Thermal Index (How To)

To figure out the thermal index, use Foreflight:

Airport: KBRY –> Weather –> Winds Aloft.

It usually makes sense for us to use the 2 pm (1800Z) prediction.

In two columns, jot down Altitude and Forecast temperature in Cº.

Alt: Temp: 

9,000     -7ºC       

6,000     -4ºC

3,000     +2ºC

surface    -----

Then add a third column by writing down the maximum temperature expected that day at the surface 
(in this example, 58ºF = 14ºC)

Alt: Temp:

9,000     -7ºC       

6,000     -4ºC     

3,000     +2ºC     

surface    ------      14ºC

Then use the Dry Adiabatic Lapse Rate of 3ºC per 1000 feet (9ºC per 3000 ft) to figure the theoretical temperature that a 58ºF parcel of air will attain as it rises thru the atmosphere, cooling as it goes.   

Since our airport is 700ft MSL, not sea level, the first calculation will be 14ºC -  (2.3 * 3º) to make the 3000ft prediction.  

2.3*3º = about 7º that will be subtracted from the max temperature for the day.

So for the 3000 ft TI, 14º - 7º = 7º.)

Alt: Temp:       Parcel

                         temp:

9,000 -7ºC      

6,000 -4ºC      

3,000 +2ºC       7ºC

surface ----- 14ºC

For the rest of the layers, just subtract 9º from each lower level temperature.

Alt: Temp: Parcel

temp:

9,000   -7ºC        -11ºC

6,000 -4ºC       -2ºC

3,000 +2ºC         7ºC

surface ------      14ºC

Now, we make the fourth column, the "Thermal Index," by computing the difference between columns 3 and 4.

Alt: Temp Parcel TI:

temp

9,000 -7ºC      -11ºC     +4

6,000 -4ºC       -2ºC                 -2

3,000 +2ºC        7ºC              -5

surface ------      14ºC

A thermal index of  between -2 and -4 is necessary to get enough lift to carry a glider.  Any larger negative number is even better. So, the index indicates good lift at 3000ft, and perhaps up to 6000ft, but not higher.