Thermal Index:  The TI is determined by subtracting the environmental profile temperature from the temperature of the dry-adiabat profile for each level of interest.  Most soaring literature indicates that a TI value of -3 reflects a good chance of sailplanes reaching the altitude of this temperature difference.  TI values of -8 to -10 generally indicate very good conditions.  TI values of 0 or greater are generally unfavorable for soaring.  The pressure level at which TI=0 is referred to as Hyd.  The RAOB software provides two options for deriving this altitude.

Mpi: for Mario Piccagli, whose equations were developed for use over U.S. mid-Atlantic states.

     ALT (ft agl) = 1580 + [0.57 * Hyd(ft agl)]

     LIFT(tpm) = 50.0 + [0.049 * Hyd(ft agl)]

Rpe:  for Russell Pearson, whose equations equations were developed for use over the southwestern U.S.

     ALT(ft agl) = 133.72 + [1.03 * Hyd(ft agl)]

     LIFT(fpm) = 41.49 + [0.07 * Hyd(ft agl)]

The above equations apply only to dry thermals, which are conditions where no clouds exist.  The effects of moisture on thermal development and cloud formation are discussed in WMO's Handbook of Meteorological Forecasting for Soaring Flight.  Although not employed in RAOB, this handbook presents several manual techniques for analyzing soaring potential with respect to atmospheric moisture.  One good indicator of low level moisture is the Convective Condensation Level (CCL), which typically identifies the height of cumuliform cloud bases, which are normally produced from surface heating and associated thermal activity.  Mario Piccagli (e.g. Mpi) developed a lift equation using the height of the CCL (CCLht), from which RAOB also displays resulting lift strength in fpm.  This equation is:

     LIFT(fpm) = -10.0 + [0.078 * CCLht(ft agl)]

Trigger Temperature: This is the surface temperature required to produce a dry-adiabatic lapse rate which will intersect the sounding at the altitude specified.

Soaring Index (SI):  Researchers have recently developed the Soaring Index which is designed to incorporate the vertical temperature gradient between the trigger temperature and the maximum altitude of thermal activity (Armstrong and Hill, 1976).  Like the above Pearson and Piccagli lift strength estimates, the Soaring Index also produces an estimated lift strength in feet-per-minute (fpm).

   Soaring Index = [3 * (Z/100)] + [10 * t]

            where:  Z = maximum thermal altitude (ft., agl)
                        t = (T : trigger temp.) - (T : maxthermal) degrees C

                        T : trigger temperature = sounding temperature at trigger altitude
                        T : maxthermal = sounding temperature at Z (max thermal altitude)
 
 

This sample (Forecast Sounding) is based on archived data from NOAA for Silvercreek Glider Club (St. Louis, MO), May 1, 2001 @ 1:00 pm (18z). 

Thermal Index (TI) is computed as an average of the Mpi and Rpe equations.