Now that we’re past the start of fall, our nights are longer than our days, and we’re still losing daylight at a rapid pace of more than three minutes each day.
Adding to that perhaps-depressing fact, we’re quickly entering the Pacific Northwest’s rainy season. While fall brings vibrant colors to our trees, longer nights and increasing moisture often set the stage for foggy nights and mornings across the lowlands.
This is especially common right after a rainstorm as skies begin to clear behind a weather system. Once winds become light and clouds start to break apart, the warmth built up during the day can escape quickly into the atmosphere overnight. Meteorologists call this radiational cooling, but it simply means the ground and the air near it cool rapidly after sunset.
As temperatures drop, the air near the surface can reach its dew point. When there’s plenty of leftover moisture from recent rain, water vapor condenses into tiny droplets, creating fog.
During the fall and winter, when stronger storm systems are absent, that fog and low cloud cover can sometimes be stubborn across Western Washington.
We also frequently develop a temperature inversion, which means the air near the ground is colder than the air a few thousand feet above it. Think of it as a lid on the atmosphere.
The inversion can trap cool, moist air near the surface, making it much harder for fog and low clouds to clear during our shorter fall and winter days.
So as we head deeper into autumn, use extra caution if you’re traveling late at night or during the morning commute, especially after a soaking rain followed by clearing skies. That’s often the perfect recipe for fog to form quickly.
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