Habitation Fog in November

FTLComm - Tisdale - Thursday, November 14, 2002
Water vapour in our atmosphere is the thing that sets our planet apart from other celestial bodies in our solar system. Not only does it produce and sustain life but it is also a constant source of pure wonder.

This morning the temperature in Tisdale was right around -21ºC and the amount of water held in the air was at the point (dew point) where the invisible water vapour would convert to visible ice crystals. that dew point was just around -20. As the morning sun rose the temperature crossed that line and in just moments the town was caught in fog. This particular form of fog is often referred to as habitation fog as it is water vapour being put into the air by furnace exhaust, cars, sewer lines, all the components of a town site.

With only a slight breeze this damp air is moved away and unable to form what is essentially a cloud but this morning the wind was just a slight movement (less than three knots) and that water vapour escaping into the still atmosphere just as the temperature reached that critical point resulted in a rapidly visible fog.

Weather men sometimes refer to this situation as "ice fog" though technically correct it hardly describes the actual condition. All of the Northern hemisphere when things get very cold and the wind is still there is a possibility of a fog condition coming from the activity of human life and hence the term "habitation fog."

This morning the condition similar to what happened in Tisdale occurred a bit earlier in Saskatoon as the temperature passed through the critical "dew point" mark and the Saskatoon quite correctly called it "ice crystals". Though some of that water vapour in Saskatoon was the result of humans that city also has a lot of open water with the river running through the centre of the place and so the resulting fog would not be called "habitation fog."

Some places are far more prone to experiencing this condition than others. Whitehorse, Yukon is surrounded by mountains, a large lake to the North and the river running through the city. These conditions can create an inversion were warm and often very dirty smog filled air can be trapped over the city and held in place by a cap of very cold air.

We normally associate habitation fog with arctic conditions but this fall the quasi-stationary arctic front that divides the frigid arctic air from the temperate moderate warm air has slipped surprisingly low across the province so that Yorkton to Prince Albert were within the arctic zone while modest temperatures of -10 and warmer were experienced South of that line.

If you check the
weather log you will discover that in the last few years such conditions are very unusual.

Oh yes, getting back to the phenomena this morning. By 9:30 the flirt with fog was gone the temperature had moved upward and the the condition disappeared as quickly as it had appeared.

Timothy W. Shire



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