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Ask Dr. Universe: When frost freezes, it makes shapes like flowers and ferns – why does it look like that?

Mead’s Olive Utley, 12, submitted this photo of a frosty fence for the A to Z Scavenger Hunt.  (Olive Utley)
Mead’s Olive Utley, 12, submitted this photo of a frosty fence for the A to Z Scavenger Hunt. (Olive Utley)
Washington State University

Washington State University

Dr. Universe: When frost freezes, it makes shapes like flowers and ferns. Why does it look like that? – Grace, 13, in Calgary

Dear Grace,

You’re right: Frost can sometimes form patterns that look like the ferns or flowers we find in nature.

Those frosty shapes we see on the surface of windows start out as water in the air, said my friend Kai Carter. Carter is a meteorologist with Washington State University’s AgWeatherNet team.

If you’ve ever had a glass of ice water, you may have noticed droplets formed on the outside of the glass. The droplets actually came from water in the air. This water condensed from the air onto the surface of your cup, which means it turned from a gas to a liquid.

This is similar to what’s happening when dew forms on grass. But frost is a little bit different, Carter said.

When frost forms, conditions have to be just right. As water from the air lands on a really cold surface like a windowpane, the water molecules freeze and join together with other water molecules to form patterns of ice crystals.

An ice crystal is made up of two building blocks: hydrogen and oxygen. These hydrogen and oxygen atoms form a hexagon shape that is a kind of six-sided ring. Even though we might not be able to see them with our eyes, these hexagon shapes can repeat in a pattern across the frosty surface of the window.

Sometimes the water molecules can form into one big sheet of frost. But sometimes things can get in the way of the water molecules. They may have to take a new path as they freeze to the surface of the glass.

If the molecules run into something like a speck of dust, salt or even a bit of washer fluid from a car window, they might change their direction. As you’ve observed, they can start to branch out into shapes that might look to us like feathers, ferns or tree limbs.

In mathematics, we call this kind of thing a fractal design. A fractal pattern repeats itself at different scales. One other place you can also find fractal patterns is ferns. The fern frond looks like it’s made up of little fern fronds, which look like they are made up of even smaller fern fronds.

Next time you see some frost, take time to observe its detailed patterns with a magnifying glass.

Even if you don’t live someplace where it gets really cold, you can make your own frost right in the kitchen. All you need is a tin can, salt and ice. Fill the tin with ice and 4 tablespoons of salt, and mix it up for a minute. Wait a few minutes and see what forms on the outside of the tin.

The salt is important because it melts the ice while at the same time helping the mixture drop below freezing.

Why do you think that might be? What happens when you add more salt or more ice? Tell us about your frosty experiments sometime at


Dr. Universe

Ask Dr. Universe is a project from Washington State University.

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