ROSSBY DIAGRAM PLOTTING ROUTINE 17 
USE OF THE ROSSBY-DIAGRAM AS A 
NOMOGRAM FOR FINDING @ AND q 
Professor Rossby has also sug- 
gested another method of obtaining 
the potential temperature and moist- 
ure content directly from the equiva- 
lent-potential temperature diagram 
which is presented at the rear of 
Rossby’s original paper.” This par- 
ticular diagram differs from that 
described above (see fig. 3) in that 
it contains, in addition to the 6, w, 
and @z (equivalent-potential tempera- 
ture) lines, the isobars and isotherms 
for the condensation level. With this 
particular diagram (and with one 
operation in multiplication, which also 
may be done graphically) @ and w 
may be readily evaluated from the 
values of p, T, and f obtained in an 
airplane sounding. Dr. Byers’ de- 
seription of this method appears 
below. It is undoubtedly a time say- 
ing process, although one may find 
some difficulty in working with a 
chart which has five sets of inter- 
secting lines. Anyone inexperienced 
with the Rossby diagram will find it 
better to practice first the basic 
methods presented above until he has 
a good understanding of the funda- 
mental quantities and operations.— 
As IN 
[The Rossby diagram or equivalent- 
potential temperature chart is avail- 
able in a form which gives the pres- 
sures and temperatures at the con- 
densation point of the air after an 
adiabatic expansion from the ob- 
served conditions of potential temper- 
ature and moisture content. A de- 
termination in a reverse manner 
makes it possible to evaluate the 
potential temperature and moisture 
content from the ordinary observed 
pressure, temperature and relative 
humidity data (See Plate I, in ref- 
erence cited in footnote 2, p. 16). 
The assumption is made at first 
that the air has just reached satura- 
tion. The point of intersection of 
the actual temperature line (under 
the assumption this is also the tem- 
perature of the condensation point) 
with the line for the given pressure 
coincides with the point of intersec- 
tion of the lines of actual potential 
temperature and saturation moisture 
content. Multiplying the latter by 
the relative humidity gives the exist- 
ing moisture content. Since the po- 
tential temperature in an adiabatic 
expansion is constant until saturation 
is reached, no correction is needed for 
this quantity. 
The graphical method suggested by 
Prof. Rossby is as follows: With the 
observed temperature expressed in 
absolute degrees, enter the diagram 
along the corresponding red tempera- 
ture line until the observed pressure 
line is intersected. Read the poten- 
tial temperature at the point of inter- 
section from the black lines which run 
horizontally across the diagram and 
the saturation moisture content from 
the black vertical lines. Multiply the 
latter value by the relative humidity. 
For this operation, a graph can be 
made consisting of saturation moist- 
ure content and actual moisture con- 
tent as codrdinates, with relative hu- 
midity lines crossing diagonally. 
Example: Pressure 860 mb. Temp. 
3-C., or 276°A. Rel. Hum. 80%. 
Follow red line of 276° to 860 mb, 
where potential temperature of 288.1 
and moisture content at saturation of 
5.5 g/ke are indicated. Taking 80% 
of this latter value, we find the actual 
moisture content, which is 4.4 g/kg. 
—H. R. Byers]. 
Millar of the Canadian Meteoro- 
logical Service has designed another 
2C.-G. Rossby, Thermodynamics applied to 
air mass analysis, Mass. Inst. of Tech., Me- 
teorological Papers, vol. 1, no. 3. 41 pp., 
1932. 
