About

The fourteen rules the oracle uses to turn pressure, weather and live readings into a thermal forecast.

The Walchensee thermal is driven by slope heating on the south-facing flanks of Herzogstand (1,731 m) and Jochberg (1,565 m). As the slopes warm through the morning, rising air draws cool air from the Kochelsee basin (600 m) to the north — compressed through the saddle between the peaks by the Düseneffekt (funnel effect) into a consistent N to NNE wind. The formula: clear night → strong overnight cooling → unobstructed morning sun on the slopes → thermal wind by midday. The oracle checks 14 conditions that favour or block this mechanism.

Detailed background (meteorology, thresholds, calibration) in the technical model document on GitHub. docs/thermal-model.md

The 14 rules

• Pressure gradient (Munich − Innsbruck)
  Munich − Innsbruck pressure delta. ≥ −1.0 hPa drives the north-to-south thermal pump.
• Föhn (Bolzano − Innsbruck)
  Bolzano − Innsbruck pressure delta. ≥ 4 hPa signals Föhn — kills the local thermal.
• Overnight cooling
  Mean cloud cover 22:00 previous day to 06:00. Above 30 % weakens radiative cooling and the diurnal delta.
• Solar radiation
  Peak shortwave radiation 09:00–13:00. Below 600 W/m² the slopes don't heat enough for a clean thermal.
• Dew-point spread
  Smallest (T − Td) gap 09:00–13:00. Below 2.5 °C solar energy goes into evaporation, not sensible heating.
• Boundary-layer height
  Boundary-layer height 09:00–13:00. Below 600 m the thermal is capped; above 1000 m = deep mixing.
• Ground moisture / rain
  Wet ground (≥ 2 mm rain yesterday or soil moisture > 0.35 m³/m³) diverts solar energy to evaporation.
• Atmospheric stability
  Lifted index 09:00–13:00. LI ≥ +10 = atmosphere too stable (capped). Thunderstorm risk is judged by a separate classifier over several afternoon convective factors (CAPE, LI, shear, CIN …) — not LI alone.
• Daytime cloud cover
  Max low-cloud cover 09:00–13:00. Above 60 % shades the Herzogstand/Jochberg slopes and stops slope heating.
• No insolation
  Hard veto: heavy daytime cloud (≥ 70 %) AND low morning solar (≤ 400 W/m²) together — no sun, no slope heating, no thermal. Either alone is only a soft hint; the combination is decisive.
• Upper-level wind (850 / 700 hPa)
  850 hPa direction at the morning peak + 700 hPa crossflow. SSE 150–210° opposes the N-thermal; > 25 kt at 700 hPa decouples the valley.
• Synoptic-flow override
  Wind at 850 hPa (~1500 m) 09:00–13:00. Above 15 kt destroys or deforms the local thermal cell.
• Thermal ignition (live)
  Live readings from the Urfeld buoy + nearest DWD station. ≥ 8 kt mean wind = thermal ignited.
• Lake / air temperature
  Air−water temperature delta. Cold lake (air warmer than water by > 10 °C) opposes the spring thermal; warm lake aids it. Buoy water temperature + Open-Meteo air temperature.

Glossary

Short explanations of the technical terms that show up in the rules and the daily summary.

Thermal wind

A local wind driven by the sun: air warmed on the slopes rises and pulls cooler air in behind it. At Walchensee this produces the reliable north wind that surfers and kiters wait for.

Slope heating

The sun warming the sun-facing mountain slopes. It's the engine of the thermal — no sun on the slopes, no rising air.

Ignition (thermal ignition)

The moment the thermal wind kicks in and grows strong enough to ride (at Walchensee from about 8 knots mean wind).

Knots (kt)

The unit for wind speed. 1 knot ≈ 1.85 km/h. Rules of thumb: 8 kt ≈ 15 km/h, 11 kt ≈ 20 km/h.

Gust

A short-lived wind peak, clearly stronger than the mean wind. Matters for kit choice and how the water feels.

hPa (hectopascal)

The unit for air pressure. The forecast cares less about the absolute value than about the difference between two places.

Pressure gradient · Δ (delta)

The air-pressure difference between two places (e.g. Munich − Innsbruck). Air flows from high to low pressure — the right gradient pushes the north wind over the Alps to the lake. „Δ“ is the symbol for that difference.

Föhn

A warm, strong south wind that sinks across the main Alpine ridge. It brings wind but displaces and destroys the local thermal. Detected from the Bolzano − Innsbruck pressure difference.

Inversion

A layer where the air gets warmer with height instead of colder. It acts like a lid and stops warm air from rising — bad for the thermal. A clear, cold night builds the cooling needed near the ground.

Dew point · dew-point spread

The dew point is the temperature at which the air would be saturated. Its gap to the actual temperature (T − dew point) shows how dry the air is. A small gap = humid air: solar energy evaporates water instead of heating the air.

Solar radiation (W/m²)

How much solar power reaches the ground per square metre (watts per square metre). More radiation = stronger slope heating.

Soil moisture (m³/m³)

The water content of the soil (cubic metres of water per cubic metre of soil). Wet ground after rain heats up poorly, because solar energy goes into evaporation instead of warming.

Boundary layer (boundary-layer height, BLH)

The lowest, well-mixed layer of air above the ground. The higher it reaches, the more room the thermal has to grow. A shallow boundary layer „caps“ the wind.

Lifted index (LI)

A measure of how stable the atmosphere is. High positive values = very stable, the thermal is capped. Strongly negative values = unstable, thunderstorm risk. The sweet spot is in between.

CAPE

“Convective available potential energy” — the energy available for rising air and thunderstorms. Like the lifted index, a stability measure: lots of CAPE means strong updraughts but also thunderstorm risk.

Upper-level wind · 850 / 700 hPa

Heights are often given as pressure levels rather than metres: 850 hPa is roughly 1,500 m, 700 hPa roughly 3,000 m. The wind at these heights decides whether the large-scale weather disturbs the surface thermal.

Crossflow

Wind aloft that blows across the valley axis. Too much of it decouples the valley from the air above and disturbs a clean thermal.

Synoptic flow

The wind of the large-scale weather pattern (highs and lows), as opposed to the local thermal wind. When it's too strong it overrides the delicate thermal.

Peirce score

A skill measure for the forecast (sensitivity + specificity − 1). 0 = no better than guessing, 1 = perfect. More on the Stats page.