This is a side-on cross-section: altitude (0–16 km) on the vertical axis, downwind distance (0–600 km) on the horizontal axis, volcano at the left edge. Three processes move every ash particle, each drawn from a real dispersion-forecasting model rather than decoration:
- Column rise & plume height — the empirical Mastin-style power law
H ≈ 0.45·Q0.241 (H in km, mass eruption rate Q in kg/s, constant calibrated against the 1980 Mount St Helens eruption) sets how high the eruption column punches before it spreads into an umbrella cloud.
- Wind advection — once a particle reaches the umbrella, it drifts downwind at the wind-speed slider's rate; time is compressed roughly 300:1 so a multi-hour drift plays out in under two minutes.
- Stokes settling — terminal fall speed follows
v = (2/9)·r²·g·(ρ_ash−ρ_air)/μ for particle radius r, then passes through a soft drag cap (~8 m/s) because Stokes' law itself only holds at low particle Reynolds number — real ash lapilli above roughly 1 mm fall in a turbulent, not viscous, regime.
Fine ash (tens of µm) barely settles and rides the wind for hundreds of kilometres — the real reason distal ash clouds close airspace far from the volcano. Coarse ash (hundreds of µm–1 mm) falls out within tens of kilometres of the vent.
The colour field is an illustrative concentration index referenced to the ICAO ash-advisory chart's three bands: low (0.2–2 mg/m³, caution), medium (2–4 mg/m³) and high (>4 mg/m³, no-fly) — not a literal forecast product. The three horizontal lines are notional cruise flight levels (FL300/FL350/FL390); a level turns red wherever the concentration beneath it reaches the medium threshold.