Augustine Volcano (1,282 m) erupts a relatively short eruption column directly from a summit vent as a pyroclastic flow sweeps down the volcano's north flank. The prevailing wind is blowing ash and gas southeast from the volcano to form an eruption cloud; surface wind is blowing ash across the ground from earlier pyroclatic flows.

Volcanic ash -- tiny pieces of glass, crystals, and rock ejected by exploding volcanoes -- blanket a much larger area and typically disrupt the lives of many more people than the more hazardous types of volcanic activity (for example, lahars and pyroclastic flows). Notification of an approaching eruption cloud is critical for preventing encounters between ash and aircraft downwind from an erupting volcano and for allowing people, communities, and industry to prepare for ashfall.

More about volcanic ash

• Tephra: volcanic rock and glass fragments
• Fact sheet: Volcanic Ash -- A "Hard Rain" of Abrasive Particles, (pdf)