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Frost above 32 degrees? - Eastern US Weather Forums

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Frost above 32 degrees?


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#1 flsch22

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Posted 31 October 2007 - 12:08 PM

Anyone have a simple explanation for how frost forms on cars/grass when the air temp. is above 32? Noticed this occuring this week. Thanks

#2 snowdude

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Posted 31 October 2007 - 12:12 PM

Yes, on a calm, clear, and cold morning frost can form whether it is 32 degrees or not. Frost forms when the DEW POINT is at or below freezing. So the dew point that morning you noticed the frost was at or below freezing.

#3 Bombogenesis Maximus

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Posted 31 October 2007 - 12:15 PM

 snowdude, on Oct 31 2007, 01:12 PM, said:

Yes, on a calm, clear, and cold morning frost can form whether it is 32 degrees or not. Frost forms when the DEW POINT is at or below freezing. So the dew point that morning you noticed the frost was at or below freezing.



wet bulb temperature...tee hee

#4 forkyfork

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Posted 31 October 2007 - 12:17 PM

areas closest to the ground have the most radiational cooling... so the ground was colder than 32

#5 snowdude

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Posted 31 October 2007 - 12:17 PM

 Bombogenesis Maximus, on Oct 31 2007, 01:15 PM, said:

wet bulb temperature...tee hee

:thumbsup:

#6 snowdude

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Posted 31 October 2007 - 12:19 PM

 forkyfork, on Oct 31 2007, 01:17 PM, said:

areas closest to the ground have the most radiational cooling... so the ground was colder than 32

yeah the temp was most likely 32 degrees directly on the ground combined with the same dew point.

#7 rainshadow

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Posted 31 October 2007 - 12:20 PM

 flsch22, on Oct 31 2007, 01:08 PM, said:

Anyone have a simple explanation for how frost forms on cars/grass when the air temp. is above 32? Noticed this occuring this week. Thanks


Air temperatures are measured at a standardized height of around 5 feet. Cold air settles at night and with light winds it will be colder at ground level than at 5 feet.

A car in essence is almost like a bridge, except for the tires, nothing else is in contact with the ground and can continue to radiate (cool) more than the surrounding air. I don't know if you have signs there about "bridges freezing before road surfaces", same principle for cars that can cool moreefficiently than the surrounding air and reach the dew point temperature.

#8 rainshadow

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Posted 31 October 2007 - 12:21 PM

 forkyfork, on Oct 31 2007, 01:17 PM, said:

areas closest to the ground have the most radiational cooling... so the ground was colder than 32


Mike,

You're too fast, when you are going to get a job here so I can retire and really concentrate on that 372 hr gfs.

#9 dendrite

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Posted 31 October 2007 - 12:28 PM

With strong radiational cooling, ground surface temperature can reach 5-10F colder than the actual 2m air temp.

Frost forms when the surface reaches saturation at a temperature below 32F. If it reaches saturation >32F dew forms. If dew forms and then the temperature continues to drop you get frozen dew.

#10 Lake Effect King

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Posted 31 October 2007 - 12:28 PM

I remember an experiment I did on a perfect radiational cooling type night, where I had thermometers at 1" above the snow surface, 5' above the snow surface and in a tree about 25' above the snow surface. It was 4am and the temp. at the 1" thermometer it was -10 F, at 5' it was -2 F and at 25' it was +12 F !!!! Smoke that was coming out of the chimneys was literally sinking immediately! (eg. BL cap was 0!)

#11 flsch22

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Posted 31 October 2007 - 12:29 PM

 rainshadow, on Oct 31 2007, 01:20 PM, said:

Air temperatures are measured at a standardized height of around 5 feet. Cold air settles at night and with light winds it will be colder at ground level than at 5 feet.

A car in essence is almost like a bridge, except for the tires, nothing else is in contact with the ground and can continue to radiate (cool) more than the surrounding air. I don't know if you have signs there about "bridges freezing before road surfaces", same principle for cars that can cool moreefficiently than the surrounding air and reach the dew point temperature.



thanks, very helpful. So if dewpoint is 25 and air temp is 32, then no frost will form?

#12 jonjon

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Posted 31 October 2007 - 12:31 PM

 dendrite, on Oct 31 2007, 01:28 PM, said:

With strong radiational cooling, ground surface temperature can reach 5-10F colder than the actual 2m air temp.

Frost forms when the surface reaches saturation at a temperature below 32F. If it reaches saturation >32F dew forms. If dew forms and then the temperature continues to drop you get frozen dew.


Cool. I always thought that frost was frozen dew. Learned something new today, thanks.

#13 dendrite

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Posted 31 October 2007 - 12:32 PM

 flsch22, on Oct 31 2007, 01:29 PM, said:

thanks, very helpful. So if dewpoint is 25 and air temp is 32, then no frost will form?
It will form on ground surfaces that have an air temp of 25F to match the frost point of 25F.

If the air temp is 32F, ground surface 25F, and dew/frost point 20F...then correct, no frost will form (unless the temp keeps dropping).

#14 Terpeast

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Posted 31 October 2007 - 12:34 PM

 flsch22, on Oct 31 2007, 01:29 PM, said:

thanks, very helpful. So if dewpoint is 25 and air temp is 32, then no frost will form?


No

The dew and temp gotta be the same, or very close, for dew or frost to form.

#15 dendrite

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Posted 31 October 2007 - 12:35 PM

 jonjon, on Oct 31 2007, 01:31 PM, said:

Cool. I always thought that frost was frozen dew. Learned something new today, thanks.
I always think of frost, dew, frozen dew the same as snow, rain, and freezing rain. Frost/snow form directly into a solid, dew/rain is liquid, fzra/fz dew is liquid that freezes.

#16 Midlo, va. home snow maker

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Posted 31 October 2007 - 12:37 PM

 Lake Effect King, on Oct 31 2007, 01:28 PM, said:

I remember an experiment I did on a perfect radiational cooling type night, where I had thermometers at 1" above the snow surface, 5' above the snow surface and in a tree about 25' above the snow surface. It was 4am and the temp. at the 1" thermometer it was -10 F, at 5' it was -2 F and at 25' it was +12 F !!!! Smoke that was coming out of the chimneys was literally sinking immediately! (eg. BL cap was 0!)


:weenie:

#17 mattmfm

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Posted 31 October 2007 - 03:26 PM

 Terpeast, on Oct 31 2007, 01:34 PM, said:

No

The dew and temp gotta be the same, or very close, for dew or frost to form.

I wouldn't say an absolute no. Just a few days back it was 32/19 and there was frost. Obviously the temperature was a bit colder near the ground, but that is still about a 10 degree gap.

#18 dendrite

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Posted 31 October 2007 - 03:30 PM

 mattmfm, on Oct 31 2007, 04:26 PM, said:

I wouldn't say an absolute no. Just a few days back it was 32/19 and there was frost. Obviously the temperature was a bit colder near the ground, but that is still about a 10 degree gap.
You can get a higher dewpoint near the ground due to vegetation. Still...I would question one of those readings. I don't see how you'd get frost with a dewpoint depression of 10F. Is this PVD data you're going by?

#19 WxUSAF

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Posted 31 October 2007 - 05:05 PM

We had a tiny bit of frost in a shallow gully on grass clippings a week ago with an official low temp of 43F/6C.

#20 mattmfm

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Posted 31 October 2007 - 05:31 PM

 dendrite, on Oct 31 2007, 04:30 PM, said:

You can get a higher dewpoint near the ground due to vegetation. Still...I would question one of those readings. I don't see how you'd get frost with a dewpoint depression of 10F. Is this PVD data you're going by?

Yes. DPs across the region were around 20 while temperatures hovered around freezing.





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