622 
Wexler [36], using the Brunt-Douglas isallobarie con- 
cept, and later by Schmidt [31]. 
Warm Anticyclone. A satisfactory explanation of the 
warm, deep anticyclone has not yet been given. The 
attempted explanations are usually along three lines: 
radiative, advective, and dynamic. The reasoning based 
on radiation goes something like this: Since the tropo- 
sphere over the warm-type anticyclone is warmer than 
the surroundings, the increased pressure must be caused 
by colder, denser air in the upper troposphere and 
lower stratosphere. The argument is then made that 
this cooling is caused by favorable radiative conditions 
over a restricted area. Those arguing advectively believe 
that the cold air results from northward advection of 
the cold equatorial upper troposphere and lower strato- 
sphere. Various dynamic reasons for warm anticyclo- 
genesis have been proposed, based on wave mechanics 
of the westerlies, lateral frictional drag, motion of 
polar domes, etc. Each of these proposed explanations 
will be discussed in turn. 
Radiatwe Theory. The explanation of anticyclogenesis 
based on radiative cooling of the stratospheric air 
[18] appears to be the least convincing. Gowan [13] 
in his investigation of nocturnal cooling of the ozono- 
sphere (where water vapor, ozone, and carbon dioxide 
are assumed to be the principal emitting and absorbing 
gases) finds the temperature decrease in eight hours 
from sunset varies from less than 0.1C at 15 km to 1C 
at 30 km, and thus concludes, “It seems certain that 
the cooling of the lower stratosphere, perhaps up to 
30 km, is not governed by radiation.” If one looks 
above 30 km (80-mb pressure), it is difficult to ascribe 
any significant anticyclogenetic effects to possible hori- 
zontal differences in the rates of radiative cooling of 
air in a layer whose contribution to sea-level pressure 
is so small. 
Advective Theory. The advective theory of strato- 
spheric cooling and anticyclogenesis appears super- 
ficially to be very attractive. As Brunt [5] states, the 
close apparent analogy between the polar and cyclonic 
stratospheres on the one hand and the equatorial and 
warm anticyclonic stratospheres on the other, tempts 
one to explain the anticyclogenesis as a consequence of 
solid currents moving from south to north. But if the 
entire atmospheric column at 50° latitude should be 
replaced by that at 10° latitude, then it can be shown 
from Wagner’s aerological averages [34] that the net 
change of sea-level pressure would be practically zero. 
If, however, differential advection were established such 
that the mass of the column at 50°N between the 
surface and 8 km remained unchanged, say by the 
presence of westerly winds, while the air above 9 km 
came from 10° latitude with southerly winds, then the 
sea-level pressure would increase by 25 mb, an amount 
more than sufficient to account for most cases of anti- 
eyclogenesis. Thus it is theoretically possible for anti- 
cyclogenesis to be explained by high-level advection 
over a long trajectory from the south. But the problem 
is more complicated than the simple arithmetic exercise 
would indicate. Aerological experience indicates that 
the extreme type of vertical shear in the flow pattern 
MECHANICS OF PRESSURE SYSTEMS 
postulated seldom if ever occurs, but that the full 
depth of the westerlies usually partakes in its meander- 
ings (as would be expected in a barotropic atmosphere). 
Thus, for example, if the southerly advection from 
10°N to 50°N took place above the top of a shallow 
(2-km) inert layer of surface air, the increase in pres- 
sure would only be 5 mb in summer and 11 mb in 
winter, amounts which are not sufficient to account for 
many cases of anticyclogenesis. 
The advective transport of warm and cold air is 
intimately connected with the dynamics of the westerly 
flow pattern. If the westerlies are zonal in character, 
there would be no north-south advection across the belt 
of westerlies. True enough, there may be advective 
transport of a shallow (2-km) surface layer of polar air 
southward underneath the westerlies; this is associated 
with a certain type of dynamic anticyclogenesis dis- 
cussed by Simmers [33] and Wexler [37]. But it is 
when the zonal westerlies break down into large-scale 
waves of great amplitude (meandering) that the most 
pronounced cases of warm anticyclones are found. The 
amplitude of these waves may become so large that the 
ridges break down into large-scale anticyclonic vortices 
to the north and the troughs change into cyclonic 
vortices to the south. Southerly advection accompanies 
this process, bringing northward over a shallow surface 
layer of polar air the warm, lower tropospheric air, and 
the cold upper tropospheric and lower stratospheric air 
characteristic of tropical latitudes. However, advection 
is not the primary cause of the anticyclogenetic process, 
but rather both it and the initiation of the anticyclo- 
genesis are the result of a major change in the character 
of the flow pattern in the westerlies. It follows that the 
primary problem becomes one of dynamics although it 
is acknowledged that once initiated, the secondary 
advective effects may contribute materially to the anti- 
cyclogenesis. 
Dynamic Theory. The dynamical explanations of 
warm anticyclones may be divided into two main types: 
First, an explanation based on the large-scale changes 
in the westerly flow pattern, as discussed briefly above; 
and second, once the “background” flow pattern has 
become established, an explanation based on cross- 
isobaric flow of air resulting from nongradient winds. 
Let us discuss briefly each of the proposed explanations. 
In the past two decades intensive. research has been 
performed on what might be called the “wave mechan- 
ics” of the westerlies. The pioneering work of V. Bjerk- 
nes and collaborators [3] and J. Bjerknes [2], was given 
further impetus by Rossby and collaborators [28] and 
was aided by greatly improved aerological coverage 
over most of the Northern Hemisphere. At the present 
time the recognition and explanation of the complex 
behavior of the westerlies have come to be accepted as 
the central problem of meteorology. Numerous articles 
in this Compendium deal with various phases of this 
problem in a much broader manner than can be given 
here. But in any discussion of anticyclonic origin, de- 
velopment, movement, dissipation, ete., it is necessary 
at first to depart from the narrow confines of the area 
usually covered by an anticyclone and obtain a broader 
