Fog season is among us; what to expect and how it works


It wouldn't be a fall or winter month without some eerie scenes or foggy valleys

PORTLAND, Ore. (KOIN) – It’s that time again, where fog is mixing into our nightly and morning commutes. It wouldn’t be a fall or winter month without some eerie scenes or foggy valleys.

Some of the photos below show a wide variety of fog across familiar PNW locations.

Why don’t we start off with what type of fog we typically see here in the PNW? Most of the time we usually have radiational fog or ground fog. The ground fog process will usually begin after a recent rainfall that creates a damp and moist ground. Then at night when the temperature starts to cool, it will reach the dewpoint (which is usually fairly high because of the recent rain). Once the dewpoint and the temperature reach similar values, the fog will develop.

Fog is essentially a cloud near the surface. What you can think of as water droplets that are suspended near the ground that will ultimately reduce visibility, consequently creating a hard time while you drive.

Let’s take a look at some of the stats that are common for Portland, in regards to fog. October through February you could consider our busiest months for fog.

Although, we typically have about six fog events in October, this month, we’ve already tallied 11. However, we haven’t had any significant dense fog events at this time. Meaning mainly just light fog and nothing that would be equal or less than 1/4 mile visibility. Now it’s common that we may have eight to 10 days in a row of fog or even three to four days of dense fog, but it has been more scattered this month.

Interested in why it’s better to use your low beam lights while driving through the fog? The suspended water droplets that are near the surface will reflect the light back, creating a tough environment. Using the low beam or fog lights will focus the light under those water droplets and it will light up the road. So it’s all about the angle of that beam of light. Your headlights or brights are higher on your car and brighter which will directly hit those droplets and reflect. Those fog lights are lower on your car and closer to the ground.

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