Why does water freeze differently when leaves are present on a shelter roof?

[Trying2Learn]

Hello!

I wait for the tram in a shelter. The roof of the shelter is made of transparent material (glass like). Leaves fall onto the roof and remain there in early winter during the first freeze.

If there are no leaves on the roof (recently cleared by wind), the ice freezes with no discernible pattern.

However, when there are leaves clumped up in the corner of the roof, the water freezes in a way the resembles the veins of a leaf.

Why is this?

 

[DennisN]

Summary:: Why does water freeze in patterns

However, when there are leaves clumped up in the corner of the roof, the water freezes in a way the resembles the veins of a leaf.

Hi @Trying2Learn!
I think it is cool that you are observant and see things like this!
I ask you:
Is a leaf perfectly flat? What I mean by this is if you would run your finger across the leaf, would you estimate it to be perfectly flat or would you feel any structure`? Would you feel the veins, for instance?

 

[DennisN]

Oh, by the way, are the observed "freezed" patterns similar to the actual leaf patterns, or are they more like random leaves with random structures? The reason I ask is because this would indicate something else than what I first thought.

 

[sophiecentaur]

I would suggest that the reason is to do with the pattern of contact of the leaves with the glass/plastic. There is a layer of wax on the surface of a leaf and that can affect the precise freezing temperature of the water that condenses on the surface. Any subtle pattern of any parameter may change the precise timing of the freezing process and that will turn up as a pattern in the resulting 'frost'.
It could be a thermal effect or a chemical effect but any pattern can end up as a frost pattern. Ice crystals form to follow the pattern of the previous crystals.
"Jack Frost" is a real artist!

 

[Trying2Learn]

Hi @Trying2Learn!
I think it is cool that you are observant and see things like this!
I ask you:
Is a leaf perfectly flat? What I mean by this is if you would run your finger across the leaf, would you estimate it to be perfectly flat or would you feel any structure`? Would you feel the veins, for instance?

Sorry it took so long to respond to this and previous.

No, not flat. But how does that impact the ice. Are you saying that the ice begins to freeze, more thickly (I do not know what that means either :-) What are you suggesting?

 

[Trying2Learn]

Oh, by the way, are the observed "freezed" patterns similar to the actual leaf patterns, or are they more like random leaves with random structures? The reason I ask is because this would indicate something else than what I first thought.

It looks just like the leaves.

And if the leaves are not present, there is no pattern of ice.

 

[Trying2Learn]

I would suggest that the reason is to do with the pattern of contact of the leaves with the glass/plastic. There is a layer of wax on the surface of a leaf and that can affect the precise freezing temperature of the water that condenses on the surface. Any subtle pattern of any parameter may change the precise timing of the freezing process and that will turn up as a pattern in the resulting 'frost'.
It could be a thermal effect or a chemical effect but any pattern can end up as a frost pattern. Ice crystals form to follow the pattern of the previous crystals.
"Jack Frost" is a real artist!

That does make sense. But here, the leaves have been dead and dried up for weeks. Still, what you wrote does make sense.

 

[sophiecentaur]

But here, the leaves have been dead and dried up for weeks.

Time wouldn't affect the situation until the leaves had disintegrated and the traces washed from the glass. In fact, I would expect any chemical effect to be more dramatic after the chemicals had leached through to the leaf surface.
PS we get a similar effect with slug trails on an apparently clean window on a frosty morning.

 

[DennisN]

No, not flat. But how does that impact the ice. Are you saying that the ice begins to freeze, more thickly (I do not know what that means either :-) What are you suggesting?

What I was thinking was that the structure of the leaves somehow effect the formed pattern, either due to the structural "imperfections" themselves or a combination of this and what @sophiecentaur wrote above.

It looks just like the leaves.

And if the leaves are not present, there is no pattern of ice.

Ok, because if they were random it would rather be due to small imperfections in the glass:
http://www.bbc.co.uk/weatherwatchers/article/38144457/what-causes-different-frost-patterns/

Do the patterns look like something like this? :
(i.e. does the frost form where there are no veins or does the frost form along the veins?)

 

[anorlunda]

Wouldn't the water film be thinner on the raised parts and thicker in the valleys?

I speculate that nucleation in water to begin freezing is influenced by many factors. It seems that all of those factors vary across the leaf. That makes it nearly impossible to name one of them as "the cause".