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I've never seen frost like this before.


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#1 rjmoses

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Posted 05 October 2020 - 05:04 AM

I went out to feed this morning (4 AM) and saw a pretty heavy frost on the truck. 

 

Came back inside and checked the temperature.  Both my unit by the door and my Davis weather station showed about 40.  The Davis chart showed that it had stayed above 40 all night.

 

Maybe I've not been paying attention, but I'm wondering how there could be frost at 40 degree temps on a black truck.  My thinking is that the truck should have had some residual heat from the day which should keep it frost free down to below 30.

 

So, how could the truck have frost on it if the temperature never got down to freezing?

 

Thoughts?

 

Ralph

(Maybe, it has something to do with same statistics as covid-19?)

 

 


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#2 somedevildawg

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Posted 05 October 2020 - 09:48 PM

Hmmmm.....something fishy in the state of IL. :o. Blame it on the dust?
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#3 r82230

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Posted 06 October 2020 - 08:12 AM

Ralph,  I have no idea of what could cause this, except perhaps truck elevation was 10'-20' below your Davis weather station.  :rolleyes:

 

But as a side note: my internet provider makes my computer 'think' it's in Chicago (or Denver or Minneapolis sometimes).  But with the Chicago one, I get this "fair tax" for all proposal pop up screen.  Realize, I'm not voting in IL, but my thought is, who is fair to?  Doesn't say level or everyone is paying the same percentage, but just fair.

 

Ya, I derailed this thread.  ;)

 

Larry


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#4 slowzuki

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Posted 06 October 2020 - 09:35 AM

On a clear night objects like your truck are radiating heat to space that is near absolute 0, when its still out, the convection heat transfer from air to the surface is slower than the radiation to space.  

 

Go out on a cloudy night and the clouds are way warmer than space and really cut the heat loss to the sky.



#5 rjmoses

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Posted 06 October 2020 - 01:53 PM

On a clear night objects like your truck are radiating heat to space that is near absolute 0, when its still out, the convection heat transfer from air to the surface is slower than the radiation to space.  

 

Go out on a cloudy night and the clouds are way warmer than space and really cut the heat loss to the sky.

That's a thought.  I could see a degree or two, but 8 degrees sounds like a lot.  And the air was 40 degrees.

 

Ralph



#6 slowzuki

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Posted 06 October 2020 - 03:33 PM

It was one of the calcs we had to do in heat transfer classes.  Radiant transfer is a difference in temperature to the 4th power, so big (400 F roughly roughly) difference in temperature drives massive radiant heat transfer on clear nights.

 

Point an IR temp gun at the sky on a clear night for fun.

 

The other factors in the calc are the view factor between the two bodies exchanging heat, so with a wide open clear sky very little warm objects returning heat.  Other factor is the emissivity/reflectivity of the surfaces, a perfect black body radiator is a factor of 1, black truck probably close to 0.8 or 0.9 as its a bit shiny.   A shiny polished mirror surface like a survival space blanket that has low emissivity would be warmer and less likely to have frost on it.   

 

That black truck that soaks up heat better in the sun also radiates heat away better to something colder.

 

That's a thought.  I could see a degree or two, but 8 degrees sounds like a lot.  And the air was 40 degrees.

 

Ralph


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#7 Gearclash

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Posted 28 October 2020 - 06:52 PM

It was one of the calcs we had to do in heat transfer classes.  Radiant transfer is a difference in temperature to the 4th power, so big (400 F roughly roughly) difference in temperature drives massive radiant heat transfer on clear nights.

 

Point an IR temp gun at the sky on a clear night for fun.

 

The other factors in the calc are the view factor between the two bodies exchanging heat, so with a wide open clear sky very little warm objects returning heat.  Other factor is the emissivity/reflectivity of the surfaces, a perfect black body radiator is a factor of 1, black truck probably close to 0.8 or 0.9 as its a bit shiny.   A shiny polished mirror surface like a survival space blanket that has low emissivity would be warmer and less likely to have frost on it.   

 

That black truck that soaks up heat better in the sun also radiates heat away better to something colder.

Being the inquisitive nitwit that I am, and having a good (Fluke) IR gun I did so.  Very interesting.  Of course then I had to try it during the day.  Not much to my surprise the same result!  Except when you point it at the sun which is also an interesting exercise.


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#8 Gearclash

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Posted 30 October 2020 - 08:00 AM

Last night I repeated the above mentioned exercise with the IR gun, only on an overcast sky.  Again, very interesting.



#9 mlappin

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Posted 09 November 2020 - 09:23 PM

Been a strange year, had a hard frost on the truck hood and windshield awhile back, figure the wife will be happy as the tomatoes will be done in finally. Nope, only got the ones on the end of the rows and the rest were fine.






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