2 AIR MASS ANALYSIS 
this cause must be negligible in com- 
parison with other causes. Further- 
more the theory of fronts and air 
masses is based upon atmospheric 
discontinuities, which, are simply 
zones of rapid transition of the vari- 
ous meteorological elements. It is 
assumed that these zones of transition 
are comparatively free from large 
scale mixing, the individual large 
scale air currents flowing side by side 
or above one another without appre- 
ciable mutual drag. 
Granting the importance of vertical 
motion in the atmosphere a discussion 
of the factors which tend to aid or 
hinder such motion is in order. This 
leads to the problem of stability. 
Here the term stability is used in its 
physical sense; if an air particle 
tends to remain in, or return to, its 
former position following a displace- 
ment, the condition is termed stable; 
if displacement results in a tendency 
to further movement of the particle 
from its original position, the origi- 
nal condition is designated as un- 
stable; and finally, if the particle 
neither resists nor assists displace- 
ment, the condition is one of neutral 
equilibrium. 
B. TYPES OF EQUILIBRIUM 
In the case of the atmosphere there 
are four principal types of equilibri- 
um to be considered when we are con- 
cerned with the vertical displacement 
of a selected particle through a layer 
of the atmosphere having known 
characteristics. These types of equi- 
librium are: 
il, Stable 
a. With respect to dry air* 
b. With respect to saturated air 
2. Unstable 
a. With respect to dry air 
[1. Mechanical] 
b. With respect to saturated air 
8. Neutral 
a. With respect to dry air 
b. With respect to saturated air 
4. Conditional 
Another case, that of convective 
equilibrium, will be treated inde- 
pendently in a future article, since it 
concerns the displacement of layers 
of the atmosphere rather than the dis- 
placement of individual particles of 
air through a given layer. 
It is obvious that if a particle of air 
is lifted it must expand against the 
decreasing pressure so that the pres- 
sure within and surrounding the par- 
ticle must be equal; if it sinks it must 
be compressed. It is assumed that 
these changes take place without the 
transfer of heat either from the mov- 
ing particle to its surroundings or 
vice versa. Such a thermally insul- 
ated process is termed adiabatic. If 
the particle expands it does work; if 
it is compressed work is done upon 
it. Thus there must be a conversion 
of mechanical energy into realized 
heat if the particle sinks, while heat 
must be converted into mechanical 
energy if the particle rises. By 
means of thermodynamics it can be 
shown that the relation between tem- 
perature and pressure in an adi- 
abatic displacement of an unsaturat- 
ed particle is as follows: 
T ( pr ) +288 
T» a p2 
where 7; is the original temperature 
of the particle at the pressure pi, and 
T, is the temperature it assumes at 
the pressure p2. This is Poisson’s 
equation. 
It is generally more convenient in 
aerological studies to refer to the adi- 
abatic changes in temperature with 
respect to changes in elevation. From 
1The term “dry air” in synoptic meteorol- 
ogy simply means unsaturated air. 
