28 AIR MASS ANALYSIS 
wedge on the Rossby diagram. If 
other soundings have been obtained 
within the warm air their character- 
istic curves should be compared with 
VI. 
Cold fronts belong to the class of 
discontinuities in which the warm air 
lying above the cold wedge is forced 
to rise, the energy being supplied 
mainly by the moving wedge of cold 
air. The most pronounced cold fronts 
are easily recognizable on the surface 
weather map as marked wind discon- 
tinuities, the well known wind-shift 
lines. On the other hand, there are 
many cold fronts not characterized 
by abrupt changes, and thus not so 
easily identified. The slope of the 
cold front surfaces of discontinuity 
is characteristically greater than that 
of the warm fronts, the values being 
of the order of about 1/50 in the case 
of the cold front compared with ver- 
haps 1/200 for the warm front. 
These are merely rough averages; 
in any individual case the slope may 
be appreciably different. 
The importance of fronts in 
weather analysis lies in the fact that 
they lead to the formation of vertical 
atmospheric motions, which in turn 
bring about regions of cloudiness and 
precipitation. It must be borne in 
mind, however, that no vertical mo- 
tion will be present at a front which 
obeys the conditions of equilibrium 
one another. This not only helps in 
identification, but indicates the modi- 
fication which has been taking piace 
within the air current. 
ELEMENTS OF FRONTAL STRUCTURE—THE COLD FRONT 
outlined in Article V of this series. 
This is equivalent to saying that in 
order for vertical motions to arise 
there must be a zone of convergence. 
The convergence may be interpreted 
as the attempt of the adjacent gir 
masses to bring about an equilibrium 
of frontal slope. 
The typical cold front* is usually 
portrayed in vertical cross-section as 
is shown in Fig. 7. In this diagram 
the vertical scale is of course greatly 
exaggerated with respect to the huri- 
zontal. The wedge assumes the form 
of a squall head in its foremost scc- 
tion owing to the fact that the cold 
air is retarded by friction at the 
surface of the earth, while at some 
distance aloft, where frictional effects 
are small, the cold air runs ahead. 
The elevation of this foremost part 
*According to Bergeron the cold front is of 
two main types: The first kind of cold front 
is an ‘“anafront’’ in which there is general 
upsliding motion of the warm air along the 
front surface and formation of a rather broad 
post-frontal Ast-Nbst system—this type will 
only occur with a retarded or slowly moving 
front. In the ‘‘katafront’’, the second kind of 
cold front, the warmer air ascends only in the 
lower layers, the warm air above 1-4 km gener- 
ally moving forwards faster than the cold wedge, 
sinking and thus turning the cloud system 
into a mainly prefrontal one of Acu-char- 
acter; this kind of cold front is by far the 
commonest type.—Buwll. Amer. Met. Soc., Sept., 
1937, p. 266.)—Ed. 
LY LED 
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iyigsee 
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LOGLDIDIEDL DOE Cy 4 
5 2 Lyng lyyy, DAS; Lie 
yyy ILE Cel pg GB 
CF Y el OLEEELELEL) 
fey LZ te of GLE 
ty, Uv. Trem LE ego Sen 
Lif ff Lb ie A or e = = mane Z gas e , ili lifil 
Cold 16 Yt & SUE i in | i Warv7 
HW elt | 
SY SSL SSL SUISSE SG, (JEST IESJ HES HSMM ES ESE 
Fic. 7. IDEALIZED SCHEME OF A CoLD FRONT (After Bjerknes). 
(Reproduced from Fig. 91, Bull. 79, Nat. Research Council) 
