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WEATHER U: Curious minds want to know reason for foggy windshields

Sections of the south shore of Nova Scotia's mainland are often shrouded in fog.  It was a very still morning at the coast when Lew Turner snapped this  beauty.
Sections of the south shore of Nova Scotia's mainland are often shrouded in fog. It was a very still morning at the coast when Lew Turner snapped this beauty. - Contributed

My grade 10 biology teacher once said: “There’s no better way to learn than to ask questions.”

I think that came about after a smarty-pants in the back of the class remarked: “Oh, Cindy must have a question.” I usually did.

Late last month John Curry emailed this great question to me:

“Saturday and Sunday mornings, I was at Exit 6 on the 103 at 8:45 on Saturday and 10:25 on Sunday. Very foggy both days. At Exit 6, suddenly, both mornings, the windshield misted up on the outside shutting down visibility nearly completely. Wipers on, problem solved. Outside mirrors misted as well and took a little longer to clear. Wipers on intermittent to Exit 7. What’s going on? Same phenomenon, same place, foggy two days running. Lots of moisture, suddenly warm to cold? Can you explain?”

The weekend of April 20 was very damp in that part of Nova Scotia: the relative humidity stayed above 90 per cent most of the weekend. The air mass was homogenous – meaning the temperature/dew point profile remained the same - unless you moved in closer to the water and added more moisture.

John’s trip started in Halifax around 8 a.m.; the temperature was 9.5 C and the dew point was 9.1; there was fog but the relative humidity wasn’t 100 per cent. As he made his way to Exit 6 on the highway running along the province's South Shore, he got closer to the water’s edge and that’s when the windshield “misted up.”

Exterior window condensation is simply dew that occurs when the window is colder than the dew point and there is lots of moisture in the air. As the highway meandered closer to the coast, John drove into an air mass that was saturated. Condensation occurred when the saturated air came into contact with the slightly cooler windshield.

At Exit 7 (Chester), Highway 103 veers off a little – pulling away from the coast, where the air was slightly drier – enough to clear the air -  as I hope this explanation did.



Cindy Day is the chief meteorologist for SaltWire Network.


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