The forecast is on the grass.
Stand on the airfield in the morning and you already hold the day's two most important numbers: the temperature the afternoon will reach, and the dew point — the temperature at which the air's moisture condenses. Their difference, the spread, is the single most predictive figure in soaring meteorology. A big spread promises high cloudbase or blue thermals; a small one promises early cumulus and, if it keeps shrinking, a sky that shades itself out.
Everything in this article is a way of squeezing those two numbers, plus one temperature aloft, for what they know. None of it replaces a proper forecast — but it tells you which forecast to believe, and it works on the back of an envelope.
The parcel argument.
A bubble of air leaving the hot ground cools as it rises and expands — almost exactly 1 °C per 100 m, the dry adiabatic rate, regardless of the weather. The air it climbs through cools more slowly with height, typically around 0.65 °C per 100 m. As long as the rising bubble stays warmer than its surroundings it keeps accelerating upward; the height where the two temperature lines cross is the lid on the day.
That is the entire theory of the thermal, and it explains the daily rhythm: every degree the surface warms lifts the crossing point higher. The morning inversion that holds everything down is not an obstacle to the day — it is the day, waiting for a trigger temperature.
Two thermometers forecast the afternoon.
Drag the forecast max down two degrees and watch an 1800 m day become a 1200 m day — the bust arrives before the first launch does. The red parcel line stops where it meets the black environment line: that crossing is the top of the lift, and expected climb strength scales with it (about 1.3 m/s here).
Trigger temperature is a start time.
The day does not begin when the sun rises; it begins when the surface reaches the temperature that lets parcels punch through the inversion to a useful height. That is the trigger temperature, and the tool prints it for you. Match it against the forecast temperature curve for the day and you have a launch time, not a guess — if the maximum only touches the trigger at 15:00, rigging at 10:00 bought you nothing but sunburn.
The same arithmetic explains the painful days when an over-forecast maximum never arrives: the trigger is never reached, the lid never lifts, and the day simply does not start. It was knowable at breakfast.
Blue, cumulus, or spread-out.
Whether the thermals carry clouds is a race between two heights. If the parcel reaches its condensation level — roughly 125 m for every degree of spread — before it reaches the environment crossing, each thermal is capped with a cumulus: honest markers, easy flying. If the crossing comes first, the day is blue: often strong, but unmarked, flown on instruments, faith and the birds.
The failure mode of a moist day is over-development: small spread, early condensation, clouds that grow, merge and shade the ground that feeds them. The engine throttles itself in the early afternoon, then the cycle may restart once the shadows clear. On such days the fast pilots fly early and treat 14:00 as the finish, not the start.
In your cockpit.
The outside air temperature probe makes the morning arithmetic testable while you wait on the grid: watch OAT climb toward the trigger you computed, and launch when the glider ahead of you stops relighting. In flight, the climb statistics page is the running verdict on the forecast — average climb and achieved tops, against what the sounding promised. When they disagree, believe the wing.
Once the day works, climbing well in it is craft of its own — that is the thermal centring article.
Calibrated outside air temperature and pressure — the live left edge of this chart, measured on your own wing.
Climb statistics that tell you mid-flight whether the day is meeting the forecast — or quietly missing it.
The map and altitude bands that turn a forecast top into a task range.