WINDROSE

WINDROSE is competition ballooning — the real FAI sport, flown at dawn for a reason. Three judge-declared goals wait across a river valley, each a canvas X in a field. Fly to each one, drop a weighted marker, and every metre between marker and centre goes on your card. Land gently near the final goal and that distance counts too. Four numbers, one sum. Lower wins. Gold is 140 m or under.

The catch: a balloon has no steering. The sky over the valley is six wind layers stacked like conveyor belts — surface drainage creeping down-river, a cross-valley westerly at three hundred feet, a hard northerly higher still, all of them slowly veering as the morning warms. You choose only your altitude. The wind chooses your course. And the course is a closed box, Albuquerque-style: ride the low layers out southwest, then climb and come home north on the gradient wind.

How to play — three keys:

• W / ↑ — hold to burn. The heat becomes a climb about fifteen seconds later; a good pilot burns before the sink.
• S / ↓ — hold to vent hot air and descend.
• Space / E — drop your marker. Drop from 500 m and the wind takes it 100+ metres on the way down; low and slow is accurate.

Watch the wind stack instrument on the right — it is the whole game. Read the streamers, the chimney smoke, the other balloons. Mind the power lines that cross the valley twice, touch down under 4 m/s, and don't run the propane dry. Press H or ? any time to see the controls.

Under the hood is a real thermal simulation at a fixed 1/120 s step: a 2,800 m³ envelope with ideal-gas buoyancy against a dawn temperature inversion (the reason real competitions launch at dawn — it is what makes altitude-holding stable), a 2.1 MW burner fighting fabric heat loss, 1,600 kg of entrained-air added mass making the lag honest, and a basket swinging on a true pendulum 14.5 m below the envelope every time you cross a shear. Markers fall as drag projectiles sampling every wind layer between you and the ground.

How this drop was made: the concept, course and words are Claude Fable 5's; the entire simulation and rendering were built by a Claude Opus 5 agent as one self-contained HTML file — three.js vendored locally, every sound synthesized from nothing in WebAudio. The medal cutoffs were measured, not guessed: 70 headless autopilot flights across five skill tiers flew the byte-identical physics core, and the medals sit where they landed.