| Thermal Index - How much thermal activity can you expect today? |
What is a thermal index?The thermal index ("TI") at a given altitude is the difference between the actual air temperature and the temperature that a parcel of air would have if it started at the surface and rose adiabatically (as it does in a thermal) to that altitude. Negative values mean that the air parcel is at a higher temperature than the surrounding air, and therefore the air will continue to rise. This would also indicate that there will be thermal activity. The more negative value of the TI, the more thermal activity you can expect. The altitude for which the TI reaches zero can be used as an approximation for the maximum height of thermals for the day.How to use the Thermal Index forecast calculator below if your are in Colorado Springs: 1. Select "Complete Report" 2. Select "Denver, CO" for the RAOB Station 3. Seclect "COS 55" for "From a city forecast" (COS is Colorado Springs) 4. Leave "Enter your own number" blank 5. Select "10000ft MSL" Click on "Submit Request" button What does it all mean? |
| In the first example below, all the numbers are
above zero. It is from an overcast day and shows that there
is likely to be very little thermal activity. You will probably be
able to fly throughout the entire day launching from 6000 ft. MSL.
The trigger temperature is the ground temperature which will produce a
TI value of -3 (a thermal strong enough to lift a glider at that
altitude). The forecast high for this day was 64 degrees and all
the trigger temperatures are above that, so you can see why thermal
activity would be low to none.
In the second example below, the expected high is 55 degrees. The trigger temperature at 6000 ft. is only 52. A difference of 3 degrees. Enough to allow an envelope of air to rise and (because the TI is almost a -3) you can expect fairly strong thermal activity. Remember, to get a quick idea of what thermal conditions are likely to be for the day run the TI forecast , note your forecast high, and see if the trigger temperature for your altitude is lower than your forecast high. The more difference, the more thermal activity you can expect. Just a side note: Where the TI is zero; this is the altitude a thermal can be expected to rise to. Bubba Forecast high taken from COS MSL *TI* Wdir@kts trig VirT 2.1 degrees/division ("`": Dry Adiabatic)
----- ---- -------- ---- . ---- ----------------------------------------- 12000 12.3 20 35 86 | 25.9 ` : 11500 11.8 85 | 27.6 ` : 11000 10.9 20 38 83 | 28.6 ` : 10500 9.9 82 | 29.6 ` : 10000 9.1 25 35 80 | 30.7 ` : 9500 8.6 79 | 32.5 ` : 9000 7.4 35 32 77 | 33.1 ` : 8500 5.8 74 | 32.9 ` : 8000 4.2 45 25 72 | 32.7 ` : 7500 2.8 69 | 32.8 ` : 7000 2.3 60 15 68 | 34.6 ` : 6500 1.8 67 | 36.4 ` : Forecast high taken from COS 25-NOV-2005 12 UTC Soaring report from DNR upper air data. Forecast high: 55 F; est. base of any clouds:10400 feet AGL. === Interpolations (temps in deg. F, altitudes in feet MSL) === MSL *TI* Wdir@kts trig VirT 0.6 degrees/division ("`": Dry Adiabatic)
----- ---- -------- ---- . ---- ----------------------------------------- 10000 1.3 350 7 57 | 32.5 ` : 9500 1.1 57 | 34.8 ` : 9000 0.9 355 7 56 | 37.1 ` : 8500 0.6 56 | 39.4 ` : 8000 0.3 345 8 55 | 41.4 `: 7500 -0.4 55 | 42.9 : ` 7000 -1.0 310 10 54 | 44.4 : ` 6500 -1.7 53 | 45.9 : ` 6000 -2.4 280 12 52 | 47.3 : ` |