I think I’m safe to say that here in Atlantic Canada, everyone has experienced fog. It can approach softly on little cat feet, or quickly barrel across the region. Although there are different types of fog, essentially fog is a cloud whose base is touching the ground and reduces horizontal visibility to less than one kilometre.
The most common type of fog in autumn is advection fog. There are two basic requirements for advection fog to form: there must be moisture in the air and cool air close to the ground. The relative humidity of the moist air mass is important and the closer to 100 per cent, the better. As for the air near the ground, it must be cooling to within 3 C of the dew point. If you think back to the weather unit in school, you might remember that the dew point is the temperature to which air must be cooled for water vapour in the air to condense to a liquid. When the air near the ground cools to dew point, the water vapour in the air will become visible as either dew or fog.
Advection fog is quite common in coastal areas and forms as relatively warm, moist air moves over cooler surfaces. Surfaces can be either land or water, each cooler than the warm and more humid air moving horizontally above it. While I’m not sure that I’m going for a swim any time soon, the large bodies of water in our region are relatively warm and some days, quite a bit warmer that the air driven by a cold northwest wind.
Speaking of wind, it often accompanies this type of fog and it’s usually a land breeze… slithering in like a long arm reaching onshore. By the way, one of my favourite literary terms is pathetic fallacy; it’s used when the weather fits the mood of a scene. Advection fog is perfect for that scary movie.
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Cindy Day is the chief meteorologist for SaltWire Network.