598 
for in daily synoptic practice. Helmholtz considered 
only the background pattern of an atmosphere moy- 
ing zonally at all longitudes. What synopticians need 
are the criteria of dynamic instability for large-scale 
atmospheric currents which are nonzonal and non- 
permanent, because they are part of a slowly moving, 
long-wave pattern, but still offer the propitious environ- 
ment for the quick growth of perturbations of cyclone 
size. 
Once the frontal wave has occluded, dynamic in- 
stability in the Helmholtz sense is eliminated, because 
of the accomplished spread of cyclonic vorticity to the 
whole cyclonic area. Indirect effects of dynamic in- 
stability may, however, be brought to the cyclone 
from the neighboring upwind anticyclonic part of the 
upper-tropospheric westerlies. In the absence of such a 
rejuvenating influence, the cyclone is left to decay by 
frictional inflow at the surface unopposed by deepening 
effects in the upper troposphere. This sketch of the 
last part of the life cycle of cyclones is even less inti- 
mately related to exact dynamic theories than is the 
existing theory of frontal cyclogenesis. The most hope- 
ful approach to a solution of the problems of the old 
cyclone is most likely to be through the study of the 
energy transformations in the cyclone and its farther 
environment. The major old cyclones occupy such big 
fractions of the volume and weight of the atmosphere 
that the “farther environment” must be taken to mean 
the whole hemispherical circulation. 
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