-2C or -12C...What's the difference?

[TripleS]

So would you change your driving because the temperature was lower?

I ask as I've seen people driving down perfectly clean/dry (looking) motorways doing 45/50 in the last couple of days when the temperature has been probably the lowest the majority of Brits have seen. Other days at around freezing- minus 2 and they quite happily drive quicker.

I'd adjust speed dependant on the surface damp/dry] more than ambient temperature - although that would involve being able to see, and significantly and effectively - change speed if the surface deteriorates. I've happily [motor] biked in -6C IIRC - but on dry roads.

But I also keep a wary eye on the car's ext. temp. gauge - it's often a useful warning of weather changes let alone frost.

It is only during the last few years that I've had a car with an outside temperature indicator, and although I welcome the device I don't feel it adds much of great practical value for me. I'd already spent a great many years learning to judge road surfaces by appearance, and by changes in how the car feels and sounds, and I still work mainly on that basis.

Best wishes all,
Dave.

[michael769]

Bear in mind the weather forcast gives "air temperatures in the shade at 1.5 meters from the ground".

Ground/road temperatures are always lower. Typically grass that is growing in sandy soil starts to get frost
on it when the air temperature is 4 degrees.

Indeed in Central Scotland frost will start to form on cars and untreated roads at about 2.5 deg and below. This can be affected by humidity though, if the humidity is very low frost will sometimes fail to form until well below freezing. For example most of last week our daytime temperatures failed to rise above -5, despite this my car which sat for about 9 hours in such temperatures remained completely clear of frost for 3 out of the 5 days.

The important figure is the dew point (some forecast sources will display it, the BBC and the met do not). If the dew point falls below 0 ice and frost will form on surfaces. If the air temp falls below the dew point fog will form.

[c8 sdd]

Thanks for that info michael.

I have one of these new-fangled weather station things with windspeed/rain/four remote sensors,etc.

One of the sensors is stuck up on the felt roof of our heavily insulated extension.

A few weeks ago the 1.5meter air temperature was showing -5.0c ... the roof temperature was showing -14.0c,
although after that a few clouds drifted over and an hour later the two temperatures were -3.0/-7.0 ...... which
just shows how slight cloud cover can cause temperatures to rise again.

[Mr Cholmondeley-Warner]

According to Transport Scotland salt starts to become less efficient below -6 (presumably it takes longer to melt snow and ice), at -9 and below it will not melt ice or snow but will still stop frost forming, and at somewhere between -12 and -15 depending on the concentration it stops working altogether.

Thanks for the info Michael. I just always assumed that salt melts ice no matter the temperature.

Mitch

Lots of science here. Scroll down to the section entitled "The phase diagram for sodium chloride solution". Here you can see that salt no longer prevents freezing below -21.1 degrees C, and that its maximum useful concentration is 23.3% (way higher than is ever likely to be achieved by road salting). At lesser concentrations there's a corresponding rise in the freezing point - left hand section of the line.

Hope that's not too abstruse and helps a little.

[waremark]

Cannot remember at all where I heard this, but I have heard that ice and snow are less slippery at very low temperatures. So when the salt stops working, it may be less necessary. Has anyone else heard that?

[TripleS]

Cannot remember at all where I heard this, but I have heard that ice and snow are less slippery at very low temperatures. So when the salt stops working, it may be less necessary. Has anyone else heard that?

I suppose the lower the temperature the less chance of having a film of water on top of the ice and snow. Hard, polished, ice with water on top is about as bad as it gets, I think.

Best wishes all,
Dave.

[c8 sdd]

Some of the snow we've had in the past week has fallen through very low temperature air, and as a result has
been extremely powdery.

This sort of snow falling onto a dry tarmac surface doesn't appear to be as slippery as our usual snow that
falls around freezing point (big flakes are indicative of partial melting on the way down, causing flakes to stick together).

Regardless, we had partially melted snow refreeze in our drive the other week and that was the ultimate slippery surface!!

[7db]

According to Transport Scotland salt starts to become less efficient below -6 (presumably it takes longer to melt snow and ice), at -9 and below it will not melt ice or snow but will still stop frost forming, and at somewhere between -12 and -15 depending on the concentration it stops working altogether.

Thanks for the info Michael. I just always assumed that salt melts ice no matter the temperature.

Mitch

Lots of science here. Scroll down to the section entitled "The phase diagram for sodium chloride solution". Here you can see that salt no longer prevents freezing below -21.1 degrees C, and that its maximum useful concentration is 23.3% (way higher than is ever likely to be achieved by road salting). At lesser concentrations there's a corresponding rise in the freezing point - left hand section of the line.

Hope that's not too abstruse and helps a little.

It's nearly 20 years since I studied eutectic mixes -- in fact concentrations of 23.3% are very common as you get to that low temperature. The solution concentration "slides down" the phase diagram as the temperature falls; an initially weak solution freezes out pure ice increasing the concentration of the remaining solution, until it reaches the eutectic point and the whole lot goes solid. At -21.1 deg C, there's an awful lot of pure ice and solid salt solution at 23.3% concentration.

Of course all of this is at standard pressure. It's several bar underneath your wheels which promotes melting (think about how ice skates work).

[TripleS]

Some of the snow we've had in the past week has fallen through very low temperature air, and as a result has
been extremely powdery.

This sort of snow falling onto a dry tarmac surface doesn't appear to be as slippery as our usual snow that
falls around freezing point (big flakes are indicative of partial melting on the way down, causing flakes to stick together).

Regardless, we had partially melted snow refreeze in our drive the other week and that was the ultimate slippery surface!!

I'm not sure about that. I reckon it's even worse when the thaw starts again, and there's a film of water on top.

Best wishes all,
Dave.

[TripleS]

....think about how ice skates work.

No thank you. I fall down at the mere thought of it.

It's difficult enough for me remaining an upstanding citizen.

Best wishes all,
Dave.

[michael769]

Of course all of this is at standard pressure. It's several bar underneath your wheels which promotes melting (think about how ice skates work).

And of course tyres are warm, and you have all that friction heating going on too. Real world physics are pretty complicated compared to the lab.

I suspect Transport Scotland's figures are based on what they observe happening on the ground, and are more to do with how well the salt is keeping roads safely passable over the network. It is certainly you would hope they have a good handle on as they set the requirements of winter maintenance on Scottish trunk roads, and it get pretty chilly up here in winter. Especially in the Highlands, the Nothern and Western Isles and Argyll.

[7db]

Of course all of this is at standard pressure. It's several bar underneath your wheels which promotes melting (think about how ice skates work).

And of course tyres are warm, and you have all that friction heating going on too. Real world physics are pretty complicated compared to the lab.

I think the heat transfer will be negligible, however, as the tyre is in contact with the ground for such a short period of time and the temperature difference is quite low. I'm not sure what frictional heating you are referring to - there won't be a lot of sliding between tyre and road - most of the heat in the tyres is rubber hysteresis.

[michael769]

I think the heat transfer will be negligible, however, as the tyre is in contact with the ground for such a short period of time and the temperature difference is quite low. I'm not sure what frictional heating you are referring to - there won't be a lot of sliding between tyre and road - most of the heat in the tyres is rubber hysteresis.

There is always some slip. Although the level of grip between tyres and a road is excellent it is not perfect and there will always be some slip albeit at a level that is imperceptible to the driver or any observers.

I have wondered how much heat transfer there is. When you see cars in thermal imaging cameras from helicopters there is quite a visible (although short lived) hot trail left on the road from the wheels. Of course the IR cameras may well be configured so as to exaggerate the raise in temperature somewhat - and of course such films usually involve vehicles being driven well beyond what is normal for road driving.

[TripleS]

I think the heat transfer will be negligible, however, as the tyre is in contact with the ground for such a short period of time and the temperature difference is quite low. I'm not sure what frictional heating you are referring to - there won't be a lot of sliding between tyre and road - most of the heat in the tyres is rubber hysteresis.

There is always some slip. Although the level of grip between tyres and a road is excellent it is not perfect and there will always be some slip albeit at a level that is imperceptible to the driver or any observers.

I have wondered how much heat transfer there is. When you see cars in thermal imaging cameras from helicopters there is quite a visible (although short lived) hot trail left on the road from the wheels. Of course the IR cameras may well be configured so as to exaggerate the raise in temperature somewhat - and of course such films usually involve vehicles being driven well beyond what is normal for road driving.

Yes, part of the time.

The driving sometimes exhibited by some of these wrongdoers might be quite reasonable in other circumstances, so I don't think that what you see is necessarily to be condemned totally.

I am not seeking to defend grossly irresponsible driving, but to be fair I think what we should do is look at the whole scene or sequence, and pick out the really bad bits and not bother with the rest.

Best wishes all,
Dave.

[Renny]

It is only during the last few years that I've had a car with an outside temperature indicator, and although I welcome the device I don't feel it adds much of great practical value for me. I'd already spent a great many years learning to judge road surfaces by appearance, and by changes in how the car feels and sounds, and I still work mainly on that basis.

Best wishes all,
Dave.

Luddite

I suppose you drive with the side window cracked open so you can hear if the tyres suddenly go quiet?

That is scary