CYCLONIC 
comes a warm front, for here the 
warm tropical air is forced to over- 
ride the cold wedge of air underlying 
it. The left-hand portion of the wave 
then becomes the cold front of the 
system—a wedge of cold air which is 
displacing the warm air. In normal 
cases the field of flow is such that 
the cold front travels faster so that 
it soon begins to overtake the warm 
front, as is illustrated at the point 
0 in (e). When this occurs the pro- 
cess of “ocelusion” is said to be 
taking place. Stage (d) represents 
the start of the occlusion. As the 
cold front meets the warm front the 
warm air lying between them is com- 
pletely cut off from the surface of 
the earth and exists only as an en- 
trapped body of air at upper levels. 
Hence it is ‘“‘occluded” (closed off) 
from the surface. The front between 
the two cold air masses is called an 
occluded front. On surface charts it 
is represented as a line, eg., the 
heavy broken line in (e) and (f). 
Owing to the fact that the air 
currents on both sides of the cyclone 
have had different trajectories, the 
temperatures, and thus densities, of 
the two cold currents are not the 
same. As the cold front meets the 
warm front a new discontinuity is 
formed which has a shape like a 
wedge whose point rests on the 
ground, and which slopes backward 
or forward depending upon which 
current is cooler, and known as a cold- 
front occlusion and a warm-front oc- 
clusion respectively. 
In the United States the air be- 
hind the cold front is generally colder 
than that ahead of the warm front, 
having had a shorter trajectory over 
relatively warm land areas. The cc- 
cluded front in this case will then 
assume the form of a cold front in 
its lower portion. If, as it sometimes 
happens, the air ahead of the warm 
STRUCTURE 35 
front is colder than the air behind 
the cold front, the occlusion will as- 
sume the form of another warm front 
in the lower layers. This latter case 
is illustrated in (h) which may be 
considered as a vertical cross-section 
through the line AB of (f). The in- 
tersection of the sloping front with 
the earth, in Fig. 8 (h), is drawn on 
the weather map as the “occluded 
front;” also, the position of the inter- 
section of the cold and warm fronts 
aloft is projected on the weather map 
and drawn as a dashed line (in 
U.S. A.) known as an “upper (cold) 
front” (see Bergeron’s model, p. 122-4). 
The type of weather associated with 
an occluded front passage will, then, 
depend largely upon the vertical struc- 
ture. If the temperatures of the two 
adjacent cold currents are fairly 
alike, the entrapped warm air will 
determine the effects observed; if the 
warm air is still being lifted appre- 
ciably precipitation may be abundant. 
It is clear that the temperature and 
moisture distribution within the en- 
trapped warm air is also to be con- 
sidered in forecasting what is to 
happen. The general laws of stab- 
ility are applicable. It frequenily 
happens that an occluded front de- 
velops into a well-marked cold front. 
This may be explained by the fact 
that the occluded front represents a 
trough of low pressure into which 
there is convergence. On one side 
of the front, say the eastern side, 
the air being drawn in is becoming 
warmer and warmer while the air 
behind the front, drawn from the 
northwest and north, becomes colder 
and colder. In this manner the dis- 
continuity at the front is intensified 
so that the upper trough of warm 
air, which was originally responsible 
for the front, becomes insignificant 
compared with the new front which 
has been generated in the lower 
