What Causes Black Ice?

Picture it. A bridge in Connecticut, January 2004. Having left New York at 4:30 am, I settled into my Honda hoping to reach Maine before nightfall. The first couple hours were uneventful until... suddenly the wheel locked, the brakes failed and my car spun haphazardly across three lanes to face oncoming traffic. Yet somehow, we didn't suffer a scratch between us.

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I was lucky, and I want to encourage readers in the northeast to keep black ice in mind as you brave the roads this morning. And since this is scienceblogs, what causes the slippery stuff anyway?

Black ice is ice that forms without many air bubbles inside, commonly occurring on roads as moisture from car exhaust condenses. Because it's transparent, it takes on the color of whatever surface it forms on--and if you can detect black ice at all, it generally looks like wet asphalt. It can also form when temperatures are above freezing meaning it's hard to be prepared. Unfortunately, four-wheel drive vehicles do not protect you from losing control and salt is also not as effective at freezing temperatures. And finally--as I observed firsthand--bridges and overpasses are often most dangerous because cold air circulates above and below elevated surfaces, making them freeze fastest.

I hope those driving on wintry roads today remember to be extra cautious.

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The best advice is to stay off the highways unless absolutely necessary.

Salt is _very_ effective against black ice (salt+water eutectic melting temperature is somewhere near -40C), but it needs to be applied correctly and in abudance.

By Alex Besogonov (not verified) on 02 Mar 2009 #permalink

Huh? The moisture from car exhaust is hot, therefore buoyant, and doesn't settle on road surfaces. Black ice is formed when the temperature drops rapidly, freezing puddles and any other water on the road surface.

Be careful everyone. Those are some nasty conditions outside.

Black ice isn't a problem when the conditions are obviously nasty. Black ice is a problem when the conditions "look OK".
Black ice usually looks just like moist-but-clear pavement.
Because it often occurs on highway overpasses, it can really surprise you, because you are often unaware of when you are on a highway overpass.
It's fun to watch the big SUV's spin off the road because of black ice. Someday these drivers will learn that twice the traction is useless when you have zero traction.

Whoa. That was close. I am sure some kind of salt would be effective against black ice. Were you driving on Friday? Then it would have become Black Friday.


Get yourselves on a motorcycle for at least a year or so. Then you won't just have a theoretical understanding of physics and in particular friction or the lack of it but a real understanding of what all road users should know about. Snow and ice (all forms), wet roads, polished metal drain covers, white lines (lane markings etc), diesel spills, polished tarmac, aqua-planing, gravel and so on.

I'd tell you all about my "best" motorcycle crash at Hyde Park Corner London (diesel!!) but I might have made my point ;-)

Douglas Coker
Ex-motorcyclist - no broken bones!!

By Douglas Coker (not verified) on 02 Mar 2009 #permalink

The salt-water eutectic temperature is actually -21.1degC, which occurs at 23.3wt% NaCl, according to


So, salting roads could quite easily become ineffective at colder temperatures, however only relatively low concentrations are required to produce a 5-10degC decrease in melting point.

By James Elliott (not verified) on 03 Mar 2009 #permalink

This leads me to a comment: One of the selling points for hydrogen fuel cell vehicles is the "clean" exhaust - water vapor. What would the result of thousands of cars producing water vapor exhaust be on a cold day in the winter anywhere the temperature gets below freezing? The mind boggles with the picture I have of the combination of fog and black ice that could occur. It might put some of the large crashes of the past to shame.

By klgregonis (not verified) on 03 Mar 2009 #permalink

We just had an accident on Sat with the black ice monster. We totaled our 2000 Honda and got a 2009 Honda with new safety features, here is the blurb from Honda's website, below. Does anyone know if any of these features are helpful against black ice?
Every Accord is designed to keep you on course and away from danger. Vehicle Stability Assist⢠(VSA®) helps sense oversteer or understeer in an emergency situation, and then adjusts brake pressure at each wheel and/or reduces engine power to help restore driver control and keep you on course. Standard 4-wheel anti-lock disc brakes (ABS), with Electronic Brake Distribution (EBD), help you maintain control during hard braking. Properly inflated tires are crucial for safe operation, so the Tire Pressure Monitoring System (TPMS) alerts the driver when a tire's pressure reaches a significantly low level.