636 
High-Level Processes 
A European school of meteorologists proposed a 
modified thermal theory [9] of pressure change in which 
the primary cause is thought to be temperature advec- 
tion in the stratosphere with secondary thermal effects 
in the troposphere. There is no doubt that there are 
strong advective fields about the tropopause and, there- 
fore, it seems logical to pay attention to the influence 
of stratospheric advection on pressure changes. A com- 
prehensive analysis of the large changes which take 
place near the tropopause has been presented by Fleagle 
[10]. However, as Fleagle mentions, these changes are 
not sufficient evidence to conclude that the “seat” of 
the pressure change resides in the stratosphere. The evi- 
dence which has been presented on the relationships 
between sea-level pressure change and high-level advec- 
tion do not support a conclusion that the cause of the 
sea-level change is to be found in advection in the strato- 
sphere. ; 
The strong changes which occur in the upper atmos- 
phere may be considered as arising from vertical mo- 
tion fields which accompany the sea-level pressure 
changes and which arise from such low-level processes 
as heating or cooling. A nonuniform field of vertical 
motion creates strong advective fields in the lower 
stratosphere as a result of the abrupt change in the 
vertical temperature gradient near the tropopause. 
Hence it may be argued that the high-level advective 
fields are the result rather than the cause of sea-level 
pressure changes. Similar explanations have been pre- 
sented by many meteorologists such as the discussion 
given by Douglas [6] on ‘‘SSome Facts and Theories 
about the Upper Atmosphere.” It should also be noted 
in connection with the high-level processes that Reed 
[25] has been successful in relating the prominent ozone 
variations to the fields of vertical motion which are 
known to accompany the sea-level pressure system. 
Tt would appear then that there is no definite evidence 
to support a hypothesis that stratospheric advection 
may be considered a cause of sea-level pressure change. 
However the evidence does not contradict hypotheses 
that other high-level processes may produce low-level 
pressure changes. For example, the observed variations 
of the temperature field across the tropopause may lead 
one to suspect that mstability in this general region 
may give rise to significant low-level changes. It would 
then appear desirable to investigate, analytically and 
empirically, processes other than advection which might 
take place in the vicinity of the tropopause and contri- 
bute to sea-level changes. As in the case of the high-level 
heating discussed previously, it will be necessary to 
consider the effect of tropospheric processes upon the 
net sea-level change which arises from some high-level 
process. 
High-Level Pressure Changes 
The motion and changes in intensity of the weather 
systems are accompanied by pressure changes at all 
levels in the atmosphere. The various constant-level 
charts present a series of horizontal slices through the 
MECHANISM OF PRESSURE CHANGE 
atmosphere whose over-all behavior is reflected by the 
sea-level chart. 
So far the discussion has concentrated mainly upon 
the problem of the pressure change at sea level. This 
approach is justified on the basis of the presence of a 
fixed boundary which should simplify the problem. 
When high-level pressure changes are considered it has 
to be recognized that air can move vertically through 
the level at which the pressure variation is being ana- 
lyzed. One method of viewing a high-level pressure 
change is to explain the change on the basis of the pres- 
sure variation at the earth’s surface and the change in 
the temperature or density of the air column from the 
surface to the level in question. This type of approach 
is adequate in view of the status of pressure-change 
theories. 
Empirical Evidence 
Because of the questionable status of the theories of 
pressure change it is desirable to investigate those em- 
pirical facts which have to be explained by a pressure- 
change theory. Figure 3 presents a schematic cross 
section through an area of pressure rise and fall and 
includes some pertinent information on the changes in 
the temperature field, the vertical velocities, and the 
vertical accelerations which accompany the pressure 
changes. The idealized picture is based upon observa- 
tional evidence obtained by the pressure-change project 
at the Massachusetts Institute of Technology and upon 
the empirical data presented by Fleagle [10, 11]. 
Whether Fig. 3 is a good approximation to a cross sec- 
COLD AIR 
ADVECTION 
WARM AIR 
ADVECTION 
200 
500 
PRESSURE IN MB 
WARM AIR 
ADVECTION 
4 
f 
x 
PRESSURE FALL 
COLD AIR °* 
ADVECTION 
1000 
PRESSURE RISE 
Fra. 3.—A vertical cross section through an idealized pres- 
sure trough. The solid and broken lines indicate the directions 
of the vertical velocities and vertical accelerations, respec- 
tively. The dotted line indicates the region of strong horizontal 
temperature gradient. 
tion through an actual weather system is open to ques- 
tion. Since upper-air observations are usually taken at 
twelve-hour intervals, much of the empirical informa- 
tion on the instantaneous field of vertical velocities has 
to be deduced from twelve-hour changes. For this reason 
it is to be expected that a computed field may give a 
somewhat erroneous impression of the magnitude and 
space variation of the true field of vertical velocities. 
