This weekend, (August 18th 2012) marks a slightly ominous anniversary in Anchorage history.

It marks the 20th anniversary of the eruption of Mount Spurr.

Spurr actually blew its top 3 times that summer. But it was the late August eruption that showered Anchorage with substantial amounts of ash.

That night in the city, drivers struggled to keep their windshields clean, while airlines temporarily gave up the struggle to get people to and from their destinations. Anchorage International Airport was forced to shut-down for 20 hours, in order to eliminate the risk that jets might be forced to fly through engine-clogging clouds of volcanic ash.

The of the big Spurr eruption day was somewhat overcast, and so -- to some people -- it seemed puzzling why the sky had suddenly grown dark.

On that late afternoon, Alaska Volcano Observatory Volcanologist Game McGimsey saw first-hand the reason for the late-afternoon darkness.

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As soon as he got word that Spurr was acting up, he hired a plane to photograph the eruption -- so scientists could learn as much as possible from it.

"Here I found myself -- in an airplane that day -- on the upwind side of an erupting volcano", McGimsey reminisced, today (Friday). Spurr, he continued, "was putting an eruption column up to 46-thousand feet. And it was just, it was just phenomenal," he said.

Also phenomenal were some of the science returns from that day.

A.V.O Geophysicist Dave Schneider had red a paper from an Australian Scientist theorizing how satellite photos might be used to track dangerous ash clouds carried at high altitude by the winds.

The need to do so was considered urgent for aviation safety. The reason was that just 3 years before, A K-L-M 747 dropped thousands of feet when it had inadvertently flew through the ash cloud of Mount Redoubt -- and all four of its engines were snuffed out by the ash. The pilot managed to re-fire the engines, and brought the jet -- and its passengers -- in for a safe landing, but it had been a close call.

Schneider worked with other scientists to see whether overhead satellites could differentiate water-vapor clouds (the puffy kind that produce rain) from volcanic ash clouds. By playing with the temperature-senstitivity of infrared images from orbiting satellites, Schneider, working alongside many other interdisciplinary scientists, found that dangerous ash clouds could, indeed, be tracked and forecast.

The work begun in Anchorage that summer, by the A.V.O and other scientists laid the foundation for techniques for tracking volcanic ash clouds -- so jetliners could be steered away from them.

Of the 169 active volcanoes in the United States, dozens are located here in Alaska. The Alaska Volcano Observatory is one of five such observatories in the country trying to minimize the risks posed by erupting volcanoes.