What Causes Black Ice?

By Sheril Kirshenbaum | March 2, 2009 8:00 am

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.

car snow.pngI 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 Discover blogs, 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.

CATEGORIZED UNDER: Culture, Miscellaneous
MORE ABOUT: black ice, roads, snow, winter

Comments (12)

  1. SLC

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

  2. Alex Besogonov

    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.

  3. Joe

    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.

  4. peter

    Be careful everyone. Those are some nasty conditions outside.

  5. Bruce

    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.

  6. 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.

  7. Douglas Coker


    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!!

  8. I recently came across your blog and have been reading along. I thought I would leave my first comment. I don’t know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.



  9. James Elliott

    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.

  10. klgregonis

    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.

  11. Laurie

    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.


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About Sheril Kirshenbaum

Sheril Kirshenbaum is a research scientist with the Webber Energy Group at the University of Texas at Austin's Center for International Energy and Environmental Policy where she works on projects to enhance public understanding of energy issues as they relate to food, oceans, and culture. She is involved in conservation initiatives across levels of government, working to improve communication between scientists, policymakers, and the public. Sheril is the author of The Science of Kissing, which explores one of humanity's fondest pastimes. She also co-authored Unscientific America: How Scientific Illiteracy Threatens Our Future with Chris Mooney, chosen by Library Journal as one of the Best Sci-Tech Books of 2009 and named by President Obama's science advisor John Holdren as his top recommended read. Sheril contributes to popular publications including Newsweek, The Washington Post, Discover Magazine, and The Nation, frequently covering topics that bridge science and society from climate change to genetically modified foods. Her writing is featured in the anthology The Best American Science Writing 2010. In 2006 Sheril served as a legislative Knauss science fellow on Capitol Hill with Senator Bill Nelson (D-FL) where she was involved in energy, climate, and ocean policy. She also has experience working on pop radio and her work has been published in Science, Fisheries Bulletin, Oecologia, and Issues in Science and Technology. In 2007, she helped to found Science Debate; an initiative encouraging candidates to debate science research and innovation issues on the campaign trail. Previously, Sheril was a research associate at Duke University's Nicholas School of the Environment and has served as a Fellow with the Center for Biodiversity and Conservation at the American Museum of Natural History and as a Howard Hughes Research Fellow. She has contributed reports to The Nature Conservancy and provided assistance on international protected area projects. Sheril serves as a science advisor to NPR's Science Friday and its nonprofit partner, Science Friday Initiative. She also serves on the program committee for the annual meeting of the American Association for the Advancement of Science (AAAS). She speaks regularly around the country to audiences at universities, federal agencies, and museums and has been a guest on such programs as The Today Show and The Daily Rundown on MSNBC. Sheril is a graduate of Tufts University and holds two masters of science degrees in marine biology and marine policy from the University of Maine. She co-hosts The Intersection on Discover blogs with Chris Mooney and has contributed to DeSmogBlog, Talking Science, Wired Science and Seed. She was born in Suffern, New York and is also a musician. Sheril lives in Austin, Texas with her husband David Lowry. Interested in booking Sheril Kirshenbaum to speak at your next event? Contact Hachette Speakers Bureau 866.376.6591 info@hachettespeakersbureau.com For more information, visit her website or email Sheril at srkirshenbaum@yahoo.com.


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