About Fog and Freezing Fog

After the forecasts for a colder and wetter December, we are now emerging from one of the most persistent stagnant air advisories in recent memory – cold and foggy, with little precipitation within a pretty interesting pattern of dense and off-and-on freezing fog. This stagnant air was held in place by an inversion – warm air sitting over cold air (the inverse of a normal atmosphere) – caused by air subsiding from that high pressure system sitting over us the last couple weeks. If air is subsiding from aloft, then the air at the surface is unable to lift and disperse – and fog which forms in that still, cold, air is unlikely to be moved out by incoming storm systems also blocked by that high pressure. So there we sat… still and quiet and cold.

Most of the whining I have heard (was some of that my own voice?) had to do with hunters attempting to scour the ground around Paradise and up on the Army’s Training Center for chukars and trouble-making cow elk. There were moments of white-out 20-foot visibility and iced-over roads and vehicles throughout our valley and across eastern Washington.

We will likely see the return of a number of foggy days before winter passes, so it will serve us well to understand the fogs of Paradise. As Staff Meteorologist for the Reecer Creek Rod, Gun, Working Dog & Outdoor Think Tank Benevolent Association, I offer the following primer.

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 droplets we see as cloud or fog. Fog is generally defined as “a visibility‑restricting suspension of tiny water droplets or ice crystals (roughly .001 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. A fog is simply a cloud too lazy to fly.

Each fog has its own beauty and its own story to share. We generally get one of four types: a radiation, or ground, fog; an upslope fog; an evaporation, or steam fog; or an advection fog.  Commonly, fogs morph from one type to another over time or distance.

Radiation, or ground, fog is our most common fog. Heat radiates away from the ground through a clear night sky, and, as the ground cools it in turn cools the air above it. When air cools below its “dew point,” moisture can no longer remain as a vapor and it condenses. The layers or “pockets” of fog are cooler and heavier than surrounding air, so they generally settle into low ground. These are the common fogs we see in canyons and low areas around Paradise on cool mornings. This is the type of fog which formed, and stayed, under the inversion layer kept in place by that high pressure aloft the last couple weeks.

An upslope fog will form if moist air is lifted. The resulting relatively constant rate of cooling will often result in condensation. Moisture in humid air moving up the Yakima River into the Kittitas Valley often condenses into an upslope fog.

Evaporation fogs are common in spring (and less so in fall). With bright sun in relatively cool temperatures, a wet surface will often evaporate quickly, creating a layer of very moist air – holding a high level of water vapor. When that moist air lifts even a foot or so into cooler air, condensation can be almost instantaneous, creating an evaporation fog. This is the early springtime fog which scurries across roadways when snow is melting: the sun-warmed pavement evaporates moisture which instantly becomes fog in the cold air just above the road.

An advection fog is simply one of the above fogs moving horizontally. When a radiation fog forms on the river and moves onto land it becomes an advection (referring to horizontal movement) fog.

The fogs with which we’ve been sharing our recent inversion (that stagnation advisory) have generally been radiation fogs.

Now, here’s how freezing fog happens. Given that the liquid droplets are so tiny, they may actually remain liquid to temperatures far below freezing – the droplets become supercooled. Most of our clouds, even in summertime, are supercooled – with temperatures below freezing – given the elevations at which they form. This time of year, our fogs are often comprised of supercooled water droplets 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 droplet after droplet bumps and freezes.

So, if you end up driving slowly, or must stop, because of fog, take a deep breath. Take a moment to think about how it developed. Recognize that it was here long before us, and will be a regular companion for the next few months.

If you must mutter at the fog, so be it. At least you can mutter at the correct kind of fog.

Happy winter.