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Basking in an Alaskan afterglow

Eruption causes longer Manitoba sunsets

By: Lindsey Wiebe

Posted: 04/15/2009 1:00 AM | Comments: 0 (including replies)

IF you've noticed more vivid sunsets in the last week or so, you have a volcanic eruption in Alaska to thank.

University of Winnipeg geography professor Bill Buhay says regular eruptions of Alaska's Mount Redoubt since mid-March have resulted in fine dust particles spewing high into the atmosphere, producing longer-lasting, wider sunsets than normal.

"We get what's called an afterglow," he said.

The brilliant reds and orange of a sunset come from the way sunlight is scattered by particles in the air. Mount Redoubt's eruption released aerosols, sulphuric acid and tiny particles of dust into the stratosphere, a layer of the atmosphere 20 to 30 kilometres above the earth's surface.

The presence of those fine particles so high in the sky means the colours can linger longer after the sun has already set, a phenomenon known as an afterglow.

"You need a volcano that has the energy to inject that fine aerosol and those particles into the high atmosphere," Buhay said. "Normally the mixing happens in the lower atmosphere."

Buhay said it's important for the particles to be smaller than the wavelengths of light for the light to scatter. Once those particles are in the stratosphere, "then you've got this potential for lots of light scattering when the sun goes down, and some beautiful sunsets," he said.

Mount Redoubt is located in Alaska's Aleutian Range, roughly 180 kilometre southwest of Anchorage.

Buhay said it's hard to say how long the effect might last, but he suspects Manitobans might see bigger and brighter sunsets for at least a few more days.

lindsey.wiebe@freepress.mb.ca

Republished from the Winnipeg Free Press print edition April 15, 2009 A2

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