(CKOM News staff)

By Libby Giesbrecht/ CKOM News

Why some frost on your windshield takes a little more elbow grease

Mar 18, 2022 | 11:27 AM

If it felt like Saskatchewan’s winter brought more windshield-scraping this year than normal, it’s because some frost is harder to scrape.

David Phillips, a senior climatologist with Environment Canada, said there are two types of frost to keep in mind: Hoar frost and frozen dew.

Hoar, or white, frost happens when the air is pretty cool, as is the dew point. Water vapour in the air at those temperatures will change right from vapour into ice, skipping the liquid phase, Phillips explained.

He said that form of frost is when ice crystals appear on the windshield.

Frozen dew forms when the air temperature and dew point are above the freezing point and condensation forms, then freezes as the air cools through the night into the morning. Like hoar frost, it starts with water vapour, but condenses into water first before forming ice.

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“When it’s really cold, you warm up the car inside, it takes a little longer for that windshield to warm up and get rid of the frost the old-fashioned way, which is not using any elbow grease,” Phillips said.

When frost is harder to scrape from a vehicle, it’s because of the temperature.

Phillips said the zero to -2 C degree range is fairly easy to scrape. But anything below -4 C involves ice crystals that are more bonded to each other. That’s when it takes more effort to scrape.

“It’s a little bit of a science of frost and frozen dew,” Phillips said.

What is frost?

Frost appears when the temperature reaches around the freezing mark. Phillips said the air has both a temperature and a dew point, which is the temperature at which air becomes saturated.

Because all air contains moisture, the dew point is often close to the temperature, especially during melting and freezing times in Canadian climates.

As air cools, water vapour in the air condenses to liquid on the surface of a car. It stays in that condition unless the temperature drops below zero.

Cooling to subzero temperatures means moisture from the air deposited on places like windshields will freeze into ice and frost, which forms on the surface of your vehicle.

Rising to frost on a car windshield is not uncommon during Saskatchewan winters — Phillips said the nature of the metal and glass used in car windshields attracts frost because they release heat and don’t conduct energy.

Cloudy nights are when people are less likely to see frost in the morning. Having a clear sky is the best conductor for making frost because any heat rises into the atmosphere, causing the temperature to fall faster and closer to the freezing point.

“It just radiates right up into that space,” Phillips said.

That’s why parking under a carport can also help prevent frost — heat will bounce off the ceiling and radiate back onto the car, keeping the overall temperature around a vehicle warmer.

“Diamond dust”

What about the ice that forms on windshields in the most blisteringly cold days of Saskatchewan winters?

While Phillips said frost isn’t likely forming in those temperatures — thanks to the dry winter air lacking any humidity — it could be a type of ice crystal in the air known as “diamond dust.”

Unlike frost, which technically forms on surfaces, those ice crystals are deposited on a windshield almost like a layer of snow.

Phillips said it has to be perfect conditions — the exact freezing temperatures and the exact timing — for diamond dust to be in the air.

“It’s almost as if the temperatures in the atmosphere where they form have to be below -30 C,” Phillips said.

Understanding the dew point can be key to knowing what kind of frost might face a person in the morning. Always lower than the temperature of the air, it’s the point where the air is cooled from vapour to liquid, before eventually turning to ice when frost appears.

The colder it is, the thicker the ice, according to Phillips — that’s because it bonds the ice particles more tightly.

Diamond dust can be pretty and seen only in certain optic settings.

Slippery morning roads

Colder air in the early morning and moisture from car exhausts are making morning commutes slicker, despite the snow finally starting to melt across the province.

Phillips said the air is the coolest closest to the ground, and it can lead to black ice.

“It’s kind of dangerous,” he said. “Until that sun comes up and warms the air and turns that frost into just moisture, there (are) some tricky moments there that you have to be watching out for.”