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A rising eruption column decelerates as it entrains and heats surrounding air, becomes neutrally buoyant near height H ≈ 0.236·Q0.25 km, and spreads laterally into an umbrella cloud that is then carried downwind while individual ash grains settle out at their own Stokes/Newtonian terminal velocity.
A 3D volcano vents a convective column that rises, bends with altitude wind, spreads into an umbrella cloud, and drifts downwind as an elongating ash plume — coarse grains fall out near the vent, fine ash travels far.
Raise Column Intensity for a taller, more voluminous eruption. Increase Wind Speed or change Wind Direction to steer the downwind plume. Slide Particle Size toward fine ash for long-range dispersal, or toward lapilli for near-vent fallout.
The April 2010 Eyjafjallajökull eruption in Iceland was only a moderate (VEI 4) event, but its fine, glassy ash and persistent northerly-to-southerly winds shut down European airspace for six days.