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Quote: "black ice forms at 37 deg. fahrenheit. "

I've seen this more than once as we discussed winter weather and am intrigued by it. My learning was always that phase change in molecules was absolute at a particular temperature and not distributed around a mean. This statement indicates otherwise.

Best science I can find in a brief google is that ice forms at or below 32 degrees F/ 0 degrees C. Never higher.

If the air temperature is higher than the freezing point, then for ice to form a water molecule must transfer energy to something colder than 32 such as a frozen roadbed, ground, or what ever.

Thoughts, comments?

What kind of chemicals (salts) are used in your area for de-icing? I'm a southern boy and don't have any experience with these issues. That being said, If a saturated CaCl2 (11.6 ppg) were used to de-ice, the freezing point of that solution is 44 deg. Throw into the mix other salts and possibly leaching of compounds from the road surface and who knows. I did a google search on black ice and didn't see anything on what you described on the 37 degree thing.

Rootin,

Are you saying that a salt/water solution freezes (changes to solid) at a temperature higher than 32F?

I thought any solution (water with additional molecules of another compound) would lower the freezing point rather than raise it.

I am no chemist and struggled mightily with 101.

Notice I said "Saturated". Freezing point (Crystalization) is depressed until you get near/reach saturation point. We use "Heavywater" in oilfield applications as a packer fluid. This can be NaCl, CaCl2, CaBr and ZnBr, depending on the final weight of liquid that is required. Sometimes is the proper ratio's are not used, crystalization occurs and the product is rendered useless as hydrostatic head is no longer being applied since the liquid is now solid. Just a thought that this may be applied to your question, although you mention a specific temperature of 37 degrees.

I am lost but I think I get the gist of it. Thanks.

M

camfarm wrote:I thought any solution (water with additional molecules of another compound) would lower the freezing point rather than raise it.

I am no chemist and struggled mightily with 101.

Well I am, and you remembered well. In fact the amount of the freezing point depression can be used to determine the concentration of the contaminant. Regarding ice at 37 degrees, well it has to be correct because you saw it on the internet, right? You can have ice on the road when the air temperature is 37 degrees, in fact you can snow ski at 37 degrees, but you can't make ice at 37 degrees. - Lee

The air temp may be 37 F, but the earth doesn't heat as fast as the air. If the earth's surface temp is at or below 32 F, and is at or below the dew point, you can form black ice. You don't even need wet roads. Moist, still air (like fog, for instance) coming in contact with frozen ground is all it takes.

I am getting it.

Many thanks to all you patient guys for explaining in Camspeak.

Mike for what it's worth, the dash computer in my '96 3 Series flashes the outside temp and chimes when you start the car if the reading is 37 F or lower. Manual says is an ice/freezing warning.

'

Rootin55 wrote:Notice I said "Saturated".

You lost me at 'saturated'. I'm very lost on this one. If I close my eyes, then the ice isn't really there! Is it?

MidlifeMark wrote:The air temp may be 37 F, but the earth doesn't heat as fast as the air. If the earth's surface temp is at or below 32 F, and is at or below the dew point, you can form black ice. You don't even need wet roads. Moist, still air (like fog, for instance) coming in contact with frozen ground is all it takes.

A few winters ago, I was getting ready to leave on my morning commute, listening to the news. They were talking about frost and ice on bridges - I ignored it thinking that wasn't on MY commute. Wrong - first bridge I got to I noticed the bike acting a little funny (it wasn't the roadster - more of a sportbike). Then it hit me (on the interstate), the bridges were FROZEN! It was a heavy frost that settled on the nice cold bridges and stayed there - a thick frosty layer of frost. tire tracks from cars were obvious on all of the bridges. Anyway, most of my commute was on the interstate and I figured with the high speed, gyro-scopic affect of the wheels and relatively 'short' bridges I was ok. Until I made it downtown, that part of 395 was a large, long bridge over the city. My exit was still covered with frost when I got to it and realized I was in for it. I slowed down to 1st gear, idle and extended my out-riggers (like my legs would do any good!). The exit was all bridge with a good turn towards the end. I was fine until I came to the apex of the turn and saw lots of 'slide' marks from cars, going right into the jersey wall. I almost lost it but managed to keep it together.

After that day, I respect frost a lot more and will actually PAY ATTENTION to the morning news.

Mike - My experiences with black ice would indicate what was said, that the air temp and what is on the surface is not the same. I think black ice is technically snow or ice that has melted and refrozen clear and hard to see. But I would guess black ice and regular ice would all slide the same!

Years ago, I took off one morning after a rain in 36 degree temps. The first few puddles were water not ice and I put it out of my mind and rode on. Until I got to a bridge over a small creek. Without even a moment to say "OH S**T", I was on my butt sliding along the road while my RS was on it's side leading the way to the ditch. The bridge, obviously was wall-to-wall ice due to the bridges being colder (just like all the NCDOT signs warn1 ) but I couldn't see it. The bike and I both finished the ride, BTW.

I remember that on all my winter rides now!

Pilots are familiar with two types of icing aloft: Rime and Clear. Rime has air mixed with it and apears white (on the ground it would be frost, snow, and sleet). Clear Ice has no air mixed in and is therefore hard to see the build-up. On the ground we see it as freezing rain.

Clear ice and freezing rain are formed when a cold but not yet freezing water hits an object that is well below 32F or 0C. On the ground it happens when a cold (near freezing) rain falls on objects that are below freezing. In the air it can happen when a plane that is flying high in sub-freezing temps decends into warmer air and rain.

The air temp when this happens depends on the temp of the rain falling through it, the temp of the objects collecting the ice, and how much thermal mass those objects have. Who wants to calculate that mess? 37F is just a good "rule of thumb".

My understanding is "black ice" is a trucker's term for invisible ice. Day or night, you will not see it even with your lights at night as it is generaly the same "shineyness" as the road surface (OK, reflectivity) and you don't know it's there until you "feel" it...maybe feel it with your ass on the way to the ditch.

I hate the ditch...

maybe feel it with your ass on the way to the ditch.

I hate the ditch...

I see you read my above post!

The ditch beats the guardrail!

The ditch beats the guardrail!

Sometimes those are the only choices...hate it when that happens.