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Saturday, July 4, 2020  Spokane, Washington  Est. May 19, 1883
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News >  Idaho Voices

Foggy weather means hazards in winter

Michelle Boss

You really have to get accustomed to much less sun this time of year. Aside from the fact we’re down to only about nine hours of daylight each day, those daylight hours are likely to be much cloudier.

For both Spokane and Coeur d’Alene, the months of November, December and January are the wettest months of the year. On days when it is not raining or snowing, the moist ground, low sun angle, and cooler temperatures will also lead to higher instances of fog in the area. Unless we have a dry northerly flow – it is easy to be fooled into anticipation with early morning sunshine. Stratus can quickly fill the sky; however, as early morning sunshine evaporates moisture from the ground and returns it to us in the form of low clouds and fog an hour later.

Speaking of fog, it is definitely a hazard that should not be taken lightly, especially when accompanied by sub-freezing temperatures. Though forecasts for dense fog don’t cause a rush on grocery or hardware stores like other winter storms do, reduced visibility and the potential for slick roads can cause many travel problems for those traveling by car or plane.

Under certain weather patterns, dense fog can also persist for days on end, and you can’t plow it away like snow. When visibility of a quarter-mile or less is expected over a widespread area, the National Weather Service will issue a “dense fog advisory.”

On average, Spokane (fog records are not kept for Coeur d’Alene) sees 101 days of fog each year. Heavy fog, in which visibility is a quarter-mile mile or less, is reported on 48 of those 101 days.

What is fog, exactly? It is nothing more than a cloud that has its base on or close to the ground. If you were out for a walk on a foggy day, you would literally have your head in the clouds. You might even get wet, as clouds are made up of millions of tiny water droplets.

The difference between clouds/fog and rain have to do with the size of the water droplets. It takes about a million average-sized cloud droplets to make an average sized raindrop, so cloud droplets are small enough to stay suspended in the air. The density of these droplets determines whether the fog will be heavy or light.

Fog dissipates when the relative humidity goes down. This occurs as drier air moves in, or as available sunlight warms the ground, which in turn warms the air above it. Water droplets evaporate – change from liquid water back to water vapor, which is invisible. Thus the fog begins to disappear, often from the ground up.

This is probably why people refer to fog as “lifting” though there is actually no physical lifting involved. Though it is often said that fog “burns off,” there is no burning involved either, just the warming of the ground and then the surrounding air by the sun.

Michelle Boss can be reached at weatherboss@comcast.net

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