Frost on roof window melts in lines

  • #1
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Hi.

On a cold morning, I saw frost melting on my roof window. Why did it melt in regular lines? Might the glass be slightly wavy?

IMG_20210414_090954.jpg
 
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Answers and Replies

  • #2
phinds
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Hm ... interesting. Glass sheets do go under rollers during the creation process so maybe one of the rollers left just the tiniest differences in thicknesses along the surface.
 
  • #3
Keith_McClary
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Is that flat glass or a domed skylight (often plastic)?
Glass sheets do go under rollers during the creation process
Before float glass, 1960s?
 
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  • #4
phinds
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Glass. I assume plastic sheets are created the same way but do not know for sure.
 
  • #5
Keith_McClary
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Could there be streaks from water running down it? These might freeze preferentially.
 
  • #6
phinds
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Could there be streaks from water running down it? These might freeze preferentially.
Seems too regular for that
 
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  • #7
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Might it be that the vacuum between the glass is leak (is there an actual vacuum? Or just some special gas?) and you get these thermal convection cells? Maybe with some nice vertical vortex structures because of the slanted angle?
 
  • #9
sophiecentaur
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(is there an actual vacuum?
No vacuum. The sheets would bend due to atmospheric pressure. There is a very modern design with very low pressure inside but it has stainless steel internal spacers. There was a posting about that on PF some while ago - not applicable here, I think.
It is a fascinating problem. I wonder if this is repeated on a daily basis and if it correlates with the windows being cleaned.
Could it be due to sunlight through bars on the roof or a similar effect due to an internal radiant heater? But it's a very fine pattern so perhaps not.
As usual, I suggest some experimentation. Stick a sheet of paper over half the window and see if the left and right sides still show the same pattern. If you can get to the upper glazing bar, try giving it a good (shiny) clean ( where you can't see); there could be some stripes of dirt / original packaging which dribble down the window. Examine the tiles above the window. Are they patterned?
 
  • #10
Mister T
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My guess is that the pattern is formed by sunlight reflecting off a corrugated metal roof during cold weather.
 
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  • #11
sophiecentaur
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My guess is that the pattern is formed by sunlight reflecting off a corrugated metal roof during cold weather.
Not impossible but the pattern is very fine (in the scale of buildings) - the spacing would need to be less than a degree from a source tens of metres away.
If the effect were to be repeated often then the OP could investigate further.
 
  • #12
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Thanks for all the replies! I observed this after an especially cold night, which are rare now that summer is coming. I might have to wait half a year for further investigation...

Could there be streaks from water running down it? These might freeze preferentially.
This might actually be a good point. There is dirt on the upper half of the window that I usually keep covered by the shutters with streaks from running water in it (hard to photograph against the sky). They are far less regular though.
 
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