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What Causes Black Ice?

By Sheril KirshenbaumMarch 2, 2009 10:00 AM


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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.

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 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.

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