing the warm. The formation of 
these clouds is somewhat as follows. 
The cold air mass has come from a 
cold source region; therefore, it is 
colder than the surface over which 
it is traveling. Since the lower 
layers are continually being heated 
the lapse rate becomes steeper until 
the dry adiabat is reached. The lower 
particles of air then rise and cool. 
This process, carried far enough, 
leads to the formation of the Cu 
or Stcu clouds, and if vigorous, re- 
sults in showers or snow flurries 
(instability showers). Soon after ihe 
front passage one may observe, 
through the breaks in the lower 
clouds, the layer of continuous cloud 
which lies along the discontinuity 
surface of the cold front. The cle- 
vation of the base of the Stcu or Cu 
clouds formed within the cold air 
remains fairly constant, but their 
height is somewhat less nearer the 
front. This is explained by the fact 
that the cold air near the front is 
more moist than the air farther back, 
owing to the precipitation which has 
_ fallen through the cold air and to 
the evAporation from the moist 
ground. 
In traveling over different regions 
there are many modifications which 
the cold front undergoes. While there 
is no substitute for practical experi- 
ence in interpreting the phenomena 
in any locality, a few more general 
principles can be given which may 
be of value. 
As a cold front moves into more 
southerly latitudes its slope decreases 
in accordance with formula (1) in 
Article V. Thus, in the southern 
section of the country, it is common 
to find the warm air not far above 
the surface, even though the position 
of the surface boundary of the air 
masses is distant. The cloud déek 
accompanying such a front is in gen- 
eral horizontally extensive. Like- 
wise, its base has a very small angle 
of inclination. In fact, in the south- 
ern section of the country the cisud 
deck associated with a slow-moving 
front of polar air frequently touches 
the ground, producing widespread 
fog. An airplane sounding shows a 
very thin layer of cold air, above 
which there is a large temperature 
inversion, the warm and moist Gulf 
air being aloft. The extensive fog 
is probably in large part due to satu- 
ration by falling rain or the mixing 
of the saturated, or nearly saturated 
Gulf air, with the cold Polar air com- 
posing the thin wedge. Precipitation 
falling from the warm Gulf air is, 
in the South, almost always in the 
form of rain. The cold air below 
the inversion, however, may possess 
temperatures well below freezing. In 
this event the rain is subcooled in its 
fall through the cold air, and readily 
freezes on striking objects. Then 
again it may freeze in mid-air ard 
fall as sleet. 
If the cold air passes over a 
warmer water surface, e.g., the Great 
Lakes, the rapid warming and addi- 
tion of moisture generally leads to 
the formation of snow flurries. Ccen- 
ditions are most favorable for these 
instability flurries when the contrzst 
between the temperature of the water 
and the Polar air is most pronounced, 
i.e, in late fall and early winter. 
