GRANDMA SAYS: Fun facts about winter fog

WEATHER BY DAY

I'm a lot like Grandma: she loved winter and she liked it to stay cold until spring.

Grandma, however, was not a fan of the “crazy ups and downs of winter,” as she called it.

When the temperatures inevitably started to climb, a fog would spread across the land in Glengarry County, Ontario. Soon, we would hear Grandma reciting one of her favourite weather sayings: "A summer fog for fair, a winter fog for rain, a fact most everywhere, in valley or on plain.”

Fog forms when the air cools enough that the water vapour encourages condensation over evaporation. Cool summer nights are often the result of clear skies and a light wind; these allow excess heat to radiate into space. Cloudy skies act like a blanket, absorbing and re-radiating the heat, keeping it in. If it is cool enough and clear enough for fog to form, it will probably be clear the next day.

Winter fog is the result of very different circumstances: it is often a type of advection fog.

Advection fog forms when damp air moves across a surface that is cooler than the air itself. It can happen over the ocean when a warm front travelling with a mild southerly circulation moves up along the coast in the winter. It also occurs when that warm south wind blows over frozen or snow-covered land. When the temperature of the air cools to its dew point, changes start to happen. First, saturation, followed by condensation that results in fog.

You might have heard people say that "fog eats snow." In a way, it's true. When fog droplets form along the surface of the snow, they release the latent heat of condensation, which helps to melt the snow.

And finally, winter fog is somewhat unique: it's not hampered by the wind. Strong winds often mix the air and can disperse, fragment, or prevent many kinds of fog. The fog that forms when milder, moist air blows over a snowpack can continue to generate advection fog at wind speeds of 80 km/h or more.

Grandma used to tell me that fog was good for my complexion; I can tell you that it's not good for my hair.

Cindy Day is the chief meteorologist for Saltwire Network.