6 replies to this topic

[greybing]

I have noticed that frost seems to nly form when clear skies and light winds are present.Now I know radiation occurs from stuff and cools the air This heat this heat then goes into space at night and in light winds and and clear skies [and presumambly the higher the humidity ,the nore forsty it will be]frost forms.
I'm clearly ignorant on this issue to some degree but ,say for examle it is -4 ,clear,no wind then and frost forms.Some cloud moves in and the tenp hovers at-2,[all else being equal,[ie wind ,or lack of] why does frost not form when this cloud moves in even when the temperature is low enough for it to still form. Say the frost formed earlier and this could stick around bu no more forms..why? .
Dew point,is this something to do with it.Am I imagining this phonomenom.
Anyway ,thanks for getting to thre end.

Edited by greybing, 04 February 2012 - 01:45 .

[greybing]

54 people look but no one has a word to say.Wow

[Terminal Moraine]

It is all to do with the dew point. I assume you are talking about hoar frost, the white crispy stuff which most people associate with frost? Hoar frost will only form if the temperature falls below the dew point and the moisture in the air is condensated onto surfaces and then freezes.
It's possible to have hoar frost with the temperature only just below freezing if the dew point is relatively high but it's also possible to have no visible frost at all with a temperature of -6c if the dew point is very low . In the first case the temperature has only to fall a small amount for condenstation and freezing to occur but in the second case, even at -6c, the temperature is not low enough for condensation to occur and there is no visible frost.
Hoar frost will occur with air temperatures above 0c as things such as cars, grass blades etc radiate heat more readily than air and can be cooled to below 0c even with the air temperature several degrees above it ( it's for this reason that ground frost is more frequent than air frost )
You might be scraping the frost from your car windscreen in autumn with an air minimum of +1c but in a dry, cold easterly in winter it will be free of frost at -4c.
I'm not quite sure what you were getting at regarding cloud moving in but, if a visible frost forms under clear skies with a temperature of -4c, and then cloud moves in and the temperature rises to -2c, the frost will be maintained provided the dew point does not fall so low that the frost evaporates ( unlikely with a cloud cover unless very dry air moves in from the Continent under cloudy skies).

[greybing]

Terminal Moraine, on 06 February 2012 - 19:48 , said:

It is all to do with the dew point. I assume you are talking about hoar frost, the white crispy stuff which most people associate with frost? Hoar frost will only form if the temperature falls below the dew point and the moisture in the air is condensated onto surfaces and then freezes.
It's possible to have hoar frost with the temperature only just below freezing if the dew point is relatively high but it's also possible to have no visible frost at all with a temperature of -6c if the dew point is very low . In the first case the temperature has only to fall a small amount for condenstation and freezing to occur but in the second case, even at -6c, the temperature is not low enough for condensation to occur and there is no visible frost.
Hoar frost will occur with air temperatures above 0c as things such as cars, grass blades etc radiate heat more readily than air and can be cooled to below 0c even with the air temperature several degrees above it ( it's for this reason that ground frost is more frequent than air frost )
You might be scraping the frost from your car windscreen in autumn with an air minimum of +1c but in a dry, cold easterly in winter it will be free of frost at -4c.
I'm not quite sure what you were getting at regarding cloud moving in but, if a visible frost forms under clear skies with a temperature of -4c, and then cloud moves in and the temperature rises to -2c, the frost will be maintained provided the dew point does not fall so low that the frost evaporates ( unlikely with a cloud cover unless very dry air moves in from the Continent under cloudy skies).

Thank you for the reply .Pretty much all of what you said I did know already .Alas my question is un answered .Possibly the cloud changes the dewpoint,as you imply,[though unlikely you say] so it is higher than the air temperature.So that the frost stops forming despite the still low enough temperature for frost. If that were the case then you would have known this though.Maybe it is all in my head.Thank you again.

Edited by greybing, 06 February 2012 - 23:26 .

[Terminal Moraine]

greybing, on 06 February 2012 - 23:23 , said:

Thank you for the reply .Pretty much all of what you said I did know already .Alas my question is un answered .Possibly the cloud changes the dewpoint,as you imply,[though unlikely you say] so it is higher than the air temperature.So that the frost stops forming despite the still low enough temperature for frost. If that were the case then you would have known this though.Maybe it is all in my head.Thank you again.

It's not possible ( under normal atmospheric conditions ) for the dewpoint to be higher than the ambient air temperature as, once the two are the same, the air is completely saturated and condensation begins.
When you refer to 'the frost stops forming' do you mean the thickness of the visible frost remains the same and does not continue to increase?
If a layer of cloud moves in to cover previously clear skies the radiation of heat from the ground will stop or be greatly reduced and the air temperature will rise. Assuming the layer of cloud is not the result of a moister airmass moving into the area the dewpoint will remain the same and, as warmer air has a greater capacity for holding moisture than colder air, condensation will cease and the hoar frost will stop forming. The hoar frost which has already formed will then remain the same provided the temperature remains below 0c.

[Interitus]

Just to add to TM's excellent explanations, even if there is no change in temperature and dew point, if the object stops radiating heat due to cloud cover the surface will assume the same ambient temperature as the air which being equal or higher than the dew point means no further condensation. As an example, if you were to introduce a colder object then frost would still form - such as taking something out of the freezer which can covered in frost at room temperatures and dew points well above freezing.

Actually in this cloudy scenario where there is limited radiation the amount of frost can decrease by sublimation when the dew/frost point is below the air temperature, no matter how cold it is.

[greybing]

Interitus, on 07 February 2012 - 20:18 , said:

Just to add to TM's excellent explanations, even if there is no change in temperature and dew point, if the object stops radiating heat due to cloud cover the surface will assume the same ambient temperature as the air which being equal or higher than the dew point means no further condensation. As an example, if you were to introduce a colder object then frost would still form - such as taking something out of the freezer which can covered in frost at room temperatures and dew points well above freezing.

Actually in this cloudy scenario where there is limited radiation the amount of frost can decrease by sublimation when the dew/frost point is below the air temperature, no matter how cold it is.

interesting