AEROLOGY OF EXTRATROPICAL DISTURBANCES 
characterized by cyclonic inflow of tropical air in the 
surface layers; the upper tropical air forms an intensi- 
fied upper ridge or, in extreme cases, a closed upper 
anticyclone, as a result of the latitudinal effect and the 
divergence in the northward ascending air. 
On daily 500-mb charts the formation, intensifica- 
tion, and final degeneration of the upper troughs and 
ridges can be studied. The disturbances, superficially 
studied, give the impression of being large waves, and 
the air flow gives the impression of beimg a horizontal 
movement along sinusoidal trajectories. A careful anal- 
ysis of the real movement of selected air parcels, how- 
ever, emphasizes the importance of the vertical com- 
ponents of the three-dimensional air flow and the ir- 
reversibility of the processes. In the most extreme form 
the irreversibility of the process appears in cases in 
which an upper trough develops into a closed “high- 
level” cyclone to the south of the strongest westerlies, 
or in which an upper ridge ends up in a closed “‘high- 
level” anticyclone to the north of the strongest wester- 
lies. Thus the life history of the upper disturbances 
shows an irreversibility similar to that which is char- 
acteristic of the polar-front cyclones. 
Figure 4 presents some of the principal types of 
upper-air disturbances associated with the deforma- 
tions of the polar front at the 500-mb level. This figure 
is a further development of Fig. 3. The polar-front dis- 
turbances on the corresponding surface map are indi- 
cated by the common symbols for warm, cold, and 
occluded fronts. Because of technical difficulties in 
presenting the smaller upper disturbances corresponding 
to the polar-front waves and cyclones, they have been 
omitted in drawing the boundary of the polar air and 
the 500-mb contour lmes. The shaded regions indicate 
the area occupied by polar air at the 500-mb surface. 
The scheme presented here consists essentially of four 
irregular long upper waves as in Fig. 3. The wave 
troughs are situated on the east coast of Asia, over the 
Gulf of Alaska, on the east coast of North America, 
and over eastern and southern Europe. The schematic 
chart does not intend to present any climatological 
distribution of the disturbances in question. However, 
the different types of disturbances are to some extent 
placed in regions of the Northern Hemisphere where 
they can frequently be observed on daily synoptic 
charts. It might, however, be pointed out that not all 
of the disturbances in Fig. 4 would necessarily appear 
at the same time over the Northern Hemisphere. The 
chart corresponds to typical winter situations. For 
other seasons the intensity of the disturbances is usually 
weaker, but in principle of the same type. 
In addition to the four large upper troughs of differ- 
ent shape, the chart also contains three cold cyclones 
on the south side of the polar front. These cyclonic dis- 
turbances are formed from previous cold troughs of the 
same type as the four large troughs in Fig. 3. The forma- 
tion is associated with a gradual “cutting-off”’ of the 
polar air from its original source region in the north 
and will be described later. 
The average latitude of the polar front at the 500-mb 
surface is about 48°N in Fie. 4. The strongest wind at 
607 
the 500-mb surface, corresponding to the smallest dis- 
tance between the contour lines, can be observed chiefly 
in the vicinity of the upper polar front or just to the 
south of it. Exceptions to this rule, however, can be 
seen in connection with the strong northward displace- 
ment of the polar front from its average position. 
Bt 
ii) 
= _ Ost] ~ 91 
Opi 
Fie. 4.—Schematic circumpolar chart for the 500-mb level 
showing several types of disturbances. Heavy line is polar front 
at 500 mb; thin lines are contours of 500-mb surface. Fronts at 
earth’s surface indicated by usual symbols. Area covered by 
polar air at 500-mb level is indicated by stippling. 
Our schematic figure does not intend to give more 
than a rather general idea of the development and 
structure of different types of disturbances. A more 
detailed description of some of them will be presented 
in the subsequent sections. There the following special 
types will be discussed: (1) the cyclone families of the 
regular type (Fig. 3, or over the Pacific Ocean in Fig. 4); 
(2) the occluded surface cyclone (represented, for ex~ 
ample, by the cyclone over the central parts of the 
United States); (3) the practically symmetric upper 
low (east of the Azores); and (4) the warm upper anti- 
cyclone (over northwestern Europe) with its series of 
cyclonic perturbations. 
In the schemes presented in Figs. 3 and 4 there is a 
general difference in scale between the polar-front dis- 
turbances connected with individual surface cyclones 
and those associated with the long upper waves. How- 
ever there are also cases where one upper long wave is 
connected with only one large surface cyclone. This type 
is illustrated in Fig. 4 by the cyclone over the United 
States. 
It seems difficult to maintain the idea of a complete 
difference in nature between the ‘long upper waves” 
and the cyclonic perturbations. Every cyclone natu- 
rally influences the upper-air flow and appears as a dis- 
turbance on the upper polar front. During the develop- 
ment and deepening of a surface cyclone there is also a 
