THE COLD 
FRONT ? 33 
COLD AIR 
WARM AIR 
(a) 
ZB 
= 
Vain ST CUZ 
(g) 
Fic. 8. FORMATION AND OCCLUSION OF A CYCLONE. 
air from the polar to the tropical 
regions to compensate for the pole- 
ward flow which must be present in 
the upper levels of the atmosphere. 
This conclusion is based upon observ- 
ations which, in the mean, show a 
definite poleward pressure gradient 
aloft. Furthermore, it can be shown 
that the interchange of polar and 
tropical air must take place sporadic- 
ally and irregularly, the polar air 
breaking out in vast tongues and dis- 
placing the warmer air in its path. 
When this occurs the original low, or 
the primary cyclone as it is called, 
Cold AU 
moves off with the system to the 
southeast, becoming a migratory 
cyclone of the mid-latitudes. 
The Polar Front, which has now 
been pushed to the south, may extend 
in a general NE-SW direction, and 
usually slows up in its more south- 
erly portion. We now have a stow 
moving discontinuity between ithe 
polar and the tropical air. This 
front, because it marks the boundary 
surface between two currents of dif- 
ferent density and is almost station- 
ary, is particularly favorable to the 
formation of wave disturbances on 
