Why at 39 degree?

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If I read this correctly, you are saying that water will freeze at a higher(warmer) temperature due to altitude. What altitude would you have to be at for water to freeze at 39° Fahrenheit? Just curious.

Dan
 

Tim$2

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You might not get much ice forming on the ground at 39 (4 Celsius) but you could definitely have sleet and possible snow falling at that temp.
 

corvettecrazy

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If I read this correctly, you are saying that water will freeze at a higher(warmer) temperature due to altitude. What altitude would you have to be at for water to freeze at 39° Fahrenheit? Just curious.

Dan

Yes, that is correct. I'm not sure of the exact number, but I do know there are no roads with a high enough elevation for water to freeze on them at 39°F.
 

TDIFTW

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If I read this correctly, you are saying that water will freeze at a higher(warmer) temperature due to altitude. What altitude would you have to be at for water to freeze at 39° Fahrenheit? Just curious.

Dan
not real sure..... im sure there is a chart out there somewhere that will make it a easy calculation.

as stated above, there may not be ice on the roads but up in the clouds it will be cold enough for sleet or snow to form before it falls to the ground. once it hits the ground it will melt but, while it was in the higher altitude it did freeze. but again there are no roads that are that high up.

basically under the right conditions, water could freeze at 39 degrees, is it anything we will see at sea level..... no, but under the right conditions water could boil at 110 degrees too.
 

MayorDJQ

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This thread is still going? Ice forms at 32*F/0*C, not 39*F/3.8*C.

The problem is that air temperature changes a lot quicker than ground temperature.

Bridges can freeze before regular road surfaces because there can be a <32* air above AND below it, making the bridge structure and road decking <32*F, while the regular road surface may still be above 32* and resist ice formation. If it's 40*F ambient, then the bridge will be 40*F and no ice can form.

The ground can be frozen solid at 0*F and water will obviously freeze on the road. If it's a nice sunny day and the air temps get up to 50*F, the a bridge will warm toward the 50*F air above & below it, if it has enough time and will resist ice formation.

If the ground/road surface is 39*F and the air temp drops to 30*F, ice may accumulate on the road surface.
 

MayorDJQ

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that's generally true, but not always.... ie super cooled liquid droplets in the atmosphere.
But those droplets started somewhere that was under 32*F, just like a snowstorm when it's 38*F. Somewhere up in the atmosphere it was cold enough to freeze water.
 

MonsterTDI09

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But those droplets started somewhere that was under 32*F, just like a snowstorm when it's 38*F. Somewhere up in the atmosphere it was cold enough to freeze water.

If the ground is 40*the snow will melt when it hits the ground.
 

philip_g

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But those droplets started somewhere that was under 32*F, just like a snowstorm when it's 38*F. Somewhere up in the atmosphere it was cold enough to freeze water.
you clearly have never seen this experiment

http://www.youtube.com/watch?v=fSPzMva9_CE

I don't care where it started or where it ends, water exists in a supercooled state well below 32 degrees. What does this have to do with the topic at hand? Nothing. But you said ice forms at 32F always, and that's not true.
 

MayorDJQ

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Pretty cool. I have never seen that video before. It also looks like it melts pretty rapidly.

So how is water supercooled in the atmosphere?
 

tcp_ip_dude

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If the dew point was in the single digits or teens, the air temp at 39F, and it started sleeting, snowing, or raining it would not be unusual at all to see a 7-10 degree temp drop in just a matter of minutes (evaporative cooling in the extremely dry air), as another poster mentioned, driving through even slight elevation changes (valleys), which tend to puddle the cooler air, could contribute to further temp drops in a short time/distance.

Given how fast conditions can change when driving at highway speed, the lag in sensor response time; I find 39F to be a reasonable precautionary alert temp. IMHO.
 
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tdiatlast

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If the dew point was in the single digits or teens, the air temp at 39F, and it started sleeting, snowing, or raining it would not be unusual at all to see a 7-10 degree temp drop in just a matter of minutes (evaporative cooling in the extremely dry air), as another poster mentioned, driving through even slight elevation changes (valleys), which tend to puddle the cooler air, could contribute to further temp drops in a short time/distance.

Given how fast conditions can change when driving at highway speed, the lag in sensor response time; I find 39F to be a reasonable precautionary alert temp. IMHO.
After 44 posts, most of them intelligent (!), I think you nailed it!
 

tdiatlast

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My apologies, steelmb. Your posts came in while I was away 4 days to attend a funeral, and somehow upon my return, I must have skipped over your most erudite posts.

Sorry I lumped you into the "most of them intelligent" crowd w/o proper credit.
 

philip_g

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If the dew point was in the single digits or teens, the air temp at 39F, and it started sleeting, snowing, or raining it would not be unusual at all to see a 7-10 degree temp drop in just a matter of minutes (evaporative cooling in the extremely dry air), as another poster mentioned, driving through even slight elevation changes (valleys), which tend to puddle the cooler air, could contribute to further temp drops in a short time/distance.

Given how fast conditions can change when driving at highway speed, the lag in sensor response time; I find 39F to be a reasonable precautionary alert temp. IMHO.
So how about no alert at all and the drive can judge the conditions instead of chiming at me every CAVU 38 degree day I back out of the garage.
 

Lug_Nut

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Hey guys?
If it's like my '05 Passat, it also dings as the temperature RISES to -4C (25F).
 

All of Us

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Never noticed mine doing that. Only dings on the way down, not on the way back up.

Dan
 
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