Trigger temperature: the most useful number before breakfast
`thermal_trigger_temp_c` is the surface temperature that has to be reached before the first thermal of the day breaks the morning inversion. Here is how it is computed and why pilots should care.
On a summer morning, the atmosphere near the ground is capped by an inversion left over from the overnight cooling. Warm air from the sun-heated ground tries to rise, but the inversion blocks it until the surface gets hot enough that a parcel has enough buoyancy to punch through. That temperature, where the first thermal breaks out, is the trigger temperature.
`thermal_trigger_temp_c` is the model's forecast of that value, in degrees Celsius, for each grid cell and each forecast hour. Check it against the actual screen temperature and you know whether thermals have started. If the forecast trigger is 18 C and the current temperature is 15 C, thermals have not started yet. If it is 19 C and rising, they just have.
How it is computed. The model takes the early-morning vertical profile of temperature and moisture, lifts a surface parcel dry-adiabatically, and finds the temperature at which the lifted parcel can reach the inversion level without going negatively buoyant. That surface temperature is the trigger. Strictly it is the convective temperature T_c from classical boundary layer meteorology, but for soaring purposes the interpretation is the same.
A few things make it more useful than raw surface temperature. It accounts for the depth and strength of the overnight inversion, which is what actually decides when thermals start. Two days with the same forecast high temperature can have trigger temperatures 3 to 5 C apart if one has a strong inversion and the other has a weak one. Pilots feel this as days that 'fire up late' even though they were forecast warm.
It is also independent of site altitude in a useful way. The field is the surface temperature at that grid cell, so a 400 m hill will have a lower absolute trigger than a valley, reflecting the thinner layer of air that needs to be heated. If you only looked at flatland trigger temperatures and assumed they held for the ridges, you would get the timing wrong.
Limits. On days with strong overnight convection (showers, a front clearing through) the concept starts to break down. The 'inversion' is gone, the boundary layer is already mixed, and thermals start as soon as the sun starts heating the ground. On those days `thermal_trigger_temp_c` will often come out close to the dawn temperature and stops being a useful gate.
It is also a forecast of the trigger, not a guarantee that thermals will actually be strong once they trigger. On cold frontal days the trigger might break at 11 am but the thermals that follow are weak and blown out by gusty wind. Trigger tells you the timing, `wstar_ms` tells you the strength, and `day_rating` tries to combine both.
A practical pattern for a pilot-facing display: show the forecast trigger, the current observed temperature at the site's nearest METAR, and the gap. Live gap shrinking towards zero is the go signal. If the gap has been growing for two hours, the day is not going to happen and you can go back to bed.
Trigger temperature is one of the three fields that should not be shown in isolation. It lives with `wstar_ms` and `cloudbase_agl_ft` in any useful pre-flight summary. Full field documentation on the API reference.