Fog-free skies and warm temperatures were not enough Monday to purge the Rogue Valley of the stagnant air now packing so much particulate matter that wood-burning restrictions remained in place early today.

Fog-free skies and warm temperatures were not enough Monday to purge the Rogue Valley of the stagnant air now packing so much particulate matter that wood-burning restrictions remained in place early today.

Jackson County environmental-health officials Monday issued their fourth straight "red day" air-quality alert, banning all burning of wood except in certified stoves inside the valley's Air Quality Maintenance Area.

That ban remains in effect at least until 7 a.m. today, but Jackson County health officials suspected that it would be extended into early Wednesday based on National Weather Service reports that no inversion-busting front is expected before Wednesday at the earliest.

The wood-burning ban comes despite temperatures rising to 58 degrees and a dearth of the cold fog that gripped the valley all of last week.

Weather Service meteorologist Mike Johnson said that's because a high-pressure ridge remains over the valley, pushing the air down close to the surface in what meteorologists call "boundary weather."

The falling air concentrates everything in it — including the tiny particulate matter emitted with wood smoke.

A view from above the valley Monday revealed a low haze stretched over Medford. It was visible from higher elevations now partly because the air has become so dry that it is unable to produce the fog normally associated with inversions, Johnson said.

"The fog's gone but the inversion's still there," he said. "It's like a lid on the atmosphere, it's that you just can't see it."

Wednesday's front appeared likely to be too weak to purge the inversion, Johnson said. The next front will likely arrive Friday, and that one's stronger, he said.

"That one Friday looks like it has a good chance of scouring out this inversion," Johnson said.

Monday's high of 59 degrees, which came at 3:45 p.m., fell short of the record high for Jan. 19 of 63 degrees, set in 1981.

The forecast for Tuesday called for a near repeat of Monday, with a high forecast at 58 degrees.

The Rogue Valley floor has been in the grip of red-day burning restrictions since Friday. Burning in conventional fireplaces and non-certified woodstoves — those manufactured before 1985 — remained prohibited Monday.

Burning in certified woodstoves, which include a "certified" label, is permitted so long as the stove emits no visible smoke, according to the Jackson County Wood Burning Advisory.

However, health officials urged all residents with alternative heat sources to use them until the inversion lifts.

The county updates its woodstove advisory hotline, at 776-9000, daily at 7 a.m.

Reach reporter Mark Freeman at 776-4470, or e-mail mfreeman@mailtribune.com.