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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Fog occurrence common in winter

Michelle Boss Correspondent

In addition to snow and cold temperatures, fog is another common occurrence during the cool season.

Fog is just a cloud that reaches the ground, so on a foggy day, it is really possible to have “your head in the clouds.” A cloud forms when water vapor (the invisible gaseous form of water) cools and condenses into a water droplet (changing from a gas back to a liquid). To get fog, a sufficient amount of water vapor near the ground and temperatures cool enough to condense that water into droplets are needed.

You might wonder why the water droplets do not immediately fall as rain. It’s a size issue; cloud droplets are very small. It takes about 1 million average-size cloud droplets to make an average-size raindrop, so cloud droplets are small enough to stay suspended in the air.

The sequence of events that often leads to fog during the cool season goes like this: snow falls, snow melts and evaporates, and fog forms. A storm system first brings snow into the area. In the wake of the storm, high pressure moves in bringing a brief period of sunshine which starts to melt and evaporate some of the snow. This process puts a lot of water vapor into the air just above the ground. At night, as temperatures cool, the water vapor condenses into water droplets and fog forms. Contrary to what is often said, fog does not technically ever “lift” or “burn off.” During the day, what sun does penetrate through the fog warms the ground, which in turn warms the air above it. The warmer air begins to evaporate the fog from the bottom up. As the fog thins, more sunshine comes through, warming the ground further and continuing the fog evaporation. If winds are light, however, and the air mass remains stagnant in the absence of new storm systems, nighttime cooling will lead to the formation of fog again. The fog that forms under these conditions is called “radiation fog.”

Fog is a hazard throughout the year because it reduces visibility for both automobile and air traffic. In the winter months, subfreezing temperatures create added danger when fog freezes onto surfaces such as roads and sidewalks, making them very slick.

Though fog occurs all across the Inland Northwest, it is more persistent in valleys or low spots where the coldest air tends to pool. These low spots may retain fog after other areas have dissipated, hence the term “patchy fog.” Heavy fog, a situation when visibilities are reduced to a quarter-mile or less, is most common in December with January being the next foggiest month. In Spokane, the nearest city where fog records are kept, there are on average 101 days with fog per year, with 48 days of heavy fog annually. The foggiest location in the United States is not too far away, at Cape Disappointment, Wash., which averages more than 100 days of heavy fog each year. The fog produced on the coast is called “advection fog” and occurs when warm, moist air from the Pacific Ocean moves over the cold California Current.

During the winter months, dense fog episodes can last for days. The thick fog limits solar heating during the day and temperatures may only rise 1 or 2 degrees. Cool temperatures and weak air movement perpetuate the foggy conditions, until the next storm system comes through to stir up the air.

With or without fog, we are climatologically in the coolest part of the year. Average temperatures will start their upward climb by the second week of January.