About Fog and Freezing Fog


The only subject on the agenda of Tuesday’s hastily-assembled meeting of the Reecer Creek Rod, Gun, Working Dog & Outdoor Think Tank Benevolent Association was the fog of Paradise.

Homeys took turns grumbling. I’d heard it all before. One or another piped up, “Look, it’s bad enough that the fog moves up off the river and I can’t see fifty feet down the road. Then all over the valley is the #$%?& ‘freezing fog’ stuff! I couldn’t even get the key into my car door this morning… So why does it do layer and layer of ice on windows and doors and stuff anyhow, huh?”

As a former TV weatherman, and staff meteorologist for our little think tank, the imprecision of their mutterings was getting under my skin. Herewith, my attempt to meet my clarification responsibilities to the association.

For a fog or a cloud to develop, air must be at or near saturation for water vapor (holding all the moisture it can hold). It must then cool enough to cause the water vapor to condense, forming the very tiny droplets we see as cloud or fog.

Fog is generally defined as “a visibility‑restricting suspension of tiny water droplets or ice crystals (roughly 1/1000th of an inch in diameter) in an air layer next to the ground.” By international convention, “fog” restricts visibility to 1000 meters (about 0.6 mile) or less. In reality, a fog is just a cloud too lazy to fly.

Each fog has its own beauty and its own story to tell. We generally get one of four types: a radiation (ground) fog; an upslope fog; an evaporation (steam) fog; or an advection fog. Often, fogs morph from one type to another over time or distance. Given that fog is a regular companion in this country, and at this time of year, I feel we owe it to ourselves to understand it.

Radiation, or ground, fog is our most common type. Heat radiates away from the ground through a clear night sky, and the ground cools. As the ground cools, it in turn cools the air above it until water vapor condenses. Since pockets of fog are cooler – and thus more dense and heavier – than surrounding air, they often settle into valleys. These are the common fogs in canyons and low areas on cool mornings.

If air is lifted, it cools at a relatively constant rate, again often resulting in condensation. Thus, moisture in humid air moving up the Yakima River into the valley may condense into an upslope fog.

At times, a water surface will evaporate quickly, creating very moist air. When that moist air moves into cooler air, condensation can be almost instantaneous, creating an evaporation fog. Many of the fogs we see developing over the river, or valley ponds, are evaporation fogs. This is also the very entertaining early springtime fog which scurries across roadways when snow is melting in spring; the sun-warmed pavement evaporates moisture which instantly becomes fog in the cold air above the road.

An evaporation fog which moves off the river, or from any other location, would be properly termed an advection fog (referring to its horizontal movement). If the fog bank is drawn up a canyon or hillside by air movement, it would become and upslope fog.

As are most of ours, the fogs on our agenda were radiation fogs. We do get the others, of course, and often an evaporation fog will drift off the river onto I-90 as an advection fog.

Given that the droplets are so tiny, they may remain liquid to temperatures far below freezing. Most of our clouds are supercooled – with temperatures well below freezing. This time of year, our fogs are often supercooled as well. When a supercooled water droplet is bumped, or touches an even colder surface, it instantly freezes.

On these cold mornings, then, water droplets touch the very cold roadway and freeze. The process is probably speeded up a bit by cars moving and swirling the air around, increasing the likelihood of contact and freezing. A similar thing happens as a supercooled fog surrounds a very cold car sitting overnight, and fog droplet after fog droplet settles or bumps and freezes.

So, if you end up driving slowly – or must stop – because of liquid or freezing fog, take a deep breath. Take a moment to think about how it came to be. Recognize that it was here long before us, and will be a regular companion over coming months.

If you must curse the fog, so be it. At least, please, curse the correct kind of fog.

Happy winter.


Written by Jim Huckabay. Posted in Uncategorized

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