B5-16 PREPARATION OF CHARTS 
find the thickness of the stratum in Table 7 using the mean virtual tem- 
perature and the section of the table lying between the pair of dashed 
lines corresponding -to the pressures at the upper and lower limits of the 
stratum. Enter the thickness value in the dashed block just above the 
constant pressure block, if provided, pertaining to the base of the stra- 
tum. Thickness values for complete strata only willbe obtained by either 
method. Whenever the observation extends to pressures less than 10 mb, 
thickness values for strata within the 12.5 to 1 mb range can be com- 
puted from the same thickness tabs used in computing the thickness 
values within the 125 to 10 mb range. In this case, however, the strata 
for which thickness values are computed will be: 12.5 - 10 - 8 - 7- 6 - 
2 Bo Bo 28 2.4 oil.Bs W waldo, 
7.2 Altitude of Standard Isobaric Surfaces. Compute the altitude of each 
of the standard isobaric surfaces by adding the thickness of each stratum 
to the altitude of the base of the stratum, progressing upward to the high- 
est standard isobar reached in the sounding. Enter the altitude data thus 
found in the altitude section of the constant pressure data block corres- 
ponding to the appropriate standard isobaric surface. 
8. Computation When Less Than Station Altitude. Whenever the sur- 
face pressure is less than 1000.0 or 850.0 millibars, the altitude of the 
corresponding constant-pressure level or levels relative to sea level 
will be computed on the back of WBAN-31A in accordance with the 
following instructions. The tables referred to in these instructions on the 
back of WBAN-31A are included in the Radiosonde Observation Compu- 
tation Tables (WBAN). 
8.1 Lines 1_ Thru_5. On the lines 1 thru 5, enter the data to tenths 
as required by the legends. These lines are used to compute the station 
temperature argument (t), which is entered on line 6 and is expressed 
by the formula: 
ye lye (2to+t_g¢), degrees Celsius, where 
to = current station temperature (°C) 
t_g = station temperature 6 hours previously (°C) 
8.2 Lines 6 Thru 14, On lines 6 thru 14, enter the required data 
as indicated by the legends on the respective lines. The entry required 
on line 9 is the surface altitude in meters m.s.1l. (see JB4-4.3). When 
the value entered on line 13 is greater than the value entered on line 
9, the value entered on line 14 will have a minus sign (-) prefixed to 
the value. 
Al S69 FMH#3 
