142 
face cold front. Thus the trough in 
the contour lines is usually found 
slightly in advance of the cold front, 
with a steep upward slope of the isen- 
tropic surface in the rear of the cold 
front. 
The parallelism of contour lines 
with well-marked cold fronts many 
times enables one to construct height 
lines in regions where upper-air ob- 
servations are sparse. The applica- 
tion of this principle to frontal-wave 
disturbances is apparent, for here 
there must be a wavelike pattern in 
the contour lines roughly parallel to 
the frontal waves in the synoptic sur- 
face chart. 
Sharp warm fronts show up in the 
contour-line pattern in much the same 
manner as do cold fronts, the gradient 
increasing abruptly at the front while 
the lines remain fairly parallel to the 
front. There are, however, many 
warm fronts on the surface weather 
maps which are not associated with 
this simple contour-line pattern above. 
Moreover, the surface maps fre- 
quently indicate an homogeneous air 
mass in the warm sector, while the 
isentropic charts show conclusively 
that the warm air is far from homo- 
geneous aloft, but rather character- 
ized by troughs in the contour lines 
suggestive of fronts. Some time after 
their appearance in the contour-line 
pattern, these troughs may appear on 
surface charts as regions of fronto- 
genesis, suggesting that such newly 
AIR MASS ANALYSIS 
formed fronts are a result rather than 
a cause of the processes in the upper 
air. It thus appears that many fronts 
on the surface weather map are in- 
duced by action taking place first in 
the upper air and later showing up at 
the ground. This, obviously, means 
that a frontogenetical wind-field de- 
velops aloft and gradually extends 
downwards. 
The above ideas are illustrated in 
Fig. 3:—The light broken lines la- 
belled z, z +1, etc., are contour lines 
of an isentropic surface. H indicates 
regions where the isentropic surface 
is high, and L regions where it is low. 
Fig. 3a represents the normal topo- 
graphy, while Fig. 3b shows a type 
of topography which is associated 
with frontogenesis within the warm 
air mass subsequent to the appear- 
ance of the trough in the contour 
lines. This latter case is presumably 
associated with frictionally driven 
anticyclonic eddies which we shall 
treat in more detail later on. 
Finally, we may use the upper-air 
wind observations as entered directly 
for the observed heights of the isen- 
tropic surface over aerological sound- 
ing stations as a guide to the configu- 
ration of contour lines. In cases 
where the pressure distribution aloft 
is largely determined by the distribu- 
tion of temperature, the winds blow 
nearly parallel to the contour lines 
so that domes or ridges are generally 
to the left of the current flow. 
Fig. 8. RELATION OF CONTOUR LINE PATTERN TO SURFACE FRONTS. 
