There's a name for that frost that covered Yellowknife last week, and it's not snow

Rime ice, a type of frost, seen settled over Yellowknife on Friday. In the last few weeks there has been little sunlight, with mostly cloud cover so far this year. (Walter Strong/CBC - image credit)
Rime ice, a type of frost, seen settled over Yellowknife on Friday. In the last few weeks there has been little sunlight, with mostly cloud cover so far this year. (Walter Strong/CBC - image credit)

What's white, frosty and seen all over Yellowknife last week?

Hint: the answer isn't snow, exactly.

There's something else quite like it however, that was forming on your car windows, on trees, and power lines.

It's called rime ice, though some might confuse it for hoarfrost. It's since been blanketed in snow thanks to the dump over the weekend.

Rime ice happens when fog particles start to freeze.

Terri Lang, a meteorologist from Environment and Climate Change Canada, explained that fog — "little clouds on the ground" — is made up of tiny liquid particles.

Those particles can exist down to temperatures of –25 C before they actually freeze, and they "are floating around the atmosphere."

But, anytime these little liquid particles run into something, they will freeze instantly and form rime icing, she said.

Submitted by Shannon Scott
Submitted by Shannon Scott

"This can go on for a long period of time, depending on how long the fog persists. And you can get quite a buildup of this rime icing, and a lot of people think 'Well, it looks like snow on the trees.' But it's actually this icing."

What makes the rime ice look so fluffy, Lang said, is its form.

"If you … go in real close and take a look, they're forming like little trees, almost little dendrites, they almost look like snowflakes too," Lang said. "If you go up to it, you can usually blow off the particles really easily."

But it's when the rime ice starts to build up that brings potential trouble.

"It starts getting heavier and it's harder to blow them off," she said.

Lang said rime ice can build up quickly and become heavy on power lines, and even interrupt power grids.

"It's so much heavier and denser than hoarfrost," she said.

Walter Strong/CBC
Walter Strong/CBC

Northland Utilities Ltd., the company that operates the city's electrical system, said it isn't concerned.

The system was "designed and built" to take on heavy weather like wet snow, ice and wind, said manager Raymond McDonald.

"The current accumulation of snow/rime ice on our power system is not a concern. Our employees inspect power lines daily and take appropriate action to address all hazards," McDonald said.

He said there are procedures in place for working safely around power lines and for removing excess ice and snow if it becomes hazardous.

McDonald added the company "responds to and addresses all customer concerns regarding low-hanging power lines."

He said he's unaware of any past incidents where rime ice impacted the city's electrical system.

Different from other kinds of frost

Lang said rime ice differs from hoarfrost, which forms under clear skies. Hoarfrost "skips" the liquid phase that rime ice goes through, and it goes right to a solid.

"So the water vapour in the air goes right into frost and on trees on cars, etc. But it's usually a lot more delicate, and it doesn't build up as much as rime icing does," Lang said.

Lang also has an answer for why it's been so cloudy in Yellowknife. So far, according to Environment Canada, there's been no sunshine in 2023 in Yellowknife, aside from parts of Friday.

"We haven't had weather systems moving through," she said. "And what that does is it makes the air really stagnant. And there's not a lot of movement in the air."

Walter Strong/CBC
Walter Strong/CBC

She said when the air gets stagnant an "inversion," is formed.

"Normally in the atmosphere, if you went up in a balloon ... the atmosphere would be warmer at the surface, and then colder as you go up. But when an inversion forms, it actually warms as you go up through the atmosphere," she said.

She said that forms a "cap or a layer" that traps everything below.

"So all the moisture, all the pollutants, everything that's there traps it in those low levels, and it can't escape because of this inversion," Lang said.

The phenomenon also explains why it's been a little warmer lately. Last week, for example, temperatures hovered between -11 C and -15 C with little to no fluctuation outside of that range.

"The cloud kind of acts like a little blanket and kind of keeps that warmth in as well," Lang said.

"It also doesn't let the sunshine from above heat anything up either. So that's why the temperatures don't vary much under this kind of cloud pattern."

The forecast for Yellowknife on Environment Canada's website shows cloudy with flurries until Wednesday, when there might be a break in the clouds.