MECHANISM OF PRESSURE CHANGE 
ing and it follows that the pressure at the base of the 
cooled region rises, in view of the additional mass, 
while the pressure at the base of the other section falls, 
as a result of a lowering of the free surface. In a similar 
manner, it is necessary to heat or cool the atmosphere 
in one region relative to the remainder of the atmos- 
phere in order to produce pressure changes. The pres- 
sure change depends upon the space variation of the 
heating or cooling and not on the magnitude and sign 
of the local temperature change. For example, pressure 
can fall in a region of cooling provided that the cooling 
is stronger elsewhere. This mechanism of pressure 
change may be referred to as a thermal theory. Various 
explanations have been offered for the source of local 
heating and cooling, such as the release of latent heat, 
the advection of warm or cold air, and the nonadiabatic 
processes. More information on the thermal theory 
will be presented in a later section. 
An alternate method of producing a change in pres- 
sure, which involves no consideration of thermal proc- 
esses, requires some outside agency to exert a force on 
the air. For example, a pressure drop may be produced 
in the center of a container of water by causing the 
container to rotate so that the drag at the walls sets 
the fluid in motion. Another illustration of this process 
is the pressure rise created at the bank of a stream where 
the water piles up at a bend. The question now arises 
as to whether pressure changes can occur in the atmos- 
phere by an analogous mechanical process. The for- 
mation of high pressure on the windward side and of 
low pressure on the leeward side of a mountain range 
may be considered a mechanical process. However, 
there appear to be no other agencies which can produce 
- significant pressure changes in the atmosphere by me- 
chanical means. 
The role of lateral mixing processes in the develop- 
ment of pressure changes has been the object of various 
studies. Rossby [26] analyzed the pressure changes 
which accompany the lateral diffusion of momentum 
in a straight current. It was concluded that the diffu- 
sion process could give rise to the formation of a low- 
pressure trough to the left and a high-pressure ridge to 
the right of the current. The pressure changes arise 
from a change in the height of the free surface. This 
study of the mutual adjustment of pressure and velocity 
distributions evidently was undertaken in connection 
with the interpretation of the so-called dynamic pres- 
sure systems, that is, the warm highs and cold lows. 
Such an explanation of pressure changes is open to 
criticism insofar as it leaves unexplained the manner 
in which the strong current was created in view of the 
continual operation of diffusion processes. Lateral mix- 
ing may be an adequate explanation of some pressure 
changes but to complete the picture it is necessary to 
explain the development of the situation which precedes 
the operation of the lateral mixing process. In his dis- 
cussion of Jeffreys’ theory [19] of atmospheric circula- 
tion, Whipple [30] offers a somewhat similar argument 
to explain the pressure distribution. The suggestion is 
made that the strong west winds aloft induce strong 
631 
west winds below through turbulence and that air is 
flung outwards and creates anticyclonic belts. Whipple 
points out that the details of the mechanism are not 
clear. 
There have also been a number of explanations of 
pressure changes which have been based upon the prin- 
ciple that the wind field changes and, therefore, the 
pressure field changes as a consequence of the mutual 
adjustment of pressure and wind distributions. The 
use of constant absolute vorticity trajectories [12] to 
predict a 10,000-ft pressure pattern is an example of 
this principle. In this case the question may be raised 
as to the applicability of the simplified vorticity equa- 
tion. The final wind field is deduced on the basis of 
many assumptions concerning the behavior of individ- 
ual air particles. There appears to be no sound reason 
why the pressure distribution should change so that 
the air particles can follow the assumed trajectories. 
Other explanations of pressure changes as a consequence 
of wind changes are to be found in meteorological liter- 
ature. The statements are frequently vague without an 
explanation of how the pressure changes so that it is 
difficult to discuss the arguments. However, these ex- 
planations usually suffer from a defect like the assump- 
tion of a simplified air trajectory or an unexplained 
change in wind speed. This group of pressure-change 
theories is often referred to as a dynamic explanation 
of pressure changes. 
A comprehensive discussion of dynamic and other 
theories of cyclogenesis has been presented by Raeth- 
jen [24]. The discussion is pertinent to the problem of 
the mechanism of pressure change since it considers the 
formation of a pressure minimum. Raethjen emphasizes 
the problem of the development of cyclonic and anti- 
cyclonic vorticity and includes an analysis of such 
dynamic theories of pressure change as the concept 
that the pressure field of a cyclone forms from its wind 
field. The significance of lateral mixing and friction as 
processes which influence the vorticity distribution is 
discussed in some detail by Raethjen. 
The various approaches to the pressure-change mech- 
anism may now be reviewed. The thermal theory 
appeals in view of its simple physical picture but pre- 
sents the problem of how the local temperature changes 
are produced. The dynamic theories which consider that 
the pressure-change field is a result of a changed wind 
field are not altogether consistent with the fact that 
wind changes arise from the accelerations which accom- 
pany local pressure changes. Of course, it is possible to 
determine the average pressure force acting on a particle 
if the velocity of the particle is known at two different 
times. However, it does not follow that the change in 
wind velocity can be considered the cause of pressure 
changes. Rather it appears more logical to view the 
changes in the wind field as a consequence of pressure 
changes. Nevertheless the wind field, in particular the 
vorticity distribution, is a vital feature of a pressure 
system. Consequently any theory of pressure change, 
such as a thermal theory, must include an explanation 
of the observed changes in the wind field. * 
