Thermal index
The cloud base page says plainly what it cannot do: it gives the height a parcel would condense at, and says nothing about whether it gets there. This is the page that answers that, and it needs the one thing two surface readings cannot supply — a sounding.
The sounding below is invented. It belongs to no station on no day, and it has an inversion in it because a sounding without one teaches nothing this page exists to show. Replace it with your own.
Top of the usable lift
above sea level.
- Best index
- Trigger temperature
- °C
- Condensation level
The surface has not reached the trigger temperature yet, so nothing is rising at all. That is a different answer from “the thermals are weak”.
| Height | Sounding | Parcel | Index | Reads as |
|---|---|---|---|---|
| ° | ° |
TI = Tsounding − Tparcel
Tparcel(h) = Tsurface − 3.0 × (h − h0) ÷ 1000
- TI
- thermal index at that height
- Tsounding
- what the sounding says the air is doing there
- Tparcel
- a parcel lifted dry-adiabatically from the surface
- h0
- surface elevation
- 3.0
- the dry adiabatic lapse rate, °C per 1,000 ft
The sign reads backwards and this trips everybody up: a MORE NEGATIVE index is a BETTER day, because negative means the parcel is warmer than the air around it and therefore still climbing. Zero is where it runs out of buoyancy. That is the handbooks’ subtraction order and this page keeps it, because a forecast you read anywhere else will use the same sign.
Reading the answer
The convention is coarse and worth knowing: around −2 is a decent day, −3 or lower is a good one, and between 0 and −1 the lift exists but is weak enough that you will work for it. At 0 the parcel has no buoyancy left and that height is the ceiling on the day.
The trigger temperature is the same calculation run backwards: the surface temperature at which the dry adiabat first goes warmer than the sounding anywhere. Below it nothing rises at all. It is, in effect, the time of day the day starts — and on a morning where the forecast max sits barely above it, the flyable window will be short whatever the index says at its best.
Blue, or marked?
Give it a surface dew point as well and the two heights can be compared, which is the whole question a soaring forecast is trying to answer.
If the condensation level sits below the top of the lift, the rising air reaches its dew point while it is still buoyant and cumulus mark the thermals. If it sits above, the lift caps first, nothing condenses, and the day is blue — the thermals are there, and nothing is pointing at them.
That comparison is why cloud base alone is a misleading number. A forecast base of 7,000 ft over lift that stops at 4,500 does not describe a 7,000-foot day. It describes a blue 4,500-foot one.
Where to get a sounding
This page deliberately ships none. A sounding is a measurement of one place at one time, it expires within hours, and a stored one would be worse than useless. Morning ascents are published by national meteorological services, and every soaring forecast provider works from model soundings — the comparison of those is a separate page, with what each covers and what it costs.
Read the heights and temperatures off whichever you use and type them in. Six or seven levels through the working band is plenty; the inversion is what you are looking for, so make sure you have levels either side of it.
What this assumes, and what it can't know
The sounding is yours and this page stores none. No climatology, no stored ascents, no forecast of its own. It does arithmetic on the numbers you read off your own provider, and the worked example is invented rather than taken from anywhere real.
A dry parcel, lifted from the surface. The model is a parcel rising dry-adiabatically with no entrainment, no mixing, and no latent heat once it condenses. Above the condensation level a real parcel follows the saturated adiabat and can go a great deal further, which is how the same sounding produces both a pleasant cumulus day and a thunderstorm.
The index says how high, not how strong. Climb rate depends on the surface, the sun angle, the wind and what is triggering. A good index over a wet field on a windy day is still a poor day’s flying.
The forecast maximum is a forecast. Everything here hangs off it, and being two degrees out moves the top of the lift by hundreds of feet. Check it against what the temperature is actually doing.
Not yet checked by anyone but me. If you instruct, or one of these assumptions is wrong, I would genuinely rather hear it than not — tell me and I will credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.