548 
cations involved in the study of the balance of angular 
momentum and of kinetic energy in the atmosphere, 
let us now consider the general circulation from the 
standpoint of a rational theory for atmospheric motions. 
By this term we mean essentially a solution of the 
equations of motion which purports to give a picture of 
the circulation or some portion of it. This is sharply 
contrasted with the study of the balance of various 
quantities, because the latter involves hydrodynamical 
principles only to the extent of formulating several 
integral requirements for the atmosphere. The investi- 
gation of how the atmosphere actually fulfills these 
requirements is then necessarily an empirical and ob- 
servational problem. 
Logically, then, the empirical picture obtained from 
the study of the balance of various quantities and from 
other observational studies should give us the funda- 
mental physical characterization of the atmosphere 
which is then to be explained in terms of some rational 
theory. But, in a larger sense, the proper physical 
characterization of the general circulation is a subject 
which up to the present has hardly been entered upon. 
We need quantitative estimates of the flow of heat, of 
kinetic energy, of momentum, etc., for the mean state 
and also when consideration is given to synoptic and 
seasonal variations. The labor involved in order to sup- 
ply this information is bound to be tremendous, involv- 
ing the cooperation of meteorological services over the 
entire globe. Nevertheless progress is being made and 
there is reason for optimism. 
In view of this state of affairs 1t is not surprising 
that the development of a rational theory for the 
general circulation has not made very much headway 
up to the present time. Thus the scope of the more 
successful efforts toward the solution of the hydro- 
dynamic equations has necessarily been severely limited 
to certain portions of the atmospheric circulation where 
the introduction of drastic simplifications still leads to 
results of interest. As an example we might cite the 
several solutions of the two-dimensional barotropic vor- 
ticity equation given by Rossby [8] and recently elabo- 
rated by Charney and Eliassen [3] and others. For 
short-period extrapolation it appears that these results 
may prove to be of more and more value in forecasting, 
even though these solutions leave unanswered some of 
the fundamental questions concerning the energy bal- 
ance when long-term effects are contemplated. 
As a general comment it can be said that really 
adequate solutions which link the mechanics of the 
atmospheric circulation to the source of energy from 
solar radiation are a desideratum for future research, 
although the situation does not warrant undue pessi- 
mism. In this connection reference may be made to a 
recent publication by Lorenz [6] which is concerned 
with the solution of the two-dimensional equations of 
motion for the region near the pole, taking into account 
friction and external heating and cooling. 
Leaving now the discussion of illustrative topics, 
what are the avenues for future progress which are 
indicated as of today? This is necessarily a subjective 
matter in which various investigators must follow their 
THE GENERAL CIRCULATION 
own inclinations and tastes, for there is no shortage of 
diverse problems which are worth while. The inclina- 
tions of the writer, as one of these investigators, are 
probably quite apparent from what has already been 
said. The underlying and oft-recurring motif of this 
essay is the need for a more complete and systematic 
empirical description of the basic physical processes 
involved in the general circulation. We are now begin- 
ning to have sufficient observational material at least 
in the Northern Hemisphere. 
Before making concrete proposals, let us take a glance 
at what is being done which may serve as a beginning 
toward this aim. In the enumeration of activities which 
follows, any omission of important work is inadvertent 
and is due simply to the writer’s lack of information. 
At the Massachusetts Institute of Technology Willett 
[18] has for a period of years gathered statistical in- 
formation concerning the general circulation in the 
Northern Hemisphere in the form of 5-day averages. 
More recently he has begun gathering similar data for 
the Southern Hemisphere. The writer has found this 
material invaluable for the study of the general circu- 
lation. The Department of Meteorology of the Univer- 
sity of California at Los Angeles has undertaken a study 
of the angular momentum balance of the atmosphere 
by means of finite difference integration procedures. 
This project will probably also include a similar study 
of the energy balance later. 
Priestley [7] of Australia has proposed a program for 
the observational study of the momentum and energy 
balance of the atmosphere, utilizing radiosonde and 
radio-wind observations directly. Samples of his analy- 
ses already prepared by him have proven to be of great 
interest, but require elaboration. 
Van Mieghem [15] of Belgium has proposed the study 
of the balance of various forms of energy, entropy, and 
momentum, emphasizing the importance of such studies 
for the progress of research concerning the general 
circulation. 
The Extended Forecast Division of the U.S. Weather 
Bureau has for some years been amassing observational 
material in the form of 5-day averages for the Northern 
Hemisphere. The Weather Bureau [14] has recently 
also been preparing in published form very complete 
daily Northern Hemisphere maps for the surface and 
500 millibars. These charts are most excellent and 
should prove to be invaluable for research purposes. 
Finally mention may again be made of the studies 
of the angular momentum and energy balance initiated 
by the writer at the Massachusetts Institute of Tech- 
nology. 
The items listed above do not purport to cover all 
important research on the general circulation conducted 
currently, but rather only such activities as are apt to 
furnish statistical information concerning the gross 
character of the general circulation. Thus various other 
synoptic and theoretical activities are not included 
even though they may in the end furnish indispensable 
insight as to the interpretation of the empirical picture 
given by the statistics. 
The general trend to be discerned is that the em- 
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