Winter driving. Awareness of diff. betn air and road temps.

[craigowl 0]

For those of you that know this already, apologies. However, if it prevents one of you guys from coming a cropper, it will be worth it.

 

Remember that in the winter half of the year, whilst the air temperature indicated by your car may indicate a temperature above freezing point (0 degrees C), under clear skies when the sun is low or below the horizon, the road surface temps can be one or two degrees colder than an indicated air temperature. (Unless the car designers have taken this into consideration and set the values lower than truth - im not sure if many/any do.)

Below is a graph showing today's shaded air temperature (about 1.2 metres above the ground in this case - the standard height for measuring air temp in Europe), and the corresponding figures recorded by a sensor on a tarmac square at ground level.

 

The 2nd graph for a day in June shows that the opposite occurs. Heat gained during the day mostly keeps the tarmac temp above the air temp, even during the night.

[dr_mat 0]

Unless the car designers have taken this into consideration and set the values lower than truth - im not sure if many/any do.

 

That's what the little "frost" symbol on the MFA display is for - to warn that although the read temp may be above freezing, there may still be ice on the ground..

 

But it's good to make people more aware of little facts like this.

[Jim 2]

Hm.. well I think thats interesting to know, and something I hadn't really ever given any thought to.

 

Thanks :)

[Scruffythefirst 0]

Arent most outside air temp sensors on cars under the front bumper at road levelish ?

[dr_mat 0]

Arent most outside air temp sensors on cars under the front bumper at road levelish ?

 

Yes, because that's one of the few places they can measure the air temp from without it being overly impacted by the heat generated from the car itself, and also not affected by the wind chill of having a sensor in a moving air stream.

 

But they're not at road level, by anyone's standards.

[craigowl 0]

Dr mat said

That's what the little "frost" symbol on the MFA display is for - to warn that although the read temp may be above freezing, there may still be ice on the ground..

Good call doc! Believe it or not, I have not seen the little frost symbol, merely because the C is cuddled up in the garage in nasty weather - that is for little Pugrot!

Anyway, that is the forum working well, we are learning things from each other. :) 8)

 

Scruffit said

Arent most outside air temp sensors on cars under the front bumper at road levelish ?

That is what I had read, too. Never actually got down on my hands and knees to look, evemn though I wear an anorak :) b However, when I have been out in the C the temperature on the MFA has corresponded well with the air temperature recorded by my air temp sensors at the 4foot level. We need to remember, though, that on clear, still nights and mornings, the temperature gradient between the ground and 4 ft above it can be really steep. An "official" air minimum temperature of, say, plus 2C can see tar/concrete temperatures of minus 1 C and grass turf figures of minus 3 C.

Any way, whatever you believe, be observant and take care.

[Musicman 0]

And bridges will be colder still overnight

[TomB 0]

Nice one. Thanks for that as I'll be doing a lot of driving over Christmas and was confidently bowling along yesterday as the outside temp was reading 3 degrees.

Cheers,

Tom

[craigowl 0]

musicman said

And bridges will be colder still overnight

 

Very good point geetarman 8, and not many people will know that! Well worth spreading the word.

Because bridges merely have air below them, which is a poor conductor of heat, they cool off a lot more on radiation nights than does anything in direct contact with the ground, such as the highway. Even on the coldest of nights there is an upward flux of heat from below the earth's surface trying to balance the loss of heat to space from the earth's surface. Car roofs and bonnets behave like bridges, too.

Further, a blanket of fresh snow, full of air, acts like a roll of insulation, stopping the earth's warmth from escaping upwards. Most or all of our lowest temperatures occur when there is a good snow cover.

Gritting and salting authorities are usually aware of the need for extra salt at certain spots - or where the bridge is made of metal - a non-corrosive deicing chemical.

Safe driving all.

After a couple of days of milder windy weather, you may have noted that colder, wintry conditions are expected before Christmas.

On Saturday, bookies were only giving 2 to 1 for snow on Christmas day. Consultants are giving them too much help!

A few years ago I dont think you saw odds as low as 2 to 1 a week before Christmas.

A couple of my former colleagues won over £1k a few years ago - it made the front page of the Scottish Sun and our boss was not pleased!

[bcstudent 0]

Arent most outside air temp sensors on cars under the front bumper at road levelish ?

 

Yes, because that's one of the few places they can measure the air temp from without it being overly impacted by the heat generated from the car itself, and also not affected by the wind chill of having a sensor in a moving air stream.

 

But they're not at road level, by anyone's standards.

 

Aren't the Corrado air temp. senders in the off-side brake cooling duct? Surely that constitues "in a moving air stream."

[craigowl 0]

Common misunderstanding about windchill temperature.

 

It does not matter whether the air is still or moving at, say, 40 knots (46 mph) - the temperature sensor, or one of the older mercury in glass thermometers will still read plus 3 C if the temperature of the air is plus 3 C.

However, the calculated "wind-chill equivalent temperature" will be about minus 7 C. 8) 8) 8) 8) 8) 8) 8) 8)

This is what the temp will "feel like" to a human being. Ice would not form except if the person's clothes were wet and the humidity of the air was extremely low. The freezing being caused by the removal of heat by latent heat of evaporation.

[Musicman 0]

... and don't forget that, taking the adiabatic lapse rate (0.6 degC/100 m) into consideration, the roof of your car will be approximately 0.009 degC cooler than that indicated by the temp sensor ;)

 

[do you work for the Met office, Craig?]

[Kevin Bacon 5]

...[do you work for the Met office, Craig?]

 

There's a member on here called 'metman' that does.....

[dr_mat 0]

Common misunderstanding about windchill temperature.

....

This is what the temp will "feel like" to a human being. ... caused by the removal of heat by latent heat of evaporation.

 

Good point, I guess I'd never thought about it!

[craigowl 0]

musicman said

[do you work for the Met office, Craig?]

Not now. Retired. My name isnt Craig, Craigowl is a hill outside my home town, Dundee.

Perhaps I should change my name to Schecter, but I still can't play it.

Pic of Craigowl to follow later.

[craigowl 0]

2 pics taken on Craigowl in about December 1965.

First shows view to north towards Grampian mountains.

Car is Cortina mk1 estate. Not mine - I didnt have car for another 5 years.

 

2nd pic is looking sw to River Tay.

[dr_mat 0]

I always liked Scotland. Pity it's so damn far away!

[Musicman 0]

Craigowl is a hill outside my home town, Dundee

 

Erk. Too cold for my tastes.

 

My brother decided moved to a village outside Aberdeen a few years ago - I think I prefer the extra few degrees in London or at least somewhere nice on the west coast like Dumfries or Portpatrick ;)

[craigowl 0]

Kevhaywire said

There's a member on here called 'metman' that does

metman is dead :cry: - reincarnated as Craigowl.

dr mat said

always liked Scotland. Pity it's so damn far away!

Berkshire pubs are 8) 8) . I have been married to Berkshire woman for 35 years, too. :)