CHAPTER A4 
ADIABATIC CHART 
Vg Adiabatic Chart. The WBAN-31 isa combined form for computing, 
recording and graphing raob data. This form consists of three sections, 
the WBAN-31A or AA, B, and C. Datafrom the recorder record are en- 
tered in the forms under the heading "Data Block." After evaluation, 
they are transcribed to the adiabatic chart for additional computation. 
Data required for punched cards and transmission purposes are entered 
in the blocks printed to the right of the chart. Baseline-check readings 
and the surface-observation data are entered in the appropriate section 
provided on WBAN-31A and AA. Constant pressure level computations 
are entered on the back of WBAN-31A and AA, for levels below the sur- 
face of the station. NOTE: Instructions that refer to WBAN-31A are also 
applicable to WBAN-31AA. WBAN-31AA is used at stations where tem- 
peratures colder than -60°C may be encountered at pressures greater 
than 400 mb. The notation WBAN-31( ), means thatinstructions are ap- 
plicable to all charts, i.e., WBAN-31A, AA, B, and C. 
1.1 Use of the Adiabatic Chart. The adiabatic portions of WBAN-31( ) 
furnish a graphical means of computing many of the functions of pres- 
sure, temperature, and relative humidity. The horizontal lines represent 
pressure, and the vertical lines, temperature. Space is provided on the 
left side of WBAN-31A and B for plotting values of relative humidity. 
The sloping lines on the chart are dry adiabatic lines. Along the lower 
edge and below the temperature scale is an altitude scale expressed in 
kilometers above sea level. All altitudes are expressed in terms of the 
geopotential meter (gpm), which is approximately equal to the geometric 
meter. In this handbook it will be understood that meters are expressed 
in terms of the geopotential meter. Short vertical brown lines, or tabs, 
printed on WBAN-31A are used to obtain the virtual temperature incre- 
ments for the strata in which they occur. Short vertical black tabs, 
printed on WBAN-31A, B,andC, are usedto determine the thickness be- 
tween adjacent standard isobaric surfaces. Since the thickness ticks are 
overprinted on the base chart, each chart must be checked for correct 
registration before the chart is usedinanobservation. Registration tabs 
are placed in each corner of the chart for this purpose. Altitude com- 
putations should not be made, using the thickness tick method, when the 
registration on the chart, at any corner, isin error by more than 0.3 of 
a printed division. If a chart is found in error by more than 0.3 ofa 
printed division, Table 7 will be used to determine thickness between 
adjacent standard isobaric surfaces. 
SES OO A4-1 FMH#3 
