AEROLOGY OF EXTRATROPICAL DISTURBANCES 
precipitation approximately coincide with the areas 
where 0f./ds < 0. These areas can, according to equa- 
tion (16), be found where the ascending warm air is 
diverging. In Fig. 10c the arrows indicate the flow of 
warm air along the frontal surface in some parts of the 
disturbance. Since the frontal contours also move, one 
cannot immediately determine the vertical component 
FRONTAL TOPOGRAPHY 
18 NOV. 1948 
0300 GMT 
30) 
FRONTAL TOPOGRAPHY & 
19 NOV. 1948 \ oi 
0300 GMT 
120 110, 100 30 Sa eEeeas ak S 
615 
cut off from the cold air in the north. This cutting-off 
process starts in the upper troposphere and gradually 
penetrates downward. The process of cutting off is not 
completely finished at the level of the 700-mb surface, 
as can be seen from Fig. 10. 
The case history of November 17-19 represents a 
common development in the United States, as was 
FRONTAL TOPOGRAPHY 
18 NOV. 1948 
1500 GMT 
FRONTAL TOPOGRAPH 
19 NOV. 1948 
Ni 
1500 GMT 
120 n10 
(c) 
(d) 
Fra. 10.—Idealized frontal contour charts, November 18-19, 1948, corresponding to charts of Fig. 8b-e. On Fig. 10c, corre- 
sponding to Fig. 8d, thin arrows indicate instantaneous streamlines at warm-air boundary of frontal surfaces. On Fig. 10d 
frontal contours are dashed where front is indistinct. 
of the air flow from the map; however, the relatively 
slow movement of the system and the large component 
of the warm air flow normal to the frontal contours in 
the inner parts of the cyclone indicate that here the 
ascending component w must be large. 
The charts for November 19, 1500 GMT present the 
fully developed upper cyclone which at that time very 
nearly coincides with the surface center of the “oc- 
cluded”’ cyclone. The charts also present a situation in 
which the cold air at the 500-mb surface is completely 
pointed out in the previous discussion of the schematic 
chart in Fig. 4. The fully developed surface cyclone of 
November 19 has all the characteristics of an occluded 
polar-front cyclone in spite of the fact that it did not 
develop from a wave cyclone. The three-dimensional 
fields of temperature, pressure, and wind presented in 
Figs. 8-10 are, however, characteristic of every “‘oc- 
cluded” cyclone whether it has passed through a regular 
process of occlusion or not. Essential for the whole de- 
velopment, of a mature cyclone is the formation of the 
