Story highlights National Weather Service issues an advisory for areas around the volcanoes

An observatory reports ash plumes 20,000 feet above sea level over the Pavlof Volcano

That and another Alaska volcano remain on second highest alert status

An expert notes volcanic eruptions over Alaska can seriously affect air travel

An Alaska volcano exhibiting "elevated seismic activity" has spewed ash clouds skyward -- as high as 20,000 feet above sea level -- an observatory reported Wednesday.

As was the case a day earlier, the Pavlof Volcano was on "watch" status on Wednesday because of heightened activity, and it was also under an orange code that relates to how its rumblings might affect planes flying over its summit. Both these alert levels are the second most serious out of four options, according to the Alaska Volcano Observatory.

The same alert levels also continue to apply Wednesday to the Cleveland Volcano, which like Pavlof is in the Aleutian Island range southwest of mainland Alaska. Lava was reported flowing Tuesday at Pavlof and Cleveland.

"They are very similar, and both have the ability to erupt," John Power, a scientist from the U.S. Geological Survey, said this week.

Pavlof is the higher of the two volcanoes, reaching to 8,261 feet, and the one that is connected to the Alaskan mainland. The Alaska Volcano Observatory's Wednesday update noted persisting "elevated seismic activity" as well as reports of steam and ash clouds.

Web camera images show the plume "rising occasionally up to about 20,000 feet above sea level," the same height at which a pilot on Wednesday reported a "dark ash cloud" stretching east-northeast.

The volcano observatory also reported a "diffuse ash plume" on Tuesday night at an altitude of 15,000 feet and visible downwind for up to 100 miles.

Surface temperatures at the 5,676-foot high Cleveland Volcano remained elevated over the past 24 hours, the observatory said, though there were no explosions detected by infrasound data in that time.

The volcano observatory continues to warn that "sudden explosions of blocks and ash are possible with little or no warning."

The observatory said Cleveland could also produce ash clouds rising more than 20,000 feet above sea level. Beyond any impact on fish, wildlife and humans, large ash clouds could negatively impact air travel.

The National Weather Service has issued a SIGMET, short for significant meteorological information, in areas around the volcanoes.

After "eruptive activity" last year at the Cleveland Volcano, University of Alaska Fairbanks scientist Steve McNutt said 90% of air freight from Asia to Europe and North America flies over Alaska airspace, and hundreds of flights fly through Anchorage's air space daily.

"We think of the Aleutian Islands as being remote and desolate" Power said, "but when you come up to 30,000 feet we are talking about 20 to 30,000 people there every single day."

Power described Pavlof as "one of the most historically active volcanoes in the Northern Hemisphere." Cleveland is also "very active," having last had a large eruption in 2001.