The first major eruption of Pinatubo on June 12, 1991; view to the west from Clark Air Base. Pyroclastic flows (clouds of ash and steam) advanced 5-15 km down the north, northwest, and southwest flanks of the volcano. Most of the remaining personnel at Clark Air Base were evacuated along with thousands of people at nearby Angeles City. However, personnel returned to Clark Air Base within a few hours of the start of this first eruption, as winds had blown the volcanic ash south of the base.