What Exactly Is An Air Frost?

[Backtrack]

Hey can someone give me an actuall definition for an air frost? I've heard it lots around the forum, but i actually don't know what one is. Can someone help me out please.

Thanks in advance

SS

[Icer]

I am not 100% sure but I think it is when you breath out you can see the air in front of you. Like a dragon blows fire we blow ice in a way.

[Greyowl]

Definition of air and ground frost from WiKi and BBC weather.

http://www.bbc.co.uk/weather/features/unde...ing/frost.shtml

http://en.wikipedia.org/wiki/Air_frost

[Backtrack]

cheers for the reply's guys, much appreiciated cheers for the links. Icer i don't think its that because frost forms at 0c and you can see it when you breathe at around 5c buy cheers for the reply

SS

[Guest]

Definition of air and ground frost from WiKi and BBC weather.

http://www.bbc.co.uk/weather/features/unde...ing/frost.shtml

http://en.wikipedia.org/wiki/Air_frost

I think that just about covers it - it is surprising how many "mini climate" variations we can have when the temperature is about freezing as in this last snow episode. Some parts of the ground lose their heat more quickly than others, heat escapes from houses into the ground, so we can sometimes end up with a patchwork of snow and mostly the thickest layers appear to be on car roofs.

We can also get the anomoly inside a stevenson screen when the wet bulb becomes frozen but the dry bulb remains above freezing. Whilst I understand the basic principle of evaporation causing cooling, I wonder how much the wind speed will affect this.

As I recall, we made a basic calculation on a slide rule (them were days!) to get the dew point but did not take windspeed into consideration but when I think about it now it must have had some effect.

Mike

[stewfox]

cheers for the reply's guys, much appreiciated cheers for the links. Icer i don't think its that because frost forms at 0c and you can see it when you breathe at around 5c buy cheers for the reply

SS

Someone mention having a air frost at min temp of 1.7c which clearly isnt right

I assume generally ground frosts can occur up to say 3c air temp ??

[Backtrack]

Someone mention having a air frost at min temp of 1.7c which clearly isnt right

I assume generally ground frosts can occur up to say 3c air temp ??

Ground Frosts can occur at temperatures even a little higher than that depending on the temperature of the ground itself and whether it is a clear night. Generaly though it is from around 2c and below. Ground frosts are rare when it's cloudy

SS

[sammie]

We can also get the anomoly inside a stevenson screen when the wet bulb becomes frozen but the dry bulb remains above freezing. Whilst I understand the basic principle of evaporation causing cooling, I wonder how much the wind speed will affect this.

Mike

So if I have this right;

The air movement determins the psychrometer constant which is the pressure of the water evaporated by a 1C cooling.

The faster the wind speed, or the ventalation around your stevenson screen the lower the psychrometer constant is going to be.

Since the lower the psychrometer constant the vapour pressure will drop, which means the RH will drop and therefore the dew point and Twet

will rise.

I think...

Edited November 2, 2008 by sammie

[Geordiesnow]

Can someone tells me the difference between an air frost and a hoar frost?? Am i right in saying a hoar frost is classed as a hoar frost if you see frost on trees for example?.

Ground frosts can occur between 1-4C i think, depend on the ground/grass temperture though.

[sundog]

Can someone tells me the difference between an air frost and a hoar frost?? Am i right in saying a hoar frost is classed as a hoar frost if you see frost on trees for example?.

Ground frosts can occur between 1-4C i think, depend on the ground/grass temperture though.

I think an hoar frost is that really kind of thick frost you can get on the grass etc and i suppose on trees too,i think you get hoar frosts when the humidity is fairly high and of course if its cold enough for a frost.

Like how much frost you get can often depend on how much humidity is in the air,if the humidity is low and the temp was -2c for example you might not get as much frost,if just say the temp was 0c but the humidity in the air was alot higher.

Edited November 2, 2008 by sundog

[Backtrack]

A hoar frost is the frost that occurs on the more say funny objects like trees i think. Is this the frost where the tempeature has to be below zero for though?

[sundog]

A hoar frost is the frost that occurs on the more say funny objects like trees i think. Is this the frost where the tempeature has to be below zero for though?

I dont think to get an hoar frost the temp has to be below 0c,ive seen hoar frosts when the temp was 1 or 2 degrees

[davehsug]

I always understood that hoar frost is basically frozen dew. I.e. air condenses & lays a small amount of moisture which freezes on contact with surfaces below 0c. For an air frost the actual temp of the air must be below 0c. Not 0c itself as that should more accurately be described as melting point rather than freezing point! Liquid water can exist at temps below 0c but ice cannot survive at or above!.

The absence of hoar frost does not mean the temp is not below 0c. If the air is very dry as often used to happen in easterlies, there is no dew or condensation, so no hoar frost.

Dave

[J07]

There's an excuse to show a rather pretty picture of hoarfrost from July 2007, taken by a chap named Geoff Tempest:

[Terminal Moraine]

I always understood that hoar frost is basically frozen dew. I.e. air condenses & lays a small amount of moisture which freezes on contact with surfaces below 0c. For an air frost the actual temp of the air must be below 0c. Not 0c itself as that should more accurately be described as melting point rather than freezing point! Liquid water can exist at temps below 0c but ice cannot survive at or above!.

The absence of hoar frost does not mean the temp is not below 0c. If the air is very dry as often used to happen in easterlies, there is no dew or condensation, so no hoar frost.

Dave

First sentence not quite right, Dave, but the rest of it spot on.

Hoar frost is always crystalline in appearance and occurs as a result of the direct sublimation of water vapour on to objects which have cooled below 0c as a result of ( usually ) nocturnal radiation. Frozen dew retains the shape of unfrozen dew but turns white or whitish due to the freezing.

There is also advection hoar frost which looks similar to ordinary hoar frost but occurs mainly on exposed vertical surfaces and occurs as a result of moist air coming into contact with objects whose temperature is already well below 0c as a result of a period of frost.

Hoar frost is not to be confused with Rime which occurs when supercooled fog or cloud comes into contact with objects whose temperature is below, or very slightly above, 0c .

[Backtrack]

I would just like so say thankyou very much to all you who replied, You have made it easier for me to understand frost in areas where i did not understand before.

I know who to ask in the future

SS

Edit: TM is hoar frost the type of frost you see on the roof of a car? The really sparkly one?

Edited November 3, 2008 by Snow-Storm

[Terminal Moraine]

Edit: TM is hoar frost the type of frost you see on the roof of a car? The really sparkly one?

Certainly is, SS. It's quite possible, and in fact frequent, for hoar frost to occur when the actual air temperature is several degrees above 0c as surfaces such as the metal roof of a car or blades of grass radiate heat and cool faster than the surrounding air. It's for this reason that a grass minimum thermometer ( one exposed to the open air just above the grass surface ) can read several degrees below the air temperature in the thermometer screen, and also why ground frost is more frequent than air frost.

[Backtrack]

Certainly is, SS. It's quite possible, and in fact frequent, for hoar frost to occur when the actual air temperature is several degrees above 0c as surfaces such as the metal roof of a car or blades of grass radiate heat and cool faster than the surrounding air. It's for this reason that a grass minimum thermometer ( one exposed to the open air just above the grass surface ) can read several degrees below the air temperature in the thermometer screen, and also why ground frost is more frequent than air frost.

 Thanks Tm i always wondered whether my temperature gauge was broken because it was reading 2-3c and the car was starting to freeze

Thanks alot for the info

SS