Weather Words: Advection Fog

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Not all fog is created equal! While you may think fog needs a calm, clear night to take shape, that’s not always the case. Advection fog is a bit more of a rule-breaker. It doesn’t mind a little wind or even cloudy skies. This type of fog forms when warm, moisture-rich air moves over a colder surface. This can happen when a mild air mass drifts over an area covered in snow or when humid, tropical air slides over chilly ocean waters. The cold ground or water cools the air above, dropping the temperature near the dew point, which forms a thick, low-hanging fog.

Another interesting tidbit about advection fog. It doesn’t just sit still. Since it forms when winds are present, it often moves with the breeze. You may find it rolling across land and sea like a wandering ghost. On the U.S. West Coast, where it's most common, it can linger for days, swallowing up coastal highways and making harbors disappear. At sea, it’s also known as sea fog and is notorious for cutting visibility to near zero, making navigating the waters tricky for boaters. And when the wind carries that fog inland, coastal towns can find themselves wrapped in an eerie, haze that can overstay its welcome far longer than your typical morning fog.

J​ennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.