624 
ment has been proposed by Priestley [24] as an out- 
growth of earlier work by Durst and Sutcliffe [8] on 
the role of vertical currents in producing nongradient 
winds. Given an increase of wind with height, an up- 
ward current will create subgradient winds and a down- 
ward current, supergradient winds. Priestley used this 
rule to explain anticyclogenesis where the downward 
vertical motions were initiated by (1) frictional outflow 
from the surface anticyclone or (2) radiative cooling 
of the tropospheric air over a large area. But, as 
Priestley showed, neither (1) because of lack of suffi- 
ciently large vertical wind shear in warm anticyclones, 
nor (2) because of lack of strong enough radiative cool- 
ing over a sufficiently large area, are by themselves 
adequate to explain anticyclogenesis as observed. Priest- 
ley believes that the two processes must operate simul- 
taneously to produce the observed pressure rises. How- 
ever, a severe restriction on Priestley’s model is that 
an anticyclone with surface frictional outflow must 
already be in existence before the nonradiatively caused 
descent can occur. According to Priestley’s reasoning, 
then, it follows that surface features, such as topography 
or thermal stability of the air, which diminish the 
surface frictional outflow of the anticyclone, weaken 
the downward currents, thus causing a lack of super- 
gradient winds and retarding anticyclogenesis. This 
result is not in agreement with the high frequency of 
anticyclones observed over physiographic depressions, 
such as the Great Basin in the western United States or 
over cold water, such as the Great Lakes area in sum- 
mer [32, p. 172]. Regarding the radiatively caused 
descent, Priestley showed that for a reasonable model, 
a cooling of the free air of 2C per day over an area 2500 
km in diameter is necessary to maintain the anticyclone 
against normal surface frictional outflow. This cooling 
value seems too high since Priestley quoted Elsasser 
[10] with respect only to the emissive cooling of the free 
atmosphere by long-wave radiation, and did not men- 
tion the heating by absorption of solar radiation, which 
amounts to 144C per day. Also, according to Hlsasser 
[10, p. 95], since water vapor is the principal emitter, 
the emissive cooling itself decreases very rapidly with 
moisture content, and this would mean that the dry 
air above an anticyclone would cool even less than 
Priestley’s assumed value. 
In a search for the presence of available energy in the 
atmosphere which might be used to create unbalanced 
currents, a sudden cyclogenesis upstream and the re- 
sulting acceleration and “banking” of the westerlies to 
the south has been suggested [21, p. 63]; but this 
explanation is merely substituting the unknown solu- 
tion of the cyclogenetic problem for that of the anti- 
cyclogenetic problem. A more promising and not un- 
related lead is that of the propagation of energy with 
the group velocity of a train of waves in the westerlies 
where air parcels move with constant absolute vorticity. 
This concept, removed from the restraints of sinusoidal 
wave motion first assumed by Rossby, forms the basis 
of the Charney-Eliassen numerical prediction technique 
of westerly waves [6]. 
The following is a summary of the dynamic causes of 
MECHANICS OF PRESSURE SYSTEMS 
anticyclones which appeals most to the writer. It is 
based on the combined effects of wave mechanics of 
the westerlies, lateral mixing, radiative cooling of the 
free atmosphere, thermal stability of the atmosphere 
(especially that of the surface layer), and the character 
of the underlying earth’s surface. Lateral frictional 
stresses will cause supergradient winds south of the 
axis of the westerlies. This will create a northerly 
component of flow which will vanish far to the south. 
The convergence of air to the south of the zonal wester- 
lies will result in the formation of an anticyclonic 
ridge. Waves in the westerlies will fracture the ridge into 
anticyclonic cells. Lateral frictional stresses which con- 
tinue to operate on the smusoidal westerlies will further 
increase the convergence into the anticyclonic cells. 
Factors which block the downward flow of the con- 
verging air in the cells and its return to the westerlies 
in the surface frictional layer will favor anticyclogenesis. 
These optimum factors are (1) greater thermal stability 
of the free air combined with low rates of radiative 
cooling of the air (as in dry air) to hinder the non- 
adiabatic descent of the air, (2) presence of a very 
stable surface layer of air, or ‘shielding layer,” which 
will prevent the descending air from coming under 
influence of the surface frictional stresses, and thus 
inhibit air transport north and south across isobars, 
and (3) presence of a large physiographic depression 
such as the Great Basin of the western mountains in 
the United States which ‘‘traps” the descending air 
and interferes with its motion away from the area. 
Of all these auxiliary conditions, the presence of a 
shielding layer seems to be most significant. The forma- 
tion of such a shielding layer is favored by local cooling 
of air over large areas such as cold water surfaces, 
the undercutting of the atmosphere by a subsiding and 
increasingly stable wedge of polar continental air, and 
by radiative cooling and air draimage in large physio- 
graphic depressions. 
An important dynamic consequence of the horizontal 
convergence of the air responsible for anticyclogenesis 
would be an increasing cyclonic circulation if one ap- 
plied the conservation of absolute angular momentum 
to the contraction of rings of air in the absence of 
external forces. Thus, if convergence is an important 
factor, one might expect to observe a cyclonic core 
embedded in the center of an anticyclone aloft. This 
has actually been observed by Simmers [33, p. 88], 
but appears to be observed rarely both because of lack 
of a sufficient density of upper-wind observations and 
the tendency of the peripheral anticyclonic circulation 
to obliterate by friction the weak cyclonic circulation at 
the core. 
It should be emphasized that, given the proper condi- 
tions, the lateral frictional stresses south of the axis 
of the westerlies are exerted on all the air flowing 
through the region in question. The supergradient winds 
thus created cause a northerly component of flow, 
probably too small to be observed directly. The result- 
ing horizontal convergence will be relieved by vertical 
motions downward in lower levels and upward in the 
upper troposphere. These latter motions will create a 
