Boise has experienced 29 days with fog since the new year started, including 14 days when visibility was a quarter mile or less, according to the National Weather Service.

Why so foggy?

First, there's a lot of moisture in the Valley from snow, sleet and rain over the past several weeks. The air is saturated, said Boise-based meteorologist Steve Parker.

Second, cold air can't hold as much moisture as warm air. So when temperatures drop, moisture condenses into the cloud droplets that forms fog. Parker said inversion conditions have set up again — that means cold air is trapped near the Valley floor.

Dense fog rolled in Saturday night after the sun went down, with visibility dropping below a quarter mile. The airport was shut down for four hours, after a small plane went off the runway. The airport reopened about 11:15 p.m. Saturday.

A couple of semis slid on ice on US 95 at milepost 15 near Marsing just before 8:30 a.m. Sunday morning. An Idaho State Police trooper who responded to the scene said visibility was down to about 50 feet, an ISP dispatcher said.

Police say Juan Iniguez, 44, of Madera, Calif., was traveling north in a 2013 Kenworth semi truck. The vehicle jack-knifed, blocking both the north and south lanes of travel. Neither Iniquez or his passenger, Pablin Bravo-Torres, 45, of Fowler, Calif., were injured.

The highway was shut down for about three hours, while the crash was cleared from the road.

The visibility at the Boise Airport at 11:15 a.m. Sunday was 2 miles, but expected to improve in the afternoon.

Highs this week will be in the low-40s, with overnight lows in the 20s. Mornings will be foggy, but sun is expected to burn it off.

An air mass expected late Thursday may be strong enough to blow away the stagnant air trapped by the inversion — and it may bring some more snow, though not much, Parker said.

Katy Moeller: 377-6413