Inland Northwest in the thick of peak fog season

Fog. The Inland Northwest is in the thick of it this time of year. November through January is peak season for this atmospheric marvel and 2016 is no exception.

Last month, Spokane International Airport recorded 20 days of varying fog conditions. On eight of those days were periods of fog heavy enough to reduce visibility to a quarter-mile or less.

That is to say, it was a dense blanket of gray known to obscure farmlands, buildings and highways from a distance of several blocks away.

Though fog “comes on little cat feet” as poet Carl Sandburg wrote in 1916, it can still barrel in like a tsunami made of mist. And while most of us associate it with coastal areas like San Francisco – the famous fog that often shrouds the Golden Gate Bridge is called Karl and even has a twitter account – the Inland Northwest gets its own version that appears less frequently and at a different time of year.

Regardless of where it is formed, fog is made up of condensed water droplets suspended in air, very much like a cloud that’s close to the ground. Instead of rolling in off the Pacific Ocean, the fog we get is mostly formed on land.

That’s because the fertile soils that make our region so agriculturally productive retain moisture after a rain or snowfall. Contributing to the moisture is the cool dense air that collects as water vapor on the valley floor.

Because we’re in a valley, a high pressure system can work like a lid to trap cooler air at ground level. Called a temperature inversion, it occurs when warmer air tops the cooler layer. During these inversion conditions, fog can form when both moisture and pollutants become trapped in the lower cold air. Stable conditions keep the air from rising, sometimes locking the fog in for days.

Fortunately, the heavy fog we saw in November was short-lived and did not cause vehicle pile-ups or major flight delays. Knowing it will probably return, keep in mind that the American Automobile Association warns that most fog-related accidents involve motorists who drive too fast.

Reduce your speed, avoid sudden stops, and increase following distance, the agency advises. Use the right edge of the road or painted road markings as a guide. And if you must pull off the road, do so as far as possible and make sure your hazard lights are turned on.

Nic Loyd is a meteorologist with Washington State University’s AgWeatherNet. Linda Weiford is a WSU news writer and weather geek. Contact: linda.weiford@wsu.edu or nicholas.loyd@wsu.edu.