16 AIR MASS ANALYSIS 
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equivalent-potential temperature. In 
practice it is most convenient to 
use the potential temperature and 
the specific humidity; hereafter when 
the codrdinates of the Rossby dia- 
gram are mentioned, they will be re- 
ferred to simply as potential tempera- 
ture and specific humidity, or in sym- 
bols as @ and q, with the understand- 
ing that the reader is aware of the 
small differences between them and 
the partial potential temperature and 
mixing ratio. 
The abscissa of the Rossby dia- 
gram, the specific humidity, is a 
linear scale (see fig. 3); the ordinate, 
potential temperature, is logarithmic. 
This construction facilitated the com- 
putation of the lines of constant equiv- 
alent-potential temperature in the 
original drawing. The position of 
any point on the diagram is thus 
obtained by its values of q and @. 
Thus if one wishes to plot an air- 
plane sounding on the Rossby dia- 
gram there is a routine involved in 
computing q and @. The reports of 
soundings transmitted over the tele- 
type system or by short-wave radio, 
contain the altitude in meters (2), 
the pressure in mb (p), the tempera- 
ture in Centigrade (7), and the rela- 
tive humidity in percent (f). The 
following working table is suggested: 
Ly 
Mi 
pa 
ay 
a 
RossBy DIAGRAM IN 
OUTLINE 
Station and date of sounding 
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Remarks 
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Pee ees | 
After q and @ are obtained the 
diagram is easily plotted. 
Abbreviations for the names of the 
stations and the dates of the sound- 
ings may be attached to the ends of 
the curves, and several curves may 
be drawn on one diagram, providing 
they do not interfere too much with 
one another. Dotted lines, broken 
lines, and the like help to simplify 
the differentiation among curves. 
The lines of constant equivalent- 
potential temperature, which are de- 
termined by means of the @ and q 
scales, are those curves sloping down- 
ward from left to right. 
The interpretation of the various 
types of curves that appear from 
plotting soundings on the Rossby 
diagram will be treated in the next 
article in tl.is series. 
Too much emphasis cannot be 
placed on the importance of the prac- 
tical experience with the methods dis- 
cussed in these articles. It is mainly 
by experience that the student learns 
to understand clearly and really make 
use of them. 
