616 
upper disturbance associated with the deformation of 
the upper front which can be followed on the charts in 
Fig. 8. This deformation of the upper front is associated 
with the formation of an upper cyclone or a very deep 
trough. During the process a large part of the cold air 
is separated from its original source region and flows 
as a diverging lower current very far to the south. This 
tliverging lower cold current can be seen on the surface 
map for November 19 (Fig. 11) in the regions west of 
the surface cold front. 
There is no doubt that many “occluded” cyclones on 
surface maps have never gone through a real process of 
occlusion although they show the same characteristic 
structure as really occluded polar-front perturbations. 
Obviously a well-marked surface front is not so essen- 
tial for the development as was generally assumed 
formerly. 
SURFACE MAP 
19 NOV. 1948 
Fria. 11.—Surface map, 0630 GMT November 19, 1948, three 
hours later than charts of Figs. 8d and 10c. 
One more point should be emphasized. By comparison 
of the consecutive 500-mb charts in Fig. 8 it can be 
seen that the area of polar air at that level decreases 
during the process of cyclogenesis and the development 
of the deep ‘“‘occluded” surface cyclone. The process of 
seclusion of the polar air at the 500-mb level thus corre- 
sponds to the occlusion process in lower layers. In the 
upper atmosphere the warm air gains area, in the lower 
atmosphere the cold air gains area. This process corre- 
sponds to the scheme for release of energy of storms 
proposed by Margules [83] in his classical studies.!! 
As a result of the process described here, the upper 
cold air (for example, at the level of 500 mb) has been 
11. The energetics of a similar process of cyclogenesis has 
recently been studied by Phillips [48]. The main difficulty in 
applying Margules’ ideas on the development of real cyclones 
depends upon the well-known fact, already emphasized by 
Schréder [57], that cyclones cannot be regarded as closed sys- 
tems. Because of this and other difficulties many meteorologists 
are critical of Margules’ theory (see for example [59]). It seems 
to the author, however, that the theory is essentially sound if 
applied correctly and with consideration for all complications. 
MECHANICS OF PRESSURE SYSTEMS 
isolated from the main body of cold air in the north. 
Since the boundary layer separating the warmer air 
masses in the south from the polar air masses in the 
north also marks the zone of strongest west wind, the 
whole process of forming upper cyclones is associated 
with a meandering of the “jet stream.” This meander- 
ing very often goes so far that it results in the forma- 
tion of an almost symmetrical upper cold low at very 
low latitudes, whereas the strong west wind becomes 
re-established to the north of the newly formed upper 
cyclone. Figure 12 shows, according to Hsieh [30], the 
Fic. 12.—Schematic diagrams showing various stages in 
formation of cut-off cyclone. Solid lines are contours of 500-mb 
surface, dashed lines are isotherms. (After Hsieh [80].) 
characteristic change of the pattern of isotherms and air 
flow in the middle troposphere during the development 
of a closed cold cyclonic vortex. This formation of 
“drops of cold air” (Kalilufttropfen) has been well 
known since aerological observations have been ex- 
tended over large areas.” Figure 13 shows a schematic 
picture of the frontal contours in such a cut-off low. 
500 mb 
Fie. 13.—Schematic diagram showing frontal contours in 
cut-off low. Hatching indicates area of heavier precipitation, 
stippling area of lighter precipitation. (After Hsieh [80].) 
A beautiful example of this type of high-level cyclone 
is the case of November 1-7, 1946 over the southwest- 
ern part of the United States. This case has been studied 
in some detail by Crocker [17] and Palmén [42]. Figures 
14, 15, and 16 are reproduced from the latter study. 
Figure 16, in particular, represents an almost ideal pic- 
ture of a cold symmetrical upper cyclone with its warm, 
low stratosphere. It should perhaps be mentioned that 
the tropopause marked as a continuous surface in Figs. 
12. See Scherhag’s textbook on weather analysis and fore- 
casting [56] for a detailed discussion of the structure and forma- 
tion of such drops of cold air in Hurope. Among other references 
Nyberg [40], and Raethjen [50] might be mentioned. 
