Winter driving: what is black ice?

Thursday, 20 June 2019

Black ice can take you by surprise - even on a sunny day. What should you do?

This story was originally published in June 2019. It is republished here in response to black ice warnings for the South Island.

We all know that ice is a big winter driving issue in the colder parts of New Zealand. But in certain regions, like the Central North Island and Central Otago, 'black ice' is a looming threat as well.

It sounds sinister and it certainly presents danger to the motorist, because it's a type of ice you can't see.

It's 'black' not because it's evil, but because it's transparent and therefore the road surface shows through.

Black ice doesn't contain the air bubbles and swirls of ordinary ice - so it's transparent and therefore hard to see.

Ice looks white because there are air bubbles and swirls trapped inside. If you're a poet-scientist, black ice is perhaps the most perfect kind because it has been created in a rare set of circumstances.

* Silly questions about motoring in wintry weather

Black ice hangs around longer on rural roads where there's not much traffic.

* How to de-ice your windscreen**

Starting with sub-zero conditions, melting snow, fog, gentle rain and even condensation from vehicle exhausts can help black ice to form (another reason to drive an EV?) - most often in calm conditions with little wind.

On a sunny but freezing day like you often get in Central Otago, heat can melt away the white stuff but what's left refreezes overnight into black ice.

How do I spot black ice?

A section of black ice will likely be short, so stay calm and hold the wheel straight until you regain traction.

Because it starts as a watery surface, black ice is very hard to spot because it's extremely thin - it might look like a harmless puddle. Don't be lax just because your car's thermometer says it's above freezing, especially in the morning: the road surface might not have had time to heat up and melt the black ice.

For that reason, be especially aware of wet-looking patches of road on an otherwise-fine day, especially if they are in an area that doesn't look like it gets much sun (a shady corner by a rock wall, for example).

Bridges are more susceptible to any kind of ice, because cold air can circulate above and below and there's not as much insulation underneath to stabilise the temperature.

Black ice can hang around much longer on rural roads, because the warmth and friction of traffic wears it away.

What should I do if I hit black ice?

In theory, driving on ice of any kind requires the same techniques.

A car can really only do three things: accelerate, steer and brake. If you're driving on ice and/or snow with very little traction, the key to staying safe is to only do one of those things at a time. Simple, right?

The problem with black ice is that by the time you've hit it, theory might mean little and panic takes over.

The difference between white ice and black ice is that the former is more likely to be sustained - you'll be aware you're driving on it, it'll go on for a long distance and you can modify your driving style to suit.

With black ice, there will be a clear beginning (even though you might not have spotted it) and a clear end - more than likely after a few metres.

So if you hit black ice and lose control, don't panic and try to fight your way back. You won't be able to because your car will have virtually no grip or traction (much less than on snow, for example).

So it's best to remain calm, try and keep the steering wheel at the straight-ahead (so the wheels are pointing the right way when you regain control) and wait for the black slippery stuff to pass. Usually - hopefully - that happens quite quickly.