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First, look at this picture:

dew

This is frozen dew/rain, it was cold this morning and it might have rained during the night but that's not really relevant I think.

Here are my questions:

  1. Why is the frozen water only on the top of this rail made of stone? Why not on the sides?
  2. Why not on the ground?

Right now I guess that for question 1 it is gravity doing its thing, that is the water falls on the sides and thus there's very little to freeze.

For question 3 I'm guessing the stone type is different and this triggers the visible freezing, e.g the stone is not porous at all while the ground is? Or maybe the ground is warmer because it is less sensible to temperature variations (the rail being much more exposed)?

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Sorry for my poor english. French is my native language.

The dew appears more on the surfaces facing the sky on a clear day because these surfaces cool more as a result of the thermal radiation. Indeed they radiate towards the sky whereas, in clear weather, the radiation received from the sky is weak. Tilted surfaces receive thermal radiation from surrounding objects.

Afterwards, it is also necessary to take into account the heating by thermal conduction, which is perhaps more important for the ground.

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  • $\begingroup$ Also noteworthy: The rail is elevated and is only supported at a few points. This means the still-warm ground can't keep the cooling rail warm, but the still-warm ground can keep the surface of the ground from dropping below freezing. $\endgroup$
    – David Hammen
    Commented Jul 21, 2021 at 15:02
  • $\begingroup$ No worries, french is also my mother tongue :-) Thanks for the explanation! $\endgroup$
    – Silex
    Commented Jul 22, 2021 at 9:40

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