Originally Posted by
dscheidt
The ground being cold enough to freeze water while the air temperature is above (sometimes well above) freezing is a regular occurrence in the midwest, the upper great plains, and the northeast. Maybe that's not how the weather is where you are, but it happens a lot elsewhere. Condensation isn't an issue, because the dew point can stay well below freezing even when the air temperature is 50F.
I'm not sure you understand dew point. The dew point can be well below freezing when the air temperature is warm
if the relative humidity (RH) is low. For example, Denver experiences 12% relative humidity (often far less) and 50°F temperatures quite frequently. The dew point under those conditions is -1°F. That means that the temperature of the air has to drop to -1°F without a change in the barometric pressure before dew (or frost at that temperature) will start to form. At 100% RH, dew will start to form at 50°F.
Condensation, by the way, is entirely the issue. You can't form ice if you don't "condense" it from the water in the air. My point during all of this discussion is that the conditions for the formation of ice on the roads are well known and aren't magical. Even if the ground
is cold enough to form ice, the air above the ground has to contain water and is usually visible in the form of fog or drizzle or some other form of moisture. To say "I hit a patch of black ice unexpectedly" is simply an excuse for not paying attention to the conditions. Look at the video in post 68, for example. Can you really say that Hypnotoad hit the ice "unexpectedly"? The road is looks wet. He has just ridden through a patch of snow pack and the surrounding ground is covered in snow. What is "unexpected" about finding the roads covered with ice?
Originally Posted by
tjspiel
But you made some assumptions in that post that weren't true. The black ice you get from exhaust in extreme cold isn't anywhere near 3 mm thick. It's extremely thin. It also doesn't form on horizontal surfaces, it's on the streets, typically in patches around intersections and not on the sidewalks nearby even. So the amount of water required is much much much less than what you're imagining. It doesn't happen at 20° or 30°, it happens at -20° or -30°, - temps you probably don't see often in Denver. Boiling water will freeze instantly when thrown in the air under those conditions.
How thick do you think 3mm is? It is "extremely" thin. It is 0.11". In my book that is "extremely thin".
We've also been through this "black ice forming from car exhaust" before. To recap: Well, see post 138. Simply put, the science just isn't on your side.
I'm amazed that people find throwing a cup of hot water (sorry but it isn't "boiling" by the time the reporters throw it) into very cold air is "surprising". The water has a lot of heat in it and it is suddenly dispersed and the droplets freeze. Big whoop.
Originally Posted by
tjspiel
It's not that hard to believe when you think about it. At those temps the exhaust from my own face will produce quite a bit of ice on the stuff around it, often including (unfortunately) the inside surface of the lenses of my goggles.
When I think about it, it
is that hard to believe...that's my point. Think about the ice formation around your face. I get ice forming on my mustache and maybe a bit on my balaclava when I wear one. That's about all. There's no ice on my handlebars or frame from my breath. On a per minute basis (resting), a human breaths out 0.27 g of water. That's more than a car puts out on the same basis. If, like you propose, a car's exhaust can be responsible for "black ice", you should see ice all over your bicycle from your breath. There's more of it and there is less mixing. Bicyclists aren't terribly aerodynamic but we are also moving slower and have less volume so we are stirring the air around us less.
As for the fogging of you goggles, that's just dew point. You raised the dew point in inside the goggles and lowered the temperature of the goggles to the point where the air can't hold the water anymore. It condenses out and deposits on the cold surface. If you remove the goggles, the air inside mixes with drier air outside and the fog goes away.
Edit: Today is kind of cold here and I was observing something that should dispel the "car exhaust condensing and causing black ice" thing. Take a look at the chimney of a house. The house isn't moving so any exhaust from the chimney doesn't mix as well as a car's exhaust does. The chimney of a house is also more insulated than a car's exhaust so the bricks or metal around the chimney will be closer to the outside temperature. Do you see ice forming on house chimneys? I can't say that I ever have even in the coldest of temperatures. If "black ice" can form from car exhaust, you should see vast deposits of ice around the tops of chimneys or on the roofs of houses. They are a stationary, and nearly constant, source of water vapor.