MECHANISM OF PRESSURE CHANGE 
By JAMES M. AUSTIN 
Massachusetts Institute of Technology 
The problem of weather forecasting has been in- 
timately related to the problem of the prediction of the 
pressure field and, therefore, to the mechanism of pres- 
sure change. For this reason meteorologists have sought 
an explanation of pressure change. However, despite 
the efforts of research workers during the past century 
it appears that there is not yet a completely satisfactory 
explanation of this phenomenon. In view of the uncer- 
tain state of knowledge it is the intention of the author 
to review briefly the more prominent theories of the 
pressure-change mechanism. 
It is important to note that this discussion refers to 
pressure changes and not to pressure systems. A pres- 
sure system and its associated wind and temperature 
fields may be considered to be the result of a complex 
series of events following pressure changes. It is diffi- 
cult, therefore, to investigate pressure changes through 
a study of the characteristics of pressure centers. For 
example, a center of a migratory pressure system is 
intermediate between a pressure rise and fall. This dis- 
tinction between pressure change and pressure distribu- 
tion is emphasized because it is believed that too much 
attention has been focused on meteorological data above 
the centers of pressure systems. 
Historical Note 
During the latter part of the nineteenth century Fer- 
rel [8] developed a comprehensive theory of atmospheric 
processes based upon his own conclusions and the ideas 
of the earlier meteorologists. Ferrel explained the devel- 
opment of a cyclone as a consequence of localized heat- 
ing which causes an outflow at high levels and, there- 
fore, a surface pressure fall. The importance of low-level 
convergence, high-level divergence, and the earth’s rota- 
tion in the development of the wind field around the 
pressure fall was well recognized. The heating was 
attributed principally to the release of latent heat with 
the condensation of water vapor in conditionally un- 
stable air. 
Several objections against this thermal theory were 
raised by Hann [14] and others. One group of objections 
which was directed against the latent heat as the cause 
of the warming could be met by allowing for other 
processes which would produce localized heating. How- 
ever, the more serious objections were based upon 
temperature observations at mountain stations which 
frequently showed high temperatures over surface anti- 
cyclones and low temperatures over surface cyclones. 
These observations were considered to contradict a 
heating or cooling hypothesis and, therefore, there was 
a search for a more acceptable explanation of pressure 
change. The group of theories which attempt to explain 
pressure changes without localized heating or cooling 
as a primary cause are frequently referred to as dy- 
namic explanations. An early example is given by 
Hann [14] who postulated that cyclones originate as a 
consequence of the damming and acceleration of the 
high-level air currents. A defect of many such dy- 
namic theories is the failure of the authors to give 
specific details of the mechanism. 
Before proceeding further with other theories it is 
important to investigate to what extent the empirical 
evidence on pressures and temperatures violates the 
thermal theory. The temperature data which were used 
to discredit Ferrel’s hypothesis were obtained in the 
vicinity of the centers of mature slow-moving pressure 
systems. With such systems the temperature field has 
been greatly modified by vertical motion. Consequently 
these data on the temperature distribution in one layer 
of the atmosphere do not invalidate an argument that 
pressure falls and rises as a result of localized warming 
and cooling, respectively. 
A survey of the early literature then suggests the 
following conclusions: 
1. A thermal explanation of pressure change was un- 
justly criticized by reference to temperature observa- 
tions taken above the centers of mature pressure 
systems. 
2. If the thermal theory is valid there must be a 
more adequate explanation of the localized heating. 
3. The observations which were believed to contra- 
dict a thermal theory greatly stimulated research on 
the so-called “dynamic” theories. 
Pressure-Change Theories 
Insofar as the pressure at a point is the weight of 
the air column above that point, it is convenient to 
discuss pressure changes by means of the hydrostatic 
equation 
im, = i “pg dz. (1) 
It follows from this equation that, if the variations of 
gravity g with height are ignored, the pressure p;, changes 
whenever there is a net change in the mass of air 
above h. This change of mass may be accomplished by 
the heating or cooling of the air without a change in 
the height of the free surface or by a change in the 
height of the free surface without the heating or cooling. 
Consider the simple analogy of a tank of water. If the 
water is cooled everywhere to the same extent there is 
a drop in the height of every pressure surface above 
the base of the tank. Now consider the case where 
one-half of the tank is cooled. Water flows imto the 
cooled region in response to the horizontal pressure 
gradients created between the two sections by the cool- 
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