A Strange Frost Tonight

[stewfox]

Do I REALLY have to go outside and take a picture of someone's car like a weirdo?

Mine this morning at +5c For the non believers

Edited November 6, 2011 by stewfox

[Backtrack]

Mine this morning at +5c For the non believers

Could of sworn it was more like 5.3C Stew.

[The watcher]

Could it be DP's below or at 0c?

[Backtrack]

Dewpoint is around 2C. The air temperature is 3.6C now.

[weatherobsuk]

Buxton was a warm 10.1'c in the sun today, crystal clear skies and barely a cloud to be seen. Past midnight now and its slowly dropping past 1.1'c steadily. With minimas another six hours to come.

[weatherobsuk]

Just been out with the camera getting some exposure images of the Moon and Jupiter. No doubt a frost starting to form on the tops of the roofs and cars, grass was still wet as was the ground so not significantly low in temp or dp's to be freezing that up..yet.

[Liam J]

Dewpoint is around 2C. The air temperature is 3.6C now.

http://www.theweathe...om/habyhints/5/

When temperatures drop below freezing and the temperature reaches the dew or frost point, the ice on the ground is termed frost or frozen dew. "Frost" can form in two ways: Either by deposition or freezing. Depositional frost is also known as white frost or hoar frost. It occurs when the dewpoint (now called the frost point) is below freezing.

Frost forms when the dew point is at or below freezing, and the air temperature comes to or below the dew point temperature.

Edited November 7, 2011 by Liam J

[Stormyking]

Frosty here well in our fridge and freezer anyways..... :L

Anyways I have seen frost form at 5c and below before as well so IMO it can happen

[Ice-Bore]

On a clear still night heat energy radiates from surfaces on the Earth in the form of longwave radiation. This infrared (heat) energy effectively radiates through the atmosphere to outerspace. As a result the surface temp of a car at night may be at or below the FP, when the OAT is actually 4-5 deg. C.

During daylight the opposite occurs and shortwave radiation, in the form of visible and ultraviolet light from the Sun, is absobed by the Earth and the surface temperature of the car may, under certain conditions, be warmer than the OAT.

Presumably the longwave radiation that takes place at night must equal the shortwave radiation during the day or we'd have a big problem..!!

[Optimus Prime]

On some objects the temperature isn't always directly proportionate to air temperature. What has been stated correctly is metal loses heat very fast.

If you get a clear, chilly night you can get a surface frost at relatively high temperature as long as some criterion are met;

• NO wind
  
• Mustn't be raining
  
• Little or no sunshine
  
• Temperature is below dewpoint
  

ect...

Compare a metal surface to say, concrete for example. Concrete loses heat more slowly (or retains heat) If concrete has been laying out in the sun for a while it will have adsorbed the suns heat thus it won't support a frost as fast than steel objects for example, even at a temperature of say -0.1c.

December 2009 was the first time this century I saw walls covered in frost from top down to the middle. That was due to a prolonged very cold spell of weather with low overnight minimum. It takes a long period of cold weather for concrete to start to freeze. Cars on the other hand support a frost really rather easily and the temperature doesn't have to be that low.

[4wd]

Frost can frequently form under clear skies well before the air temperature gets anywhere near freezing.

The typical home thermometer won't be sited strictly 'correct' by met office standards so accuracy of readings is open to doubt - but I would expect it to be reading the temperature at it's location no more than 1C out (either way!).

As an example of how siting affects readings, I have an electronic one at the back door which in similar conditions can read as much as 3C higher than a Vantage Vue more correctly sited away from the house.

The Vue can still read 4C higher than a thermometer near the ground in clear and still conditions.

Edited November 7, 2011 by 4wd

[johnholmes]

The post by 4wd really explains why frost forms on grass and cars before the temperature reaches zero C. I'm not sure why 1 or 2 of you find this hard to understand but please ask questions. There is a difference between what is called 'ground frost' and air frost. I do think that the initial value quoted with frost on the car was too high, its always difficult to get correct readings as amateurs, hard enough at times in the professional situation. I would not really expect ground frost to show above about 3C but it is very dependent on a whole load of factors.

[johnholmes]

http://weatherfaqs.org.uk/node/26

the link above gives as good an explanation as you will get anywhere-a highly reliable web site

[Backtrack]

Thanks for that 4WD and John.

As soon as I moved my temperature away from the wall (which was radiating heat) the temperature fell by about 1C. Unsure if that helps!

[johnholmes]

Thanks for that 4WD and John.

As soon as I moved my temperature away from the wall (which was radiating heat) the temperature fell by about 1C. Unsure if that helps!

yes it shows that the value you quoted was too high B, the thermometer needs to be away from any area likely to create anomalies like occurred with you last night. Like I said its not easy to get the position of the thermometer correct for we amateurs, small gardens, lots of other houses and buildings close by, trees etc etc.

[Backtrack]

yes it shows that the value you quoted was too high B, the thermometer needs to be away from any area likely to create anomalies like occurred with you last night. Like I said its not easy to get the position of the thermometer correct for we amateurs, small gardens, lots of other houses and buildings close by, trees etc etc.

Since I live with my parents, the best I can do is put it on the wooden gate outside at around 4ft. It's right next to a wall and surrounded by houses.

This will probably be affecting the temperature will it not?

[Liam J]

So I was partly right in saying that BT's readings would have been to high for frost to form, Dew P 2c air temp 4/5c.

Thanks for moving the sensor BT and getting the correct reading.

Edited November 7, 2011 by Liam J

[Backtrack]

So I was partly right in saying that BT's readings would have been to high for frost to form, Dew P 2c air temp 4/5c.

Thanks for moving the sensor BT and getting the correct reading.

I recorded that 4.5C after I moved it. And in reality, it's still in a bad position, and was probably still too high.

[Liam J]

I recorded that 4.5C after I moved it. And in reality, it's still in a bad position, and was probably still too high.

it just shows how buildings etc radiate the heat that gets stored through the daytime.

I have quite an open garden out back and I've got one of my sensors on the wooden fence about 2m above the ground away from any buildings, this time of year it's always in the shade.

I also have one between my garage and next doors garage on the fence and this one was showing +2.5c higher than the other two.

Edited November 7, 2011 by Liam J

[picog]

I recorded that 4.5C after I moved it. And in reality, it's still in a bad position, and was probably still too high.

That's strange because I also had ice/frost on the car (dark grey) when it was about 4C at 8pm last night. I actually went out to touch it to check that it really was frost/ice. It was not until about 11pm that the actual air temp fell below 0C.

Edited November 7, 2011 by picog

[TonyH]

A frost on the lawn the other evening with an air temp of +4c.

I have seen ground frosts reported from official weather stations when the air minima have been above +6c.

[greybing]

My rubbish thermometer is about 4metres above ground because im one floor up and it is also very close to the brick work of my bulding ,though air is able to circulate fully around it .Would my positioning of ithave an influence on my readings and if so what could I do to improve ,ie ,pertianing to position etc,thanks.Also I make sure no sunlight falls on it though it is not wind proofed.

Edited November 7, 2011 by greybing

[johnholmes]

My rubbish thermometer is about 4metres above ground because im one floor up and it is also very close to the brick work of my bulding ,though air is able to circulate fully around it .Would my positioning of ithave an influence on my readings and if so what could I do to improve ,ie ,pertianing to position etc,thanks.Also I make sure no sunlight falls on it though it is not wind proofed.

you could try, as you seem to suggest, putting some insulation around the thermometer, both to keep the reflected heat from the wall affecting it and direct sunlight, provided you can keep air flowing around it. Maybe another way is to take your readings at set times each day and compare them to the nearest accepted weather station. Then simply correlate your readings with that?

Edited November 7, 2011 by johnholmes

[greybing]

not the surface of the car, also interesting that cars of different colours will form frost at different times on the same night

I have noticed this also ,I thought I was going crazy but eventually thought about it and ,yes ,its true.

you could try, as you seem to suggest, putting some insulation around the thermometer, both to keep the reflected heat from the wall affecting it and direct sunlight, provided you can keep air flowing around it. Maybe another way is to take your readings at set times each day and compare them to the nearest accepted weather station. Then simply correlate your readings with that?

The last thing you mention is something i do.Thankyou for your input.