574 
where the last integral now represents the contribution 
of mean meridional circulations of the Hadley type to 
the poleward energy flux. 
It should be remarked that, in a stable atmosphere, a 
meridional cell of the so-called direct type produces a 
poleward flow of energy, while one of the indirect type 
produces a flow in the opposite sense. This can easily 
be shown from the form of the last integral in (26). 
Very preliminary estimates by the writer of the value 
of the third term on the right-hand side, made from 
geostrophic wind data for mdividual Northern Hemi- 
sphere maps for various levels seem to suggest that this 
contribution is somewhere in the vicinity of one-half of 
H at 40°N latitude. All in all it would thus appear that 
the contribution of the mean meridional circulations 
is small or even negative in middle latitudes, although 
very little reliance can be placed on the figures given 
or on the value of H obtained from the data given by 
Bjerknes. Suffice it to say that in all cases reasonable 
orders of magnitude are obtained, which in itself is 
somewhat encouraging. 
Concluding Remarks. Most classical models for the 
general circulation of the atmosphere have followed 
along the lines originally proposed by Hadley [4] in that 
they assume the existence of large convectively driven 
closed circulations in meridional planes, at least m the 
average conditions. The development of the mean zonal 
motions is then ascribed to the effect of the earth’s 
rotation on these primary circulations. In such a scheme 
the meridional circulations are a necessary mechanism 
in the production of kinetic energy. Also, according to 
this model the necessary meridional transport of angular 
momentum could be achieved if the poleward branches 
of the circulations carry more angular momentum than 
the returning ones at other levels. 
For a number of reasons modern meteorologists have 
come to view models of the Hadley type with skepti- 
cism. A discussion of the basis for this current skepti- 
cism has been recently given by Rossby [7]. The writer 
rather inclines to the view that, although some mean 
meridional circulations in all probability do exist, their 
role in the energy balance and in the horizontal trans- 
port of angular momentum, at least in middle latitudes, 
may be overshadowed by the characteristics of other 
types of motion. Thus, following an original suggestion 
by Jeffreys, the writer has pointed out elsewhere [8] 
that the transport of angular momentum could be 
achieved through the observed properties of horizontal 
motions. This contention has since received a certain 
THE GENERAL CIRCULATION 
amount of corroboration by the observational studies 
of Widger [11]. 
The general views expressed in the present paper 
indicate that atmospheric meridional circulations like- 
wise may not be essential for the global energy balance. 
Much more could be said if a more satisfactory ap- 
praisal were available for the magnitudes of the terms 
appearing in equation (26), since the values given are 
useful only for purposes of orientation. Further work 
in this direction is currently in progress at the Mas- 
sachusetts Institute of Technology. 
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