AEROLOGY OF EXTRATROPICAL DISTURBANCES 
our experience. On the other hand, there is probably no 
doubt that the period of eyclogenesis must be preceded 
by a building-up of the kinetic energy of the upper 
west-wind belt and that thus certain types of disturb- 
ances also appear as concentrations of already existing 
kinetic energy. 
The principle of dynamic instability may perhaps 
result in new efforts to solve the problem of the in- 
stability of the west-wind belt. A considerable amount 
of literature on that problem already exists [62]. The 
aerological analyses of weather situations with distinct 
fronts has revealed the existence in the atmosphere of 
zones with a width of 100-500 km (or sometimes even 
more) in which the criterion of dynamic or inertial in- 
stability is fulfilled. The fact that these zones actually 
exist suggests the need for further investigation of this 
type of imstability. Another approach to a solution of 
the problem of instability has been proposed by Rossby 
[54] in his discussion of the meridional displacement of 
cold air masses. 
If the complexity of the cyclone problem is con- 
sidered, it does not seem likely that any satisfactory 
theoretical solution can be achieved in the near future. 
One is also forced to this conclusion by the fact that 
“extratropical cyclones” obviously do not represent any 
well-defined single atmospheric phenomenon but more 
likely a whole group of phenomena. Therefore perhaps 
no single explanation is sufficient. 
The complexity of the atmospheric phenomena asso- 
ciated with extratropical cyclones does not mean, how- 
ever, that no general features characterize disturbances 
in the westerlies. Some of the most characteristic ones 
have been presented in this article. Further detailed 
synoptic investigations of selected types of disturbances 
will certainly improve our knowledge and gradually 
result in a better understanding of the dynamics of the 
atmosphere. In the opinion of the writer, a detailed and 
careful synoptic investigation of typical weather situa- 
tions on a very large scale—approaching hemispheric 
dimensions—is the only method which can furnish us 
with the facts which are necessary both for a better 
understanding and for further theoretical study. 
One of the greatest difficulties in synoptic meteorol- 
ogy is that the concept of causality is extremely diffi- 
cult to explore. To give some examples: Horizontal 
acceleration appears as the small difference between 
two large terms, acceleration due to the horizontal 
pressure field and acceleration due to the earth’s rota- 
tion. Similarly, the vertical acceleration is the small 
difference between the opposing accelerations due to the 
vertical pressure field and to gravity. Again, all pres- 
sure changes are small differences between two or three 
large terms, as can be seen from the tendency equation. 
Finally, the thermal wind equation represents a bal- 
anced condition between a circulation due to the verti- 
cal solenoid field and one due to the earth’s rotation. 
Because of inevitable errors in all meteorological obser- 
vations it is extremely difficult to get a sufficiently 
exact determination of quantities needed for computing 
the nonbalanced conditions in an atmospheric situation. 
Through comparison of successive situations some clues 
619 
concerning the nonbalanced parts of the movement 
can be achieved. The time difference between consecu- 
tive upper-air observations, especially between radio- 
sonde observations (now twelve hours in most regions), 
is too large to permit a satisfactory determination of the 
time derivatives. Special synoptic investigations with 
several stations operating on time intervals of 2-3 hours 
would therefore be extremely valuable. 
REFERENCES 
1. Bannan, J. K., Grawam, R. C., Sawyer, J. S., and Suep- 
PARD, P. A., Large Scale Vertical Motion in the Atmos- 
phere. Recent Research at Central Forecasting Office, 
Dunstable.’ Quart. J. R. meteor. Soc., 75: 185-188 (1949). 
2. Berceron, T., “Uber die dreidimensional verknitpfende 
Wetteranalyse.’’ Geofys. Publ., Vol. 5, No. 6 (1928). 
3. —— “Richtlinien einer dynamischen Klimatologie.’’? Me- 
teor. Z., 47: 246-262 (1930). 
4, Berecren, R., Bourn, B., and Rosspy, C.-G., ‘‘An Aero- 
logical Study of Zonal Motion, Its Perturbations and 
Break-Down.”’ Tellus, Vol. 1, No. 2, pp. 14-87 (1949). 
5. BsERKNES, J., “Exploration de quelques perturbations at- 
mosphériques 4 l’aide de sondages rapprochés dans le 
temps.” Geofys. Publ., Vol. 9, No. 9 (1932). 
6. —— “Theorie der aussertropischen Zyklonenbildung.”’ 
Meteor. Z., 54: 462-466 (1937). 
7. —— and Sorpere, H., ‘“‘Life Cycle of Cyclones and Polar 
Front Theory of Atmospheric Circulation.” Geofys. 
Publ., Vol. 3, No. 1 (4922). 2 
8. ByERKNEsS, J., and Panmnn, E., “Investigations of Selected 
European Cyclones by Means of Serial Ascents. Case 4: 
February 15-17, 1935.” Geofys. Publ., Vol. 12, No. 2 
(1937). 
9. —— ‘‘Aerologische Analyse einer Warmfrontflache.”’ Beitr. 
Phys. frei. Atmos., 25: 115-129 (1939). 
10. BserKNEs, J., u. a., “Synoptisch-aerologische Untersuch- 
ung der Wetterlage wihrend der Internationalen Tage 
vom 13 bis 18 Dezember 1937.” Veréff. geophys. Inst. 
Univ. Lpz., (2) Bd. 12, Nr. 1 (1939). 
11. Bserx«nus, V., u. a., Physikalische Hydrodynamik. Berlin, 
J. Springer, 1933. 
12. Brunt, D., “Some Problems of Modern Meteorology: I. 
The Present Position of Theories of the Origin of Cy- 
clonic Depressions.”’ Quart. J. R. meteor. Soc., 56: 345- 
350 (1980). 
13. CHarney, J. G., ‘“‘The Dynamics of Long Waves in a Baro- 
clinic Westerly Current.” J. Meteor., 4: 135-162 (1947). 
14. —— and Entassen, A., “A Numerical Method for Pre- 
dicting the Perturbations of the Middle Latitude Wester- 
lies.”” Tellus, Vol. 1, No. 2, pp. 38-54 (1949). 
15. Cuaupuury, A. M., Some Dynamic and Aerological Aspects 
of Meteorology over Indo-Pakistan. Ph.D. Thesis, 65 pp., 
Dept. Meteor., Univ. of Chicago, 1949. 
16. Cressman, G. P., “On the Forecasting of Long Waves in 
the Upper Westerlies.”” J. Meteor., 5: 44-57 (1948). 
17. Crocxnr, A. M., ‘‘Synoptie Applications of the Frontal 
Contour Chart: The Motion of Selected Lows, 5-7 No- 
vember 1946.’ Quart. J. R. meteor. Soc., 75: 57-70 (1949). 
18. —— Gonson, W. L., and Penner, C. M., ‘Frontal Contour 
Charts.”’ J. Meteor., 4: 95-99 (1947). 
19. Drerant, A., ‘‘Primire und sekundire, freie und erzwun- 
gene Druckwellen in der Atmosphire.”’ S. B. Akad. Wiss. 
Wien, Abt. Ila, 135: 357-377 (1926). 
20. Hapy, E. T., ‘Long Waves and Cyclone Waves.”’ Tellus, 
Vol. 1, No. 3, pp. 33-52 (1949). 
21. Ficker, H. v., “Beziehungen zwischen Anderungen des 
