VANCOUVER, Wash. -- Cold weather combined with the ongoing release of water vapor generated an impressive steam plume Tuesday at Mount St. Helens, which has been undergoing a low-key eruption since 2004.

"The warm, moist air rising from the lava dome condenses in the cold air and creates a cloud," said volcanologist Willie Scott at the Cascades Volcano Observatory in Vancouver, about 160 miles south of Seattle and 50 miles from the mountain.

Cold, still air Tuesday made it visible from farther away. The white plume emitting from the snowy peak could be seen clearly against a blue winter sky from downtown Portland.

Vapor has been rising from the volcano since before it rumbled back to life more than two years ago, extruding lava into the crater created when the mountain's top blew off in May 1980, flattening forests for miles and killing 57 people.