Air Temp e.g +2c BUT road surface - degrees ?!!!!

[Pilatus]

Any one else had nasty experience of air temp showing e.g +2 on in car thermometer BUT road like a skating rink,extremely dangerous situation . Can any one explain the air temp being + degrees BUT surface temp -degrees causing icing?

 

[Frank-the-Wool]

Our roads were like that yesterday morning, my wife did a pirouette around the lanes while taking the grandchildren to school!
I think it is where we get very low temperatures and then the wind increases suddenly from the west causing the air temp to suddenly rise but the ground temp is still way below 0'C.

 

[Pilatus]

Our roads were like that yesterday morning, my wife did a pirouette around the lanes while taking the grandchildren to school!
I think it is where we get very low temperatures and then the wind increases suddenly from the west causing the air temp to suddenly rise but the ground temp is still way below 0'C.

It is very very dangerous situation that can catch us all out,especially if air temp themometer in vehicle is reading +

 

[MachineryDude]

I reckon they'd be quite a lot colder. Try it out, stick your tongue out, put it on the road and have a look

 

[Dry Rot]

Black ice. Ice on the road surface at or just below freezing. Air temperature rises but the road surface remains cold. Pressure of the tyres on the road produces heat which melts the top layer of any ice forming a film of water. That acts like oil and...whoosh!

Up in the hills, we used to drive at 40mph on packed snow. Provided you didn't change speed or corner violently, that was reasonably safe. A different thing when the temperature rose slightly! Even if the road is black and appears safe, watch out for frozen puddles on the side of the road indicating freezing conditions. It isn't called black ice for nothing.

 

[Phil P]

Most vehicles with a thermometer will give an ice warning at +4, I’m sure that’s because surface ice can already be forming due to wind chill etc.

 

[Pilatus]

So despite someone saying wind chill
does not make surface freeze, it does.
I created a thread tha other day asking about wind chill.

 

[Phil P]

So despite someone saying wind chill
does not make surface freeze, it does.
I created a thread tha other day asking about wind chill.

I’m not pretending to be an expert on wind chill but if it made no difference why would they bother to tell you about it on the weather? And it’s usually about 4* less then actually temp which would make sense as at +4 a wind chill would take it to 0.
But I’m only guessing

 

[Goweresque]

I doubt that positive air temperature with a minus wind chill factor would allow ice to form, if the air temperature hadn't dropped below freezing at some point previously. Whereas if it has frozen hard, then the air temp rises, the ground is a large heat sink that will take a considerable time to be lifted above freezing. So after a cold night, the air temperature can rise above freezing the next day, but the ground remain frozen for some time.

Basic physics says water cannot be cooled below the temperature of the air blowing over it, regardless of how fast that wind blows.

https://www.uu.edu/dept/physics/scienceguys/2002Feb.cfm

 

[Gong Farmer]

Black ice. Ice on the road surface at or just below freezing. Air temperature rises but the road surface remains cold. Pressure of the tyres on the road produces heat which melts the top layer of any ice forming a film of water. That acts like oil and...whoosh!

Up in the hills, we used to drive at 40mph on packed snow. Provided you didn't change speed or corner violently, that was reasonably safe. A different thing when the temperature rose slightly! Even if the road is black and appears safe, watch out for frozen puddles on the side of the road indicating freezing conditions. It isn't called black ice for nothing.

Interesting how the definition of black ice has changed over the years. I remember it as specifically ice that forms over an oil patch (in the days when most cars leaked oil onto the road). Then tyre pressure melts the ice as described above and you have water on top of oil. Called black ice because it was harder to see than conventional ice patches.
These days black ice is used to refer to any ice on the road, and I suppose everyone drives so fast now no-one would see it even if it was pink.

 

[phillipe]

Bit of road which doesnt get any sun for month's will be a lot colder ,

 

[Dry Rot]

Interesting how the definition of black ice has changed over the years. I remember it as specifically ice that forms over an oil patch (in the days when most cars leaked oil onto the road). Then tyre pressure melts the ice as described above and you have water on top of oil. Called black ice because it was harder to see than conventional ice patches.
These days black ice is used to refer to any ice on the road, and I suppose everyone drives so fast now no-one would see it even if it was pink.

Not much oil spilt on the hill roads in the Scottish Highlands. At least, not enough to cause a skid. I suspect the definition differs more due of location. When I lived in the hills, if the road was black it usually meant it was free of snow or ice and we'd drive as normal. The danger is black ice which looks like a normal road surface but is actually a thin layer of ice. Our roads didn't get gritted either so it was doubley important to keep an eye out for the signs of frost.

If I remember my physics correctly, isn't "wind chill" another way of explaining the latent heat of vapourisation?

 

[slackjawedyokel]

Objects freeze when the air temperature is over zero due to the radiation (ie. energy) all objects emit. It was recently explained in New Scientist better than I could explain:
https://www.newscientist.com/lastword/mg23631481-000-touch-of-frost/