Another winter driving hazard to worry about

July 18, 2024

We are constantly being warned about the danger of black ice on the roads when we are driving in cold conditions. It is a real danger because it can look like just a damp patch, but the advice is always to be suspicious of tree shadows and other dark parts of roads when driving in cold weather and to slow down in case the car loses traction. Hitting black ice is not a good experience and avoiding it is highly recommended. Those orange signs around Oberon warning of snow and ice on the roads are there for a very good reason.

Black ice isn't the only danger when driving in cold weather, however. There is another phenomenon called "freezing fog" that can be just as dangerous. This is when fog can cause a layer of ice to suddenly form on the windscreen.

The American Meteorological Society defines the phenomenon as "A fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze".

Everyone who has done a basic science course at school knows that the freezing point of water is zero degrees Celsius. This doesn't mean, however, that water always freezes when the temperature is below that. It is possible for water to enter a state called "supercooling", defined by the Dictionary of Earth Sciences as "The cooling of a liquid to a temperature lower than its normal freezing temperature".

Fog consists of an enormous number of very small droplets of liquid water. When the air temperature is below zero those droplets enter a supercooled state just waiting for something to trigger freezing, and when a car hits the fog the car's windscreen is where the freezing happens. The windscreen is suddenly covered in a thin layer of ice and it might not be possible to clear it with the wipers. The driver is suddenly blind and it's worse if driving into the sun when it's low in the sky in the morning.

Fog frozen by a clothesline. I wasn't about to try to photograph it on a windscreen.

So what can you do to reduce the danger? The first step is the same as for black ice - be aware of the cold weather conditions, drive to those conditions, and be ready for something to happen. No matter how good your car's heater is it will take time to warm the windscreen and until it does the outside of the screen will be as cold as the air outside. Keep your windscreen clean (dust and other particles increase the likelihood of triggering freezing). If you see that you are approaching a patch of fog, turn the wipers on before you get there (and use the washers if they aren't frozen). If it happens, slow down and try to get to the side of the road. And like the road sign says, remember those words from author Douglas Adams:

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