This week we’re responding to a listener who asked: What the fog? Why has there been so much fog in Anchorage this winter?

Anchorage has had a record amount of dense fog this winter. The main culprit is a dramatic temperature inversion between the mountains and the city. On average, it’s been nine degrees warmer on the upper hillside than in town.

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Since Dec. 1, Anchorage has seen 26 days of fog with visibility of a quarter mile or less. Climatologist Brian Brettschneider said you have to go back to 1950 to find a winter that comes even close to the number of days with dense fog.

“Looking back through the records, all the way back to 1925, there’s no other winter that has this many days of dense fog,” Brettschneider said. “There have been winters with more days of fog that maybe wasn’t as dense, particularly 1950, but no winter has been this close in terms of the number of dense fog days.”

There are a couple of things at play, but Brettschneider attributed the fog to strong temperature inversions, where it’s warmer a few thousand feet in the mountains than it is on the valley floor.

“If you look back at weather balloon soundings, you have to go back to 1950 to find a year that has a bigger temperature inversion than 2017 so far,” Brettschneider said. “When you have a situation like that, the air doesn’t have any buoyancy, it can’t rise. So if you have moisture trapped at the surface that has nowhere to go. You need the cool air aloft or you need some wind to push it out, and we’ve had neither.”

That trapped moisture comes from Cook Inlet. The snow is contributing too.

“We don’t think about snow evaporating, but when we do get snow, some of it evaporates into the air and we do get moisture. So we have this confluence of moisture sources and then we also have high mountains that really trap the moisture in place,” Brettschneider said.

So as winter warms up with more sunlight, will the fog start to go away?

“You might have noticed in the daytime now there’s less fog. So with the higher sun angle, the sun’s energy is able to warm the surface up and when you warm the air, it’s able to hold more moisture, then you don’t see the fog,” Brettschneider said. “As it cools back down at night, the ability of the air to hold moisture decreases and it saturates.”

So when the sun gets higher in the sky, there is less fog during the day. In December and January, the short days can make fog stick around all day.

Brian Brettschneider is a climatologist in Anchorage who closely tracks Alaska climate data and trends. Alaska’s Energy Desk is checking in with him regularly as part of the segment, Ask a Climatologist. Do you have a question for Brian? Go ahead! Ask him.