B5-20 PREPARATION OF CHARTS 
EXTRA POLATED | 
25 km. Constant Pressure Blocks ao i 
1 km. sips, a ee 
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Figure B5 - 8. Extrapolation of Pressure Altitude Data. 
NOTE: points A, B, C, and D are labled for 
illustration and need not be labeled on WBAN-31(). 
10. Pressure Altitude Curve. A pressure altitude curve, based on 
pressure, temperature, and relative humidity data, is computed for each 
raob and drawn on the adiabatic chart. It provides a means of determin- 
ing the altitude of significant levels and of other data entered on the 
chart. The pressure altitude curve is also used as a sighting line for 
location of errors in altitude computations. These altitudes are in geo- 
potential meters above mean sea level. 
10.1 Plotting the Pressure Altitude Curve. Plot the surface altitude on 
the surface level against the altitude scale printed along the lower edge 
of the adiabatic chart. In the same manner, plot the altitude of each 
standard isobaric surface and the altitude of the level at the maximum 
altitude of the sounding. Connect the successive plotted points by straight 
lines and label the curve "PA" at top and bottom of each section of the 
adiabatic chart, Whenever the observation extends to pressures less than 
10 mb_ data for the 10 tol mb range will be plotted on the same WBAN- 
31C used for the 100 to 10 mb range. The pressure altitude curve per- 
taining to the 10 to 1 mb range will be drawn in red. 
a. To reduce the likelihood of a computation and/or plotting error 
when an observation terminates shortly after transferring 
from one adiabatic chart to another or when it is necessary 
to displace the pressure altitude curve, each portion of the 
pressure altitude curve should be based on-at least three 
points. For example, if an observation terminates at 82 mb, the 
Po lSiG9 FMH#3 
