Whether it approaches softly on little cat feet, or rolls in like a spooky movie, everyone has experienced fog.  Although there are different types of fog, essentially fog is a cloud on the ground which reduces visibility to less than 1km. The basic requirements for fog to form are moisture in the air – the closer to 100% humidity the better, and the air near the ground must cool to within 3 C of the dew point – this is the temperature to which air must be cooled in order for water vapor in the air to condense to liquid.  When the air near the ground cools to the dew point, the water vapor in the air will become visible as fog in the air or dew on the ground.

A temperature difference between the cold, snow covered ground and the air above was responsible for this morning's fog -  advection fog.  Advection fog 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. This type of fog will often appear to roll into an area and have a forward movement.

The southerly winds behind a warm front to the north, are pulling warm moist air over our region.  We can expect this to persist until late Friday or Saturday morning.  At that time, a cold front will push through, with a wind shift and a cooler brand of air.

I’ll have all your weekend forecast details tonight on CTV News.



Chief Meteorologist

Cindy Day