Mt. St. Helens is a stratovolcano located in Washington, U.S.A erupted on the 18th May 1980. The eruption, classified as a VEI 5, produced an eruption column 24 km (15 miles) high and emitted 1.3 km3 of ash, depositing ash across the Pacific Northwest. One of the most damaging features of this eruption was due to a sector collapse on the northern side, producing a 2.3km3 landslide and resulting in a lateral blast eruption producing large pyroclastic flows. (illustrated by the sequence below)
The mean diameter of ash particles that fell to the ground downwind of Mt. St. Helens during an 8 hour long eruption on May 18, 1980 are shown in the below graphs. Peak wind velocity during the eruption varied between 80 and 140 kph (50 and 86 mph) as measured 400 km (249 miles) downwind of the volcano at approximately 12 km (7 mi) above sea level.