Ash that’s disrupted air travel in northern Europe and Scandinavia demonstrates that volcanoes can be at least as menacing in the sky as they are on the ground.

In the Pacific Northwest, the Vancouver-based Cascades Volcano Observatory closely monitors volcanoes such as Mount St. Helens partly because of the hazard they present to aviation. U.S. Geological Survey geologist John Pallister noted that more than 100,000 airline passengers cross the Cascades daily.

“That’s a small- to medium-size city in the sky every day that is threatened by these kinds of events,” Pallister said.

On March 8, 2005, Mount St. Helens shot a plume to an altitude of 36,000 feet — easily within cruising distance of airliners passing by.

On Dec. 15, 1989, a Boeing 747 flew through what appeared to be a normal cloud during an eruption of Alaska’s Mount Redoubt. The silica-rich ash melted and then solidified within the hot compartments of the engine casings, stalling all four engines. Pilots managed to restart the engines and landed safely in Anchorage, narrowly avoiding a crash.

On-the-ground monitoring and satellite identification of ash clouds has improved since then, Pallister said, but they still present a hazard to aircraft.

“You can’t tell by looking whether a cloud at altitude contains ash or not, especially if you’re flying in an overcast,” he said. “Often these things are embedded in weather systems, so it’s not surprising these things are encountered.”

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