Sixty Fiver

Your understanding of how black ice forms is lacking... -18C / 0F is that magical point where de-icers stop working and water vapour from exhaust can condense and freeze.

It might not look wet because of the lighting or lack thereof, and there may even be a little sand to make you think things are okay and then you gently touch the brakes to find that things are smooth as glass.

For the record, my Jeep is all wheel drive with ABS and excellent winter tyres and I have 30 years of accident free driving in extreme weather conditions... this morning was a highly cautious driving experience when I left the packed and icy streets where the AWD works really well to the streets that had been cleared and had built up black ice at every intersection.

cyccommute 

You can still see it. It's not "invisible". Rime ice (the type that would form from water vapor freezing out of the air at 0°F) traps a lot of air in it and appears white. True "black" ice is appears as a wet surface because the surface is either wet or it is reflective or both.

Saying that you "were just driving along and hit a patch of black ice that I didn't see" is the motorist's equivalent of a JRA in bicycling.

Hypno Toad

Kinda funny, the Minneapolis people are all saying the same thing about black ice - commonly form at sub-zero temps from auto exhaust at traffic lights. I know that CO gets cold enough to see these conditions, not sure why you're arguing with the MN peoples ?? I could understand someone in LA, but you should understand this version of black ice. It is damn hard to see (it is not white at all) and the uninitiated/inattentive will be caught off guard - and at the wrong time, when your hitting the brakes for a red light.

tjspiel

Rime ice is different. You'll set it on chain link fences, trees, etc. I'll see it on my person too from my breath freezing. Black ice on streets from car exhaust doesn't look like that at all. Maybe it would if left to itself but it's getting constantly driven over.

You also have to remember that morning commutes in the winter often occur before sunrise or just after it. Lighting is low and uneven. There are shadows. The roads themselves typically have a grayish hue from salt residue and will be darker in some places and lighter in others. You can see black ice when your eyeballs are 6 inches away, but it's pretty easy not to recognize it as ice when further away.

cyccommute 

No, my understanding of how "black" ice forms isn't "lacking". Ice, of any variety, can form below 0°C...i.e. the "freezing" point of water. If the temperature is 0°C and there is air is at the dew point, ice can form. The water coming out of the air can be rain or snow or fog but if the temperature is below the freezing point of water, it is reasonable to expect some kind of ice formation. You drove on snow packed and icy streets today. Was any of it "invisible"? Have you ever been JRA or JWA and slipped on ice that you couldn't see or didn't have any reasonable expectation of knowing was there. dscheidt said "there are lots of situations where the ice has formed where none of your conditions [are met]." None may mean something else to him but at least one of those conditions has to be met and that is a temperature below the freezing point of water. Having a moisture source is another. If the temperature is above the freezing point, ice can't form and if there is no moisture source, ice can't form.

Nor is -18°C a "magical point" for either de-icers. -18°C is a point where only one deicer becomes ineffective and even that depends on the concentration of the deicer. Sodium chloride is ineffective below -7°C in "normal" concentrations. Magnesium chloride is the one that becomes ineffective below -18°C. Calcium chloride is ineffective below -32°C. And change the concentration of anyone of those salts and you'll change the freezing point depression.

I'm even dubious about the water vapor from exhaust causing the problem. Assuming 2,2,4-trimethylpentane (isooctane), a gallon of gasoline has a mass of 2.7 kg. Combusted, it makes 3.8kg of water and 8 kg of carbon dioxide. Assuming 26 miles per gallon, .14 kg of water per mile of driving (sorry about mixing units but it's easier to think that way). For every foot of that mile, that car is putting out 0.027g of water. That's not a lot of water per foot. A cubic foot of air can carry 1.2g of water per cubic meter at -18°C. That's the exhaust of 44 vehicles assuming that the water stays in one place which it wouldn't. It's going to be mixed with a lot of dry air from everywhere around the car. The water may come out of the air but I doubt that the exhaust from vehicles is the major source.

Sixty Fiver

Still not getting it after a lot of years...

On one of my morning routes the daily traffic volume is 40,000 cars which sit at lights, idling... bring the temperature down to that where the de-icer stops working (our locality is generally -18C) and that water vapour condenses on the roadway, does not melt, and makes it like a curling rink that has just been misted.

We know when water freezes and that snow can be compressed and polished... that compressed snowpack and ice that has formed is highly visible and those roads are also rough and rutted with no sanding so drivers expect them to be slippery and most drive accordingly.

Getting back to that 40,000 car a day route, the cumulative effect of cold temperatures and high traffic volume equals black ice (we have had no precipitation) and drivers do not realize how slippery the roads are until they touch their change speed by accelerating, braking, or change lanes.

When the sun hits and temps come up few degrees the whole situation changes save for those shaded areas where the road just looks like a clear road... carnage usually ensues.

tjspiel

It's typically formed at intersections where multiple cars are stopped waiting for lights and this occurs many times over the course of a rush hour. The tail pipes of lots of vehicles direct the exhaust right down to the street. Again, these cars are stopped so the exhaust is not getting disbursed over a large area as it would if the cars were moving.

cyccommute 

No, they aren't saying the same thing. I'm not saying that black ice doesn't form. What I am saying is that it is rare and is all to often used as an excuse. I've had my share of experiences with ice of many different varieties. What I would call "black" forms when the road is wet from freezing drizzle or from refreezing of melt water. I've never seen "invisible" ice. If it is cold and there is moisture in the air, there is a reasonable expectation of that ice is present. Far too many people use "black ice" as an excuse for "I was driving too fast and not paying attention".

cyccommute 

No, rime ice isn't different. It is, essentially, ice that forms from the gaseous form (aka water vapor) to the solid. It's the opposite of sublimation. If water vapor from car exhaust could form "black" ice, it would form it using the same mechanism passing from vapor to solid without going through the liquid phase.

You are going to have to try harder than that. I wasn't born at night nor was I born last night. I've seen enough ice to know that ice isn't invisible any any of its forms...unless it isn't there.

tjspiel

I think you're confusing rime ice with hoar frost. Rime ice is formed from small liquid water droplets (like fog). Car exhaust is very visible in cold temps so a substantial portion of the water vapor is condensing before freezing. It's not going straight from gas to solid, or at least not all of it. In either case, black ice from car exhaust looks nothing like hoar frost or rime ice.

The ice that's getting formed on the street is different from fog freezing on a tree branch. A water droplet may freeze on contact with the street (like it would a tree) but it's getting hit by the warm exhaust and alternating bursts of cold air so it melts and refreezes perhaps many times while a car is sitting there. That is not the same process that produces rime ice or hoar frost. At least that's my theory. I could be wrong but as I've said before, they don't look the same.

Why are you arguing with people who experience this every winter?

lopek77

Is that special ice you guys talking about....cold?
Little cold, very cold, just cold, cold, freezing cold???

kickstart

I found some of the clipless vs platforms arguments interesting as both sides are arguing half truths. I learned taking some motorcycle racing training that attempting to "save it" is a good way to get seriously injured and one should "rag doll" when going down, but being attached is no advantage either as its best to not get tangled up with the bike.
The arguments about how certain types of ice may or may not be visible are rather entertaining as if its a black or white situation.

cyccommute 

Let's do a little math, shall we? Let's assume that all the cars are the same size and, for simplicity, that they occupy a space that is 30' by 11' or 330 square feet. Let's also assume that the cars are sitting in rows 3 across. For 40,000 cars, that stretches to 400,000 feet or, roughly 75 miles with an area of 13,200,000 sq ft. Since cars have a dimension in the y direction, let's assume 5 feet tall so they have a volume of 66,000,000 cubic feet. Shifting over to metric, that 1,800,000 cubic meters. We'll assume that the cars are essentially empty space (not completely valid but they have a higher volume of empty space than filled space). If each one of those 44,000 cars is just idling and not going anywhere and they are getting 26 miles to the gallon, they are all putting out 0.027g of water in their location. That's 1100g of water for all the cars. That's not a lot of water in a very large space.

In real life, the volume of air is unconstrained and constantly mixing because the cars are moving. Air with less moisture (or even more) would be moving around the cars constantly. Plus the exhaust itself contains some heat.

Maybe you don't have stupid people where you live but we aren't that lucky. Whether it is people who come to Colorado from elsewhere or just people who are clueless, we have a large percentage of people who never "drive accordingly". Hence my comment about black ice being used as the motorist's example of a JRA. "I hit a patch of "black" ice on the road. Who knew that could happen?"

I'm still not buying the idea that car exhaust can be blamed for "black" ice. I live at 5280 feet...about twice as high as you do. The air pressure is lower here so the water carrying capability of the air is much lower. I can double that altitude less than an hour to the west by car. If black ice were to from from car exhaust, it would take less water to form here, i.e. fewer cars, and much less water from car exhaust to form higher up. Winter temperatures in Leadville (10,560 ft) should cause water to precipitate out of the air constantly but I've never heard anyone say that "invisible" black ice is formed from car exhaust.

Another part of the problem is that there is a huge reservoir of air that carries water in it. Air at 0°C can carry 4.89 g/cubic meter. Cool the air to -15°C and the carrying capacity of the air drop by 67% to 1.58 g/cubic meter and it's not linear. That water is going to come out of the air and settle on cold surfaces and freeze. That's a whole lot better explanation to accept than "car exhaust".


But the exhaust is a lot hotter than the surrounding air and rises, mixing with the ambient air. The cars moving through the air provide further mixing. There are simpler explanations. Bridges, for example, tend to ice readily. Part of that is because the bride has a lower thermal mass than the road ways and so cool more rapidly but bridges are commonly over waterways. Water in river and lakes saturate the air and, when the temperature drops, the water has to go somewhere. A bridge is a convenient condenser. As water freezes, it releases heat causing the air around the bridge to rise. Colder, denser air moving to fill the void and condenses the water on the bridge which starts the cycle all over.

Bridges in Colorado don't ice as much as they do in your area for a couple of reasons. Our higher altitude results in a lower pressure and less water in the air. Our rivers...largely because of our altitude and lower water content...are smaller. A "major" bridge over a river in Colorado is, maybe, 90 feet long. Take the Mississippi River where the I-35w bridge crosses it, cut it down by a quarter and reduce the depth to about 18". Thats a "major" waterway around here. There just not enough water to saturate the air around the Platte "River" so that we get a lot of bridge icing from the cold.

I suspect that neither you nor Sixty Fiver see black ice formation during the day while traffic is moving. The kind of ice you are seeing forms over night when traffic is light and the air isn't being mixed as turbulently. There could be some increased water content lingering from the cars after traffic stops but, compared to the water available in the atmosphere, it's still minor.

noglider 

I've never seen invisible anything.

(Not disagreeing with you. Just playing with your words. Or maybe that was your point.)

tjspiel

Bridges do ice up easier (most of them not even over open water in the winter) but I'm not talking about bridges. Yes hot air rises, eventually - if there is no force moving it any other way. Exhaust is often directed down.



Look. This is known phenomena. Not an urban legend or something we are making up.

cyccommute 

No, I'm not confusing them. They all form from the same mechanism. Hoar frost and rime ice both form from water condensing out of the atmosphere. Droplet size has little to do with the mechanism and water vapor could be consider very, very small droplets.

"Black ice" as defined by the American Meterological Society as

Definition 4 is the one that I'm referring to and is always recognizable by a wet appearance. I've never seen "dry" water ice.

Watch the exhaust from cars the next time you are out in the cold. Even in the coldest of conditions and even when the tailpipe exhaust is at it's thickest, the fog rises up. It doesn't go down.

Um, I do live where we have winter. I experience rime ice (aka freezing drizzle), hoar frost (aka freezing fog), freezing melt ice, snow, snow pack, polished snow pack, ice from compacted snow pack and every form of snow and ice that you experience. The only thing I've never seen is this "invisible" "black" ice you guys are talking about. Granted, Colorado weather can be quite goofy...it was 65°F a week ago and back in November we dropped 50°F in 2 hours and 75°F in 12 hours...but that goofy weather would cause us to have more of the "invisible" black ice from car exhaust than you do. When you drop 50°F in 2 hours all kinds of things come out of the sky.

cyccommute 

The water only has to be partially open over water to have air saturated with water vapor. Rivers are usually warmer than the surrounding air as well...they have to be because water doesn't flow when it is frozen.

As to the exhaust, I would put the number of tailpipes that direct car exhaust downward in the minority to the number that direct it sideways or on a flat trajectory, not that it matters much. The exhaust is going to rise up out of the pipe very rapidly...much faster than the small amount of water in a given volume of exhaust could freeze on the ground.

Show me a credible source that's not "some guy on the internet"...I can find one...and I might believe you. Until then, yes, this is an urban legend. It has all the hallmarks..."everyone knows this", "there are lots of sources that say this", etc.

As it stands now, there just isn't enough water in car exhaust nor a high enough concentration of cars in any given area for me to believe it. There are also alternative explanations that make far more sense You might as well say that human breath is responsible for ice on sidewalks.

tjspiel

Lots of bridges ice up without being over water at all, -overpasses for example.

Whether the exhaust is directed down or not, a lot of it comes in contact with the pavement. It doesn't all "rise up out of the pipe very rapidly", - probably because it's condensing. If you've been on the streets in very cold weather you will see it and my guess is that it's more than you realized. Enough to cause problems over the course of a rush hour.


For ice to form you need water, - which has to come from someplace. Black ice as I'm describing it is typically found at intersections on dry, very cold days. Again, I'm not talking about bridges. Where does the water come from? Why intersections more than anyplace else? What do you suppose is happening to the water coming into contact with the pavement in the picture above?

Don't know about sidewalks but the fraction of my human breath that ends up inside my goggles can ice up them up on an extremely cold day, - and it's not Rime ice. It's smooth. Rime ice does form on my balaclava though.

Simply google "exhaust" and "black ice". You can decide if you find any of those resources credible.

alan s 

Black ice is just clear ice, with a somewhat shiny surface, over a black undersurface such as the road or other pavement. Doesn't really matter where it came from, a car tailpipe, refrozen runoff, freezing fog or drizzle. It can be hard to see, but it is not invisible. I encounter the stuff all the time, and have never fallen as a result, because I can see it. It's not actually invisible.

lopek77

Here is the reason why they don't make many V8s anymore, and why they started making hybrid and electric cars...all in the name of safety during winter time. Less exhaust gasses = less black ice. How clever is that?
You can't have small engines in semi trucks, that's why their exhaust pipes are pointing up - simple fix, but that's also the reason why bridges and overpasses are always full of black ice first... Nothing is perfect I guess.
@cyccommute You are one of my favorite posters here, but I'm so dissapointed you didn't know that exhaust story. Nobody is perfect I guess...

lopek77

I didn't add that black ice formation is only started by cars exhaust gases....that's why we can only "see" it on roads.
OTOH, term of Gray Ice refers to sidewalk icing. It starts from dog gases.
How do I know all of that? DISCOVERY CHANNEL!

Sixty Fiver

From mid November to early March we often find ourselves saying "It has warmed up to freezing"... our daily high temperatures usually does not pass that mark until the middle of March when winter releases it's grip and the ice would stay save for the use of de-icers that are used on the roads.

Our sunny winter skies are also attributed to the fact that is is often too cold for clouds to form and when it gets really cold it is a sky of endless blue... and without precipitation or fog the ice still forms on the roads.

Black ice is a real phenomenon and not some myth although half of the people in North America will probably never experience it... some interesting stats are that the stopping distance on ice is 9 times that of dry pavement and the warmer and and closer to freezing, the more slippery ice gets. We need to get into analyzing ice at a molecular level to figure that out.

tj - Minneapolis and Edmonton have very similar winter climates and challenges... we know that black ice really isn't black but crystal clear, how and when it forms, and how to deal with it or when to expect it more.

Last night the temperature dropped to -18C and I knew what to expect on my morning drive... the intersections are sanded and traction is improved but that zone behind the busiest intersections gets as slippery as a road can get (if you are driving).

This is where cars wait in a cloud of condensing exhaust.

wolfchild

Any experienced winter cyclist should expect black ice, it common sense that winter will produce black ice when you least expect it. Even if you don't see the black ice at least take some precautions and be prepared for it. Don't slam your brakes hard at the last second and don't take corners at crazy insane speeds when there is a risk of black ice or when the road conditions are slick.

lopek77

All this fancy talk by us - just regular cyclists, but did you know that science still didn't figured out why stuff gets slippery when wet? I'm not kidding!

Daniel4

A while back we had a sunny weekend of above freezing which melted some snow and created some runoff. Then we had a night of sub freezing temperature. When we got up Monday morning, my driveway was all iced up. Not thick, not white, just a very thin layer of ice that you can't see and that was impossible to walk on without taking very small steps.

But riding was no problem at all just as long as you anticipate ahead to make sure you are braking moderately.