October ushers in the season of fog in the Puget Sound region

This month is about spooky ghosts and goblins — and fog. Fog can occur throughout the year, but October is the number one month of the year for fog. In fact, Olympia ranks as one of the foggiest places on the globe, with October at the top of the list of foggiest months.

As autumn progresses with longer nights and shorter days, fog becomes more prevalent this time of year. During weather patterns with clear skies and light winds, such as that coming up heading into this weekend, temperature inversions become established, and fog forms – often with poor visibility.

A temperature inversion is when the air aloft is warmer but cooler near the surface, such as in valleys. An analogy would be a mountain lake during summer. The water near the surface is warm, but if you jump in, it is much colder just a few feet below. This same temperature profile in an inversion occurs on a grand scale in the atmosphere.

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A temperature inversion can occur any time of year in western Washington but is most frequent in the fall and winter seasons. Cooler air is heavier and tends to slide into low-lying areas like valleys. It is quite common for temperatures in the valleys to be quite cool, while up at higher elevations, such as in Clearview or North Bend, it is much warmer. That is a temperature inversion, and fog forms under the inversion.

Last week’s plentiful rainfall provided a source of moisture from the wet ground for fog to develop during the night and into the morning hours. As each day gets shorter through autumn, the sun’s ability to dissipate the morning fog becomes more difficult as the sun’s angle gets lower in the sky. At some point soon, fog can persist throughout the day.

There are times when the fog becomes quite thick, creating hazards from poor visibility. For instance, during these time periods, motorists should slow down, maintain safe following distances, and watch more intently for pedestrians and cyclists.

Strong high-pressure aloft weather patterns during the fall and winter seasons often result in steep temperature inversions. The air well aloft will have freezing levels perhaps above 10,000 feet, while temperatures in the Puget Sound region struggle above the freezing mark. Now that – is a strong temperature inversion!

In December 1985, this kind of weather pattern created the last significant long-winded dense fog episode. Thick fog persisted for several weeks, and some suggested particles from less regulated in that era wood smoke played a key role.

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The fog was so thick that air operations at SeaTac Airport were suspended for many days, significantly disrupting holiday flight plans. Flights into or out of SeaTac were detoured to Portland, where the east wind blowing out of the Columbia Gorge kept that airport wide open but left them with too many airplanes to park.

On the evening of Christmas Eve, though, enough east winds from the Cascade passes slowly eroded the fog to create holes in the blanket, allowing SeaTac Airport to reopen just in time for Santa to make his rounds.

With Halloween just around the corner and this month being number one for fog in the year, perhaps it is time to roll out the 1980 thriller film – The Fog.

Follow Ted Buehner, the KIRO FM news meteorologist on Twitter