PREPARATION OF CHARTS B5-25 
curve from its condensation level to the 500 mb surface, and then sub- 
tracting algebraically this temperature from the true temperature of the 
air at the 500 mb surface as indicatedby the raob. The remainder, with 
appropriate algebraic sign, is the stability index. Positive values connote 
stable conditions, and negative values unstable. The degree of stability 
is indicated by the magnitude of the difference. 
The thickness of the layer of air used in the computation varies from 
station to station, because of the difference in altitude. Computations will 
begin ata base level appropriate to the altitude of the station and selected 
from Table B5-1. 
Table B5-1. Base Level Appropriate to Station Altitude 
Station Altitude 
(for raob purposes) Base Level 
Less than 1000 gpm 850 mb 
1000 to 1400 gpm 800 mb 
1401 to 2000 gpm 750 mb 
Tea Computations. Computation of the stability index will be as follows: 
a. 
SO 
Compute the dewpoint temperature (to the nearest tenth of 
degree) at the base level using the temperature and relative 
humidity values from the plotted curves vn WBAN-31A. Re- 
cord the dewpoint temperature on the right side of the form 
between the 50° line and the constant-pressure data blocks 
opposite the level. Draw a circle around the value. When the 
base level is the 850 mb surface, use the dewpoint tempera- 
ture recorded in the 850 mb constant pressure data block. An 
additional entry will not be required. 
On a pseudo-adiabatic chart, at a pressure corresponding to 
that of the base level, locate the dewpoint temperature found 
in (a) and the temperature (to the nearest tenth of degree) of 
the sounding at the same pressure. Project the dewpoint tem- 
perature up the mixing ratio curve which passes through it and 
project the temperature of the sounding up the dry adiabatic 
curve. Find the point of intersection of the two curves. Which 
is the level of lifting condensation. If the lifting condensation 
pressure is less than 500 mb, do not compute a stability in- 
dex but record it as "dry." 
FMH#3 
