542 
the Keplerian era. It behooves us to marshal and cor- 
relate our observations into as precise and consistent 
a scheme as possible in order that we may know in 
sufficient detail what is to be explained. 
From what has been said it might seem to the reader 
that meteorology must still undergo a rather protracted 
period of development before results can be expected at 
the final fruition of this process. Although there is 
reason to expect this pattern of events in the philo- 
sophical aspects of the subject, it should not be for- 
gotten that the main practical use of meteorological 
knowledge, the preparation of weather forecasts, is at 
present largely an empirical procedure. As such, every 
improvement in our empirical information about the 
atmosphere enhances in some degree the possibility of 
improved forecasts, even though satisfactory under- 
standing still remains to be achieved. Here only the 
intellectual aspects of problems are touched upon, leav- 
ing any possible practical applications for treatment 
elsewhere. 
Following the general plan implied in what has been 
said, let us begin by examining how the general cir- 
culation, as we observe it, achieves internal dynamic 
consistency in several important respects. As will be seen, 
this is merely the extraction from data and interpre- 
tation of certain information, and does not in any sense 
constitute an explanation of why the facts are as they 
are found. In order to concentrate attention on the 
most basic processes, let us first consider the mean 
state, leaving the temporal fluctuations in the general 
circulation as a problem of much greater difficulty to 
be touched upon later. 
If an observer equipped with suitable instruments 
were to measure the motions of the atmosphere from 
some extraterrestrial vantage point, in the same manner 
as we measure the motions in the sun, he would proba- 
bly be impressed by the irregularities of the details, 
but at the same time he would discern that there is a 
pronounced general drift of the air from west to east 
relative to the earth in middle latitudes, extending 
from the surface to the stratosphere and even beyond. 
On the other hand, in the more equatorial regions (and 
at times near the poles, at least the North Pole) he 
would discern a drift from east to west from the sur- 
face to great heights. This situation immediately poses 
perhaps the most important problem concerning the 
general circulation. The specific question involved here 
is how the belts of westerlies can maintain their high 
rate of rotation in the face of the retarding effect of 
surface friction, flanked as they are by oppositely di- 
rected winds on at least their equatorial sides. No 
really satisfactory rational theory for this state of 
affairs has yet been given, although some deductions 
can easily be made concerning the nature of certain 
aspects of the mechanism which is necessary to main- 
tain these existing motions. 
The retarding effect of the surface frictional forces 
on the middle-latitude westerlies may be looked upon 
as a continuous abstraction of absolute angular mo- 
mentum from the atmosphere in these regions. Accord- 
ing to simple principles of Newtonian mechanics, this 
THE GENERAL CIRCULATION 
drain can be compensated only by an equivalent flow 
of angular momentum into the westerly belts. Likewise 
the surface frictional effect in the regions of the easter- 
lies may be interpreted as a flow of angular momentum 
from the earth into the atmosphere. In order that the 
angular momentum so transferred into the easterly 
regions should not progressively accumulate and de- 
stroy these wind systems, it is necessary that an equiva- 
lent flow out of these regions should exist. 
The inescapable conclusion is that the accounts are 
balanced by a flow of angular momentum from the 
easterlies in the more tropical regions poleward to the 
westerly belts (the polar easterlies are of relatively 
small importance in this connection). Such a flow of 
angular momentum, let us say northward at the north- 
ern border of the tropical easterlies, can be measured 
in terms of an equivalent tangential stress acting across 
a vertical surface parallel to the latitude circle. A crude 
estimate of the value of this stress may be made from 
existing information concerning the surface frictional 
forees on the easterlies to the south. The result is of 
the order of 50 to 100 dynes em. Molecular and 
small-scale eddy viscosity cannot transmit stresses of 
this magnitude under the existing conditions, so that 
very large-scale nonzonal components of motion must 
furnish the necessary eddy-transfer of angular momen- 
tum. We thus come to the very pertinent observation 
due to Jeffreys [5], namely that the large nonzonal 
components of motion in the atmosphere are necessary 
for the maintenance of the average zonal components. 
Most classical models for the general circulation as 
well as some more recent ones (see for example Rossby 
[9]) have followed along lines originally proposed by 
Hadley [4] in that they assume the existence of large, 
slow, convectively driven closed circulations in meridi- 
onal planes. The development of the mean zonal mo- 
tions is then ascribed to the effect of the earth’s rotation 
on these primary circulations. According to this view 
the necessary transport of angular momentum could 
be achieved if, for example, the poleward branches of 
the meridional circulations carry more angular momen- 
tum than the returning ones at other levels. 
For several reasons many modern meteorologists have 
come to view models of the Hadley type with skepti- 
cism. In the first place, the warmest regions of the 
atmosphere are not usually found in the tropics as 
most schemes of this kind visualize, but rather some 
distance away from the equator. Also, at best it is 
difficult to account for the great extent of the westerlies 
in the atmosphere on any such basis. Finally, there is 
a suggestion in the climatological distribution of pre- 
cipitation and in the poleward flow of air in the friction 
layer for the existence of a slow meridional circulation 
with an upward branch in middle latitudes and a 
downward branch toward the subtropics, as originally 
suggested by Bergeron [2]. Such a “reverse”’ cell with 
equatorward flow aloft would, due to the action of 
Coriolis forces, tend to establish east winds in the 
1. It is here assumed that the main transfer of momentum 
takes place in the troposphere. 
