Dear Straight Dope:
A winter driving question: Over the years I've heard a lot of people attributing automobile accidents to the presence of "black ice." Most of the accidents attributed to the "black ice" phenomenon seem to be single car accidents-you know, the kind of accidents police chalk up to driver error. Thus, when I hear people say "Oh, it wasn't my fault-I must have hit a patch of BLACK ICE!" I am naturally skeptical. It sounds like they were forced off the road by some covert government organization. So my question is--what is black ice, and just how prevalent is it? Does it really cause all the accidents attributed to it?
Save your skepticism for do-it-yourself cryonics or Jerry Brown’s newest plan to revitalize Oakland. If you lived in Maine instead of California, you’d know that black ice is very real. Then again, maybe you wouldn’t. Half the people around here drive as if it’s a myth.
Black ice, also called glare ice or clear ice, is a thin layer of ice on the roadway. Any ice is dangerous to drive on because it’s so slippery (more later on why), but black ice is especially insidious because a road covered with it looks merely wet, not icy. Black ice isn’t really black, of course, but it’s so thin and transparent that the dark color of the pavement shows through. And it really is dangerous to drive on. Statistics from Sweden show that the automobile accident rate there is five times higher on roads covered with black ice than on dry pavement, four times higher than on wet pavement, and twice as high as on pavement covered with packed snow.
How can you tell if there’s black ice on the road? The best way I know is to try to walk on the pavement before driving. If you fall on your ass for no apparent reason, you’ve got black ice (or you’ve been drinking too much, in which case you shouldn’t even think about driving). The other way to tell is to count the number of cars in the ditch while muttering to yourself “I wonder why all those cars are in the ditch; it’s not that slippery.” (I once had the misfortune of riding with a driver who said just that).
The slipperiness of a road is usually measured as the stopping distance, which is the distance required to bring a vehicle to a stop from a certain speed (often 20 m.p.h., but not always). Stopping distance on black ice has been measured to be about nine times greater than on dry pavement. And before you run out to buy an SUV, you should know that four-wheel drive vehicles won’t help much for stopping on black ice. They suffer from the same lack of stopping traction as a two-wheel drive car, though they may make starting on ice somewhat easier. Ordinary snow tires, as the name implies, are designed for snow and not for ice. On black ice, they are no better than ordinary tires, and in some tests very slightly worse. Studded tires (tires embedded with metal studs to grip the snow and ice) are somewhat helpful, reducing stopping distance on ice by about 20% compared to ordinary tires. Tire chains are more helpful, reducing stopping distance by about 30% to 50%. But even with tire chains, stopping distance is still several times greater than on dry pavement with ordinary tires. Before you run out to buy studded tires, you should know that some jurisdictions prohibit or restrict their use because of the damage they do to the roadway.
Black ice is most common at night and very early in the morning, when temperatures are lowest and traffic lightest. It is thin enough that it often melts soon after the sunlight hits it, but it can persist much longer on a shaded stretch of road. Your local highway department can spread sand to provide better traction on ice, but the stopping distance on ice treated with sand is still almost triple that on wet pavement. They may opt instead to use chemicals to melt the ice. One of the cheapest, and certainly the most well-known, is salt (sodium chloride), but it has some drawbacks. It works poorly at temperatures well below freezing, it hastens corrosion of metal, and it can damage concrete. Calcium chloride is an alternative that works at colder temperatures and is less damaging, but it’s more expensive, and at temperatures near freezing it works much more slowly than salt. Other chemicals have been tried as well, such as magnesium chloride, trisodium phosphate, and even urea. (No word yet on whether highway workers ever spell their names in the ice with urea.) In any case, you shouldn’t assume a road has been treated. Highway departments can’t always keep up with new ice formation, especially on secondary roads.
That brings us to the question of where all this ice comes from. If the road surface is below the freezing point, any water that comes into contact with it can freeze. Rain that spreads out and freezes on contact is known as freezing rain. There can also be freezing mist, freezing drizzle, and even freezing fog. Meteorologists call the transparent layer of ice that results “glaze.” A similar but more opaque coating of ice is called “rime.” There is one source of rime that ancient mariners (and mariners who hope someday to become ancient) should be acquainted with: freezing spray (drops of water that are whipped up by wind and waves and which can freeze on contact with the parts of a vessel above the waterline).
Other sources of moisture can also form black ice. If water melts from a snow bank during the heat of the day and flows onto or across the road, it can refreeze at night, leaving a small patch of ice on the pavement. In places where cars idle (such as driveways, toll plazas, and intersections), automobile exhaust can be an important contributor. At cold temperatures, auto exhaust (like your breath) contains a visible fog of water droplets, formed when invisible water vapor condenses into droplets. Some of these fog droplets may then freeze when they come in contact with the roadway within a few feet of the tailpipe. More water condenses on the inside surface of the tailpipe and drips out, leaving a thicker but more localized patch of ice directly beneath it. Bridges and overpasses are another danger spot. Since they do not receive as much heat from the ground and lose more heat to the air, they can drop below freezing even when the rest of the roadway doesn’t. Adding to the danger is the fact that highway departments are often reluctant to spread salt there for fear of hastening corrosion of the bridges’ metal parts.
So the colder the temperature is, the more dangerous it is to drive, right? Nope. Ice will always be slippery, but it’s actually more slippery near freezing than it is at colder temperatures. Near 0 F, stopping distance on ice is significantly less than at 32 F (between 15% and 50% less, depending on whose figures you accept). The conventional explanation has to do with why ice is slippery in the first place. You’ve probably heard that ice is slippery because a body moving over it melts a thin film of water that it then glides on. It follows that ice is less slippery when very cold because less water can be melted. Controversy developed over whether pressure or friction was responsible for the melting, but that was pretty much resolved in favor of friction because pressure would only work near freezing.
The problem is that this explanation may be wrong, or at best only part of the story. No one disputes that a thin film of liquid water does often form (under a skate blade, for example), but at least one researcher has found that the presence of liquid water (perhaps because of the viscosity of water or its surface tension) can actually increase friction. (Anyone who has ever tried to walk on ice covered by puddles of water will question whether that is generally true). Over the years, many scientists have argued against the water-film explanation, saying that ice, even in the absence of an identifiable film of liquid water, is somehow inherently slippery. As far back as the nineteenth century Michael Faraday believed the surface layer remained liquid well below 32 F. Some later scientists argued that ice was a solid lubricant (like graphite), and they proposed various hypotheses to explain the mechanism of its slipperiness. But it wasn’t until recent years that we’ve had the tools to investigate more closely. As it turns out, Faraday may not have been too far off the mark.
The molecules in the interior of a crystalline solid (like ice) are constrained from moving freely by the geometry of their bonds with other molecules. In ice, each molecule normally is bonded to four others. But imagine a water molecule on the surface of the ice. It has no molecules above it to bond with, so it can vibrate much more vigorously than the tightly bound molecules in the interior. It is behaving somewhat like a molecule in a liquid, in that it is rather free to move up and down and (to a lesser extent) back and forth, but somewhat like a molecule in a solid, in that it is generally constrained from moving away from the molecule(s) it is bonded to. Neither solid nor liquid, these relatively free molecules are said to constitute a “quasi-liquid” layer that accounts for the low friction of ice.
How does all this explain why ice is less slippery when very cold? In the real world, the quasi-liquid layer is not just one but several molecules thick. Near freezing, the layer is thicker because of the relatively high disorder (more molecular motion). At colder temperatures disorder is less, the layer is thinner, and the molecular movements are less vigorous and so less liquid-like. The quasi-liquid model and the more traditional liquid-film model both explain the facts. Which explanation is the right is still controversial. It may be that each is important depending on the circumstances.
"Black ice” can also refer to a type of ice formed on lakes and rivers. Like highway black ice, river/lake black ice is transparent (hence the name), but unlike highway ice it can be ten feet thick or more. It forms relatively slowly under calm conditions so most of the individual ice crystals can orient themselves with their long axes parallel (and vertical). The slow growth and close packing of the crystals allows most of the impurities (including dissolved gases) to be excluded from the forming ice. This results in an ice so transparent that photosynthesis can continue beneath a considerable thickness of it, provided it’s not covered by opaque ice or snow. Opaque ice on lakes and rivers is formed when a slush of snow and water freezes (called “white ice”) or when supercooled water freezes rapidly while being agitated by current or wind (called “frazil ice”).
And the do-it-yourself cryonics I mentioned? It’s just an old New England folktale I remember from way back. But how the heck do you explain Jerry Brown?
Handbook of Snow: Principles, Processes, Management & Use, edited by D. M. Gray and D. H. Male
“Melting Below Zero” by John S. Wettlaufer and J. Greg Dash in Scientific American, February 2000
Send questions to Cecil via firstname.lastname@example.org.
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