Figure 4: Schematic model showing hail-like growth of ash aggregates during explosive wet volcanism.

Shaded colours indicate the dominant water phase coexisting and interacting with the airborne volcanic ash and gases. (i) Rapid coalescence of wet ash occurs in the presence of liquid water, within the warm updraft(s) of the rising, turbulent plume. (ii) Freezing takes place in the colder margins extending above the −20 °C isotherm of the background atmosphere (dotted line), which varies locally depending on geographical location and season. Fallout and turbulent re-entrainment may recycle frozen aggregates back into the warm core of the volcanic plume, leading to additional stages of wet growth and layering of ash particles. (iii) Weakening updrafts at the end of eruption lead to gravitational fallout of the larger aggregates, leaving behind a dilute cloud of fine-grained ash particles not detected by weather radar.