Originally Posted by
tjspiel
Lots of bridges ice up without being over water at all, -overpasses for example.
Yes, lots of bridges ice without being over water but it's not "car exhaust"
Originally Posted by
tjspiel
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.
Yes,
some water from car exhaust will condense out but it's a minor component. Let's say you have a city that is 30 km square (19 miles). Now, go up 60 m (200 ft), that a volume of 5.4 x10^10 cubic meters. Let's assume that the air is at-18°C (0°F). It can carry 1.2 g per cubic meter or the total amount of water available is 64,800,000 kg assuming complete saturation of the air.
Now let's stuff 44,000 cars into that volume. Let's make each one burn 1 gallon of gasoline. That 167,000 kg of water that those 44,000 cars put out. The cars contribute 0.25% of the total water in the air. It's, almost literally, a drop in the bucket.
Now let's look at the ice that could form over that 30 km area. Let's assume 3 mm so that the ice is actually completely covering the craggy surface of the pavement. I'm also going to assume that the entire area is covered with 3mm of ice including all horizontal surfaces of the buildings. The total volume of a 900,000,000 square m x 3mm object is 270,000,000 mL. Water weighs in at just around 1g per mL which means that you have 270,000 kg of ice covering the area. You are about 100,000 kg short of the amount of water you need if
every single car combusts one gallon of gas. And you'd have to wring every drop out of the exhaust.
There's an ocean of moisture laden air above the cars and even the 270,000 kg of ice is but a drop in that ocean. What I contest is the statement that car exhaust is
the major contributor. It's a minor contributor at best.
[QUOTE=tjspiel;17587939]
Originally Posted by
tjspiel
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?
The water comes from the 65 million kg of it floating around in the air. Again, some of it is from car exhaust but the atmosphere contributes more than enough on its own.
Originally Posted by
tjspiel
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.
Rime ice can be crystalline or it can be thick glaze ice.
Originally Posted by
tjspiel
Simply google "exhaust" and "black ice". You can decide if you find any of those resources credible.
I did. All I get is something that has all the hallmarks of an urban legend.
Originally Posted by
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.
Exactly what I've been saying. Like I said above, black ice is a convenient excuse. "I was just driving along and skidded on a patch of invisible ice." It JRA for motorists.