ROSSBY DIAGRAM INTERPRETATION 21 
air is a line of constant equivalent- 
potential temperature. This amounts 
to saying that the lines of constant 
6, are also the saturation adiabats. 
In a rough fashion it is evident that 
the @, lines must slope as they do; 
the potential temperature increasing 
in the saturated air particle because 
of the realized (latent) heat of con- 
densation, and the specific humidity 
falling because of the condensation 
and removal of the liquid water. 
The fact that an adiabatic process 
with unsaturated air is represented 
by a point makes the Rossby diagram 
particularly adaptable to modern 
‘synoptic analysis. A thin layer of 
air having a uniform distribution of 
temperature and moisture may be 
represented on the diagram as a 
straight line. Let this line be CD 
in fig. 5. If the entire layer CD be 
raised or lowered the temperature 
and relative humidity of each particle 
of air within the layer will be 
changed because of the adiabatic ex- 
pansion or compression. But the po- 
tential temperature and specific hu- 
midity of each particle of air will 
remain unchanged, so long as conden- 
sation or evaporation do not take 
place, and for this reason the layer, 
eompressed or expanded, will be rep- 
resented on the diagram by the same 
line CD. It is important to note 
that what has been said refers to one 
stratum of air, no new strata being 
introduced or removed during the 
process. The line element CD may 
then be considered as the charac- 
teristic curve for the given layer. 
If an entire aérological sounding is 
plotted on the Rossby diagram, it 
may be considered as the charac- 
teristic curve of the air column 
through which the sounding has been 
made. 
In the definition of an air mass, 
horizontal homogeneity was stressed. 
Soundings, then, made at different 
places within a source region and 
remaining within the same air mass 
should exhibit nearly identical char- 
acteristics; the characteristic curves 
should be similar. Frequently the 
air masses, even at the source regions 
and particularly after travelling 
some distance, are subjected to forces 
which lead to appreciable vertical dis- 
placements. When this occurs un- 
equally in different sections of the 
air mass the homogeneity with re- 
spect to level tends to be destroyed. 
The surfaces of constant potential 
temperature and constant specific 
humidity become curved instead of 
horizontal, and plots of temperature 
or moisture against elevation appear 
markedly dissimilar. It follows that 
it is difficult to identify and follow 
air masses by means of these dia- 
grams. The characteristic curves on 
the Rossby diagram, on the other 
hand, will have overlapping parts for 
the same column of air regardless of 
the extent of the expansion or com- 
pression, providing no condensation, 
evaporation, or introduction of a new 
air mass has taken place. The reader 
may refer to papers published by the 
Meteorology Course of the Massachu- 
setts Institute of Technology and to 
Harvard Meteorological Studies, No. 
2, for examples of this characteristic 
guality for various air masses on the 
Rossby diagram. 
Fig. 5 illustrates characteristic 
winter curves for two air masses: 
one having its source over Northern 
Canada (Pc—Polar Canadian) the 
other from the Gulf of Mexico (TG 
—Tropical Gulf).* The numbers, be- 
side points on the curve, represent 
elevations in kilometers above sea 
level. The Pc curve exhibits the pro- 
nounced coldness, dryness, and stab- 
*For description of these air masses see the 
article by Prof. Willett in the back of this 
booklet. 
