20 AIR MASS ANALYSIS 
IV. 
In the preceding article the me- 
chanics of plotting the Rossby dia- 
gram were discussed, and it may be 
said here that the curve joining the 
individual points is called the char- 
acteristic curve for the air column. 
The present article deals with the 
significance of various curves on the 
diagram and the changes in appear- 
ance of these curves as the more 
common atmospheric processes, such 
as vertical displacement, occur. At 
this point it is only fair to remind 
the reader that the following dis- 
cussion is, from the standpoint of a 
quantitative analysis, rather unsatis- 
factory in that it is an attempt to 
describe processes which are better 
expressed in more exact mathematical 
terms. Yet it is hoped that the 
following will serve as an outline for 
those who have a limited knowledge 
of mathematics and physics. 
In the Rossby diagram the char- 
acteristic curve for an unsaturated 
particle of air, A (see fig. 5) having 
a given potential temperature (() 
and specific humidity (q) and being 
displaced vertically, is represented as 
310 
THE ROSSBY DIAGRAM—INTERPRETATION 
a point. Owing to the fact that its 
potential temperature must remain 
constant, the point A cannot be dis- 
placed from the horizontal line repre- 
senting its potential temperature. 
Furthermore, the specific humidity 
remains constant during the adiabatic 
process with unsaturated air, and thus 
the point A cannot be displaced from 
the vertical line representing constant. 
specific humidity. It is evident, then, 
that the characteristic curve of a dry 
particle of air which is undergoing 
adiabatic transformation reduces to 
a point on the diagram. For a given 
element of air this point will be rep- 
resented by the same coordinates (@ 
and q) until the level of condensa- 
tion has been reached. From this 
level on it is assumed that the water 
produced by condensation drops out 
immediately—in other words, the 
process is considered pseudoadiabatic 
(pseudo, since there is a small amount 
of heat removed from the air by the 
falling water). From the definition 
of equivalent-potential temperature 
(6,) it is clear that the characteristic 
curve of the saturated mass of rising 
300 
290 
260 
Fic. 5. CHARACTERISTIC CURVES ON THE ROSSBY DIAGRAM. 
