B5-14 PREPARATION OF CHARTS 
For example, if the relative humidity is 100 percent, the mean tempera- 
ture will be displaced to the right by an amount equal to the full distance 
between the vertical tabs to obtain the mean virtual temperature; if the 
relative humidity is 50 percent, the mean temperature will be displaced 
one-half the distance, etc. When the mean virtual temperature of a com- 
plete stratum is being determined - thatis, one bounded by standard iso- 
baric surfaces - the brown tabs to be used are those printed on the iso- 
bars between the standard isobaric surfaces. When the mean virtual 
temperature of an incomplete stratum is being determined - that is, a 
stratum bounded by only one standard isobaric surface - the brown tabs 
to be used are those nearest the midpoint of the stratum. 
Lo Determination of the Thickness and Altitude of the Strata. Find the 
thickness of the stratum between the surface andthe next higher standard 
isobaric surface (and the altitude, meters m.s.1l., onthe latter) in accord- 
ance with the following instructions, except that whenever the surface 
level coincides with a standard isobaric surface, the scale method de- 
scribed above will be used. 
a. Enter the surface altitude (see JB4-4.3)inmeters above mean 
sea level immediately to the right of the 50°C vertical tem- 
perature line and opposite the 900 mb pressure of WBAN-31A. 
b. Use the values found in Table 6 and 6A to find the value cor- 
responding to the surface pressure to the nearest 0.1 mb. 
(Note that pressure is recorded to 0.1 mb. in ''Surface Ob- 
servation at Release'' block.) Enter this value beneath the 
surface altitude entry. This value will be the thickness of the 
stratum, uncorrected for temperature, between the surface 
level and the next higher standard isobaric surface. 
c. Find the temperature correction in Table 9 for the thickness 
in (b). The mean virtual temperature and the thickness of the 
stratum are the arguments for determining this correction. 
d. Enter the correction found in (c) under the uncorrected thick- 
ness of the stratum found in (b), prefix a minus sign to it if 
the mean virtual temperature is below 0°C .Ifthe correction is 
negative, subtract it from the uncorrected thickness; if it is 
positive, add it to the uncorrected thickness. Place the sum or 
remainder below the correction found in(c) and above the sur- 
face altitude entered in accordance with (a). 
e. Enter the sum of the surface altitude andthe corrected thick- 
ness of the stratum in the altitude section of the constant 
= 1-69 FMH#3 
