Originally Posted by
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.
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.
Originally Posted by
Sixty Fiver
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.
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?"
Originally Posted by
Sixty Fiver
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.
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".
Originally Posted by
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.
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.