26 AIR MASS ANALYSIS 
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Fic. 6. IDEALIZED VERTICAL CROSS-SECTION OF A WARM FRONT 
(GE 
— 
— = ES SS Si —— = 
(After Bjerknes). 
(Reproduced from fig. 90, Bull. 
fronts. Cold fronts, aggressive 
wedges of cold air which force warm 
air upward, will be taken up in the 
next article. Discontinuities charac- 
terized by a downward component of 
warm air flow along a surface of cold 
air are given the name surfaces of 
subsidence. They also will be treated 
later. 
In the case of a warm front the 
cold air acts as a barrier to the 
warm flow, the resulting vertical dis- 
placement leading to the formation 
of clouds and perhaps precipitation. 
Figure 6 shows the characteristic cir- 
culation, cloud forms, and precipita- 
tion area generally associated with 
a well-defined warm front. In the 
eastern part of the United States the 
over-running warm air is most likely 
to have come from the Gulf of Mex- 
ico or the tropical portion of the 
Atlantic Ocean, while the underlying 
cold air is most frequently an air 
mass of Polar Canadian origin. This 
combination of air masses represents 
extremes of warmth and moisture 
on the one hand and of coldness and 
dryness on the other. The interac- 
tions of these two currents are re- 
sponsible for most of the winter 
precipitation of the eastern part of 
the country. 
Actually there are many modifica- 
tions which are introduced into the 
ideal scheme shown in Figure 6. 
Modifications may be induced by the 
Nat. Res. Council, No. 79.) 
earth’s surface features over which 
the front is traveling, and the vertical 
structure of the warm and cold air. 
The effect of surface friction in a 
retreating cold wedge (a warm 
front) tends to flatten the wedge in 
its lower sections. Thus, for some 
distance ahead of the warm front it 
is sometimes observed that the slope 
is almost horizontal. When this is 
true, a point well ahead of the front 
will generally be found at which the 
slope of the discontinuity increases 
comparatively rapidly. On the syn- 
optic surface chart such a distribu- 
tion may appear as another front, 
particularly in the precipitation field, 
for the steep slope causes consider- 
able ascent of the warm air. 
The vertical structure of the warm 
air with respect to temperature and 
moisture distribution is important for 
types of cloud and precipitation. For 
example, if the warm current be 
stable and dry, as is frequently the 
case with currents of Tropical Pacific 
air, after crossing the Rocky moun- 
tains, condensation forms may be 
entirely lacking, and considerable 
ascent of the warm air will be neces- 
sary to produce condensation. On 
the other hand, if the warm current 
is conditionally unstable, the ascent 
over the cold wedge may become 
vigorous enough to form thunder- 
storms. If the current be convec- 
tively unstable the lifting of layers, 
