ROSSBY DIAGRAM PLOTTING ROUTINE 13 
adiabatically to 1000 mb, the tem- 
perature, 72, which it assumes at 
this pressure is by definition the 
potential temperature, which we shall 
eall 6. Thus 
T, pi \9-288 
TENS ( 1000 ) 
1000 \ 0-288 
or G=T: 
pi 
From this formula the potential 
temperature is readily computed. 
1000 \ 0.288 
Tables giving the factor =.) 
may be constructed to facilitate this 
computation. In practice, however, 
it is generally more convenient to 
obtain the potential temperature by 
means of the adiabatic chart. It is 
assumed that the reader has access 
For convenience of 
to such a chart.* 
mb. ~ 
500 
-30° 
reference, fig. 2 will serve to show the 
essential features of this diagram. 
The ordinate consists of the pressure, 
which, in the latest type of adiabatic 
chart, is plotted to the power 0.288 
(i.e., 9-288 is the ordinate). The 
reason for using 9-288 is that when 
plotted in this fashion against a 
linear scale of temperature, the lines 
representing dry adiabats become 
slanting straight lines (though they 
converge upward slightly). This is 
seen from Poisson’s equation. In the 
older Stiive adiabatic chart the pres- 
sure is plotted on a logarithmic scale, 
so that there is a slight curvature to 
the dry adiabats.* 
A little manipulation of Poisson’s 
equation explains the other features of 
*Various older forms, such as the Hertz, 
Neuhoff, and early Stiive diagrams, are still 
found in many physics and meteorology texts. 
The more convenient form, Stiive’s newer 
psuedo-adiabatiec chart, is now used by most 
meteorologists and weather services.—Ed. 
Fig. 2. ADIABATIC CHART (Stiive’s Tp" **) 
