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Freezing fog, rime and hoar frost

Some winter mornings, every branch, fence wire and blade of grass is coated in sparkling white. It’s one of the prettiest things winter does, and two very different processes can cause it.

⏱ 5 min readSeason: late fall to early springWhere: river valleys, low spots, open prairie

Hoar frost: frost that grows from the air

On a clear, cold, calm night, water vapour in the air can turn straight into ice crystals on twigs and fences without ever becoming liquid water first. As Environment and Climate Change Canada meteorologist Terri Lang puts it, it “goes from a gas to a solid.”

It grows as delicate, feathery needles and plates.
It’s very light and falls off at a touch or a puff of wind.
It’s harmless: too light to build up and do damage.

Freezing fog and rime ice

Fog droplets can stay liquid well below 0°C (32°F). This is called freezing fog. When those supercooled droplets drift into a tree, a power line or a car, they freeze on contact. That ice is rime.

Rime looks more like frozen droplets, built up on the side facing the wind.
It’s heavy and hard to knock off.
After days of fog it can build up thick enough to bring down branches and power lines and cause outages. The worst rime tends to form around -4°C (25°F) to -7°C (19°F).

On the roads: freezing fog can leave a thin, nearly invisible glaze of ice on roads, bridges and sidewalks, and it cuts visibility too. Slow down and leave extra room.

Can you tell them apart? “Looking at it, you may not be able to tell the difference,” says ECCC’s Terri Lang. A good clue: if there was thick fog, it’s probably rime; if the night was clear and still, it’s probably hoar frost.

Where to look around here

Cold air drains downhill at night and pools in low spots, so the Oldman River valley and coulees often get fog and frost when the uplands don’t. Some of the best frost photos come on a clear, calm morning after a foggy night, just as the sun comes up. Get out early: a little sunshine or wind and it’s gone.

Southern Alberta angle
Freezing fog and rime are common on the Prairies when cold air settles in for days under a high-pressure system. A chinook usually ends it fast.

Key terms

Hoar frost: Feathery ice crystals that grow straight from water vapour on clear, cold, calm nights.
Freezing fog: Fog made of supercooled water droplets, below 0°C (32°F) but still liquid.
Rime: Ice formed when freezing fog droplets freeze on contact with objects.

Learn more

Source: CBC News, quoting Environment and Climate Change Canada meteorologist Terri Lang.

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