Frost flowers on ice cream: When you start thinking about phenomena and something really annoying, all of a sudden, becomes really cool.

Frost flowers on ice cream. You must have seen them before: They sometimes occur when you’ve had some ice cream, put the left-overs back in the freezer, and take them out again. And there you have it: Water-ice crystals all over your lovely ice cream! Completely annoying because, obviously, they only taste like water and mess up your whole ice cream experience (or is that only me)?

You know I’m kinda fascinated with ice crystals on frozen blended strawberries, but last time I had some, there weren’t only crystalline structures, but there was frost on it:


Frost occurs when water vapour freezes without going through the liquid phase. Look at the awesome crystals!


Once I started thinking about the process that formed the ice and realised that those were actually frost and not just ordinary ice crystals, they all of a sudden stopped being annoying and instead became something that I kinda look forward to finding when I open a tub of my frozen blended strawberries. Because the structures are different every time, and really really pretty! And also how awesome is it to know that those ice crystals formed from water that wasn’t even liquid? Yes, this is the kind of stuff that makes me happy! :-)

Frost flowers – when water vapour freezes to ice without going through the liquid phase. Examples “at sea”

Frost flowers on the ice of the Schlei in Schleswig. By Mirjam S. Glessmer

Frost flowers! I learned about those in the context of Arctic and Antarctic ice formation. I kinda assumed that ice flowers only formed in salt water, because I remember hearing about how the ice needles that form wick up brine and that, due to their large surface (which you will remember noticing in the last post where we looked at them forming on trees), they are super important in the air-sea exchange of all kinds of stuff,  like for example bromine. So imagine my excitement when I saw them growing the other day!

Frozen Schlei river in Schleswig

Frost flowers are really pretty by themselves, but they also tell us a lot about recent weather conditions. For example, they only form when the air is A LOT colder than the water/ice surface. Do you know the snowy ice crystals you sometimes find on the inside of ice cream containers when you’ve opened and refrozen them? Yep – same thing! I even suspect that the ice crystals I was talking about in this post are also frost flowers.

Frost flowers

I find it really fascinating how they are distributed over the larger surface of the Schlei river.

Schlei river in Schleswig coated in frost flowers

Here, for example, you see them forming on the edges of ice that has been broken up by some mechanical process. Judging from their placement, I would suspect that they only formed after the ice was broken and some of the pieces tilted up.

Cracked ice and frost flowers

Here, they were probably everywhere, but then the ice got broken up and some parts submerged. When the water there refroze, no snow flowers formed, just “normal” ice. However, the existing snow flowers seem to have continued growing!

Ice with frost flowers. Partially submerged and then refrozen into “normal” ice

The really interesting thing is that frost flowers don’t actually form from the water that is freezing below, but from water vapour in the air. Which, btw, explains why they can form on benches, ice cream lids or trees (all of which would be really difficult if they could only form on open water surfaces).

Ice with frost flowers. Partially submerged and then refrozen into “normal” ice

Above you see a larger part of the Schlei’s surface: Seems like there used to be frost flowers everywhere, but when the ice broke up, some of it got pushed out of the water, and as such preserving the frost flowers and letting them continue to grow. Meanwhile, other parts got flooded and only normal ice formed there. Maybe because the temperature gradient at that point wasn’t large enough any more?

Isn’t this just beautiful??? I could watch this all day, every day.

Frozen Schlei river in Schleswig with frost flowers

But let’s look at some more details. No idea why that patch of frost flowers formed there! But they seem to always start in small patches, which eventually grow together if the conditions are stable enough over long enough periods of time.

Frost flowers on ice

Here, we see the opposite situation to the one a couple of pictures up: “Normal” ice had formed, and then was broken up. And then, when the crack froze over, frost flowers formed!

Frost flowers growing in a crack in the ice

Very cool stuff!

Frost flowers

Yep, I would still just sit there and watch!

Frozen Schlei river in Schleswig

Frost flowers – when water vapour freezes to ice without going through the liquid phase. Examples on land

Frost flowers on the ice of the Schlei in Schleswig. By Mirjam S. Glessmer

What happens when water vapour freezes to ice without going through the liquid phase? Frost flowers!!!

That’s when trees suddenly look like this:

Frosted tree.

Btw – the stem of that tree is painted white! That’s just to confuse you a little but…

But let’s take a closer look. This is what the branches look like: Tiny ice needles growing on the individual pine needles! And the orientation of the image below is correct. They are growing to the side!

Frosted tree.

You can clearly see them all growing to one direction, to one side!

Frosted tree.

When you take off a bit of frost, this is what it looks like. Needles, but with a fractal 3D structure! Since what happened here (water vapour freezing without becoming liquid in between) is basically snow forming on the surfaces down here instead of in the clouds up above, it isn’t too surprising that snow is exactly what the frost bits feel like.

A piece of frosting. This picture isn’t blurry – the ice needles have a fractal 3D structure!

Look below, you can clearly see the frost only growing to one side (and this picture is the right way up, too!):

Frosting on tree branches

Doesn’t it make you want to sit there and just watch?

What a nice picknick spot!

Although every time the slightest of breezes comes, this is what happens:

Tree being de-frosted by wind

Also really cool: These plants growing on a balcony behind a glass railing. Only the tips have been frosted!

Plants on balcony with frosted tips

And if you were wondering what this post has to do with oceanography, check out the image below. Can you spot it?

Frozen Schlei river in Schleswig

Can you spot it now? No, not my niece (although she is pretty cool, too!), the frost flowers!

Schlei river in Schleswig with frost flowers

We’ll talk about those next time :-)