I guess people on the BR are more aware than most, but the temperature's dropped below freezing tonight and it seems to have caught out the local authorities. Major A-roads around Oxford are un-gritted and covered in ice, and I've already seen one car in a ditch this evening while on a midnight ride.
That is all.
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Thanks David
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Yep, -1°C here in South Leicestershire at 5am. Smaller local roads are slippery, but the ice is visible - it is not black ice. Major roads and Motorways are clear, though.
It fascinates me that every year people are surprized by ice when driving in the winter?!?!?
I've never come a cropper on snow/ice in 22 years of driving. 4 of those I lived in Switzerland.
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Just out of interest, can someone tell me the temperature at which a damp road surface starts to freeze?
I was heading home late the other night. The car had been standing outside a friend's house and wasn't frozen and the aftermarket outside temperature sensor showed above freezing but the road surface was definitely turning icy.
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At an air temperature of about four degrees I would suggest.
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Yes, that sounds about right.
Beware all...
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Does 'wind chill' make a difference to the freezing point of water then?
gb now enters bunker for duration.(cant remember who stirred the washer jet debate over wind chill)
Only caught out once by black ice 20 years ago, and luckily i didn't get involved in the multiple pile up that followed, more luck than judgement i assure you, i just slithered past, very scary took about half a mile to stop from 60mph. It had rained for about 2 days non stop, and very suddenly the sky cleared and everything froze at about 2 am. That was by the 'Caxton Gibbet' very open there.
I think black ice is probably the most dangerous of any surface problem (except maybe spilled diesel on a wet road).
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put a portable fan in your chest freezer and see if the thermometre moves? wouldnt of thought so, but on a similar point why do fan assisted ovens need to be set at a lower temp to avoid burning the food?
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Does 'wind chill' make a difference to the freezing point of water then?
No - Number Cruncher gave a very convincing scientific explanation last week in the frozen washer thread.
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No - Number Cruncher gave a very convincing scientific explanation last week in the frozen washer thread.
I know PU i just couldn't resist it.....sorry.
PS it was me who suggested it anyway...double sorry.
Edited by gordonbennet on 06/01/2008 at 15:06
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MJ's method might a simpler one than NC's to remember though !
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I'd concur with somewhere around 4 degrees - seems to be the point at which most OAT guages ping, show a snowflake etc. Bear in mind though that you can still be caught out by local frost pockets where various factors (shelter, north facing, standing water etc) lower the temperature over just a few metres. We have one locally 200 metres from end of our road where lane crosses the Grand Union canal in a well sheltered hollow. Car can be ice free at home but trigger ABS and/or slither alarmingly if slowing for bridge.
Rare for a winter to pass without the bridge parapets loosing a few stones.
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Just out of interest can someone tell me the temperature at which a damp road surface starts to freeze?
Not really. It depends on many things. It can often be that the air temperature falls below freezing, but the road surface does not freeze - the road is a resevoir of heat, after all - this is often seen when, for instance, your car is iced up, but the road is not. This can also work in reverse - after a cold spell, the road will take longer to warm up than the air, which is where the very misleading 4 degree declerations come in. The prescence of any salt will affect the freezing temperature to some extent, too.
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Yes -VW think it's 4 degrees C (or 39F in proper degrees) as the outside temperature indicator goes ping when the air temp goes down to this level. And a little star comes on, so today, Epiphany Sunday, the wise men had better watch out.
Edited by Avant on 06/01/2008 at 15:02
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Any motorcylist will tell that there is a perceptible smell in the air when there's ice around. That and God's ice detection devices (the finger tips)
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"Just out of interest, can someone tell me the temperature at which a damp road surface starts to freeze?"
"At an air temperature of about four degrees I would suggest."
"seems to be the point at which most OAT gauges ping"
Since I think it was me that first raised the point about windchill only applying to the human skin I hope you will allow me to ask
1. Why would road surfaces begin to freeze at an air temp of 4deg? Especially if your temp sensor is only a metre above that surface? It could only (possibly) happen in perfectly still air, any slight breeze (or even perhaps the air disturbance caused by traffic) would mix the air sufficiently to keep ground temp above freezing wouldn't it?
2. Since 0 deg is both the melting point of ice and also the freezing point of water the temp has to be below 0 deg for the water on a surface to freeze doesn't it??
3. The presence of salt will reduce the freezing temp still further, while (I think) causing more condensation on the road surface (something to do with the saturated vapour pressure of air in contact with salt??)
4. What temp an OAT gauge pings at is surely set by the manufacturer (mine flashes at 3 deg, not 4 I think) as a warning, not to say that freezing is actually taking place??? Bit like me setting my alarm for 10 minutes before I actually have to get up tomorrow!
I await education on these points!
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Phil,
Cannot answer all these questions but can go some of the way.
1. Temp sensor on my Xantia (and presumably yours!) is under the nearside door mirror. Even if it were only inches above the road I suspect it can be affected by heat from the cabin/engine - so the mfr build in a margin for error. The OAT guage will not track the real air temperature immediately; the display is "buffered" so it's not up and down like a fiddlers elbow in sun/shade.
2. Water freezes at 0 degrees C but temperature can vary by several degrees over a few metres - see my example above regarding the local canal bend (and I had a similar issue when I lived in Watford also by the canal)
3. Salt/grit is not universal - another issue at the canal bend.
4. I think the alarm analogy is exactly right. The ping point is set to give a warning of the point where, if all the margins for luck are against you you might just hit ice.
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