Plain English Meaning

Tiny pieces of pulverized rock and glass blown high into the sky by a volcano. The pieces are sharp and gritty, and the cloud they form can travel a long way before settling.

Full Definition

A cloud of fine, hard, glassy particles and corrosive gases ejected into the atmosphere by an erupting volcano. The particles are abrasive rock and mineral fragments, often less than 2 millimeters across, that can remain suspended in the atmosphere for days and drift hundreds or thousands of miles from the source on prevailing winds.

Context Anchor

Seen in weather briefings, SIGMETs, instrument flying guidance, and flight planning near areas affected by volcanic activity.

Derivation

From Latin volcanus (the Roman god of fire) and Old English æsce (the residue left after burning). The word '' is misleading here -- it suggests soft, -like , but volcanic ash is actually finely shattered rock and glass.