Once Upon A Snowflake…

If you live in a place where it snows, is not it fun to play in the snow, admire the beautiful snowflakes, and build snowmen? We, as snow and ice scientists, think so too, but there are many more reasons why we study snow. Snow influences the lives of all people and animals on Earth by providing water, offering fun activities, serving as housing, and, most importantly, helping to keep Earth’s climate in balance.

Snow covers about one-third of all land area on Earth every year! In some regions close to the Earth’s poles, like Greenland and Antarctica, the snow never melts away and gigantic piles of snow and ice build up, making the landscape completely white. Did you know that snowflakes themselves also constantly change their appearance and shape throughout their lifetimes? Sometimes, snowflakes can change so much that they change from frozen water (solid) to water (liquid) or vapor (gas)! In this article, you will learn about the steps in a snowflake’s journey on Earth (Figure 1), starting in the clouds when snowflakes are formed.

Snowflake Ballet in the Clouds

High up in the clouds, where it is cold and damp, water vapor begins to freeze around tiny dust particles. That is the start of a snowflake. The freezing continues, and snowflakes grow into different shapes and sizes: plates, needles, or the very fragile but characteristic star-shaped forms. The shape of a snowflake depends on how cold and damp it is when it forms [1]. But all snowflakes are hexagons, which means they always have six corners. A newly formed snowflake has spiky arms and can be as tiny as an ant or as big as a coin. Snowflakes eventually accumulate on the ground as snow. Fun fact: scientists think that no two snowflakes look exactly alike!

Dancing in the Wind

Fallen snow creates a soft blanket, but not for long! In windy weather, snowflakes are picked up by the wind and can be transported over long distances. In this blowing snow, the snowflakes bump into each other, breaking off their spiky arms and becoming rounder and smoother. So, when you look at snow, you see a mix of pointy flakes with long arms and rounded flakes, depending on the wind and weather. Similar to when you hide behind an object to stay out of the wind, snowflakes collect around bumps in the ground or other wind-breaking barriers and pile up. These pile-ups are called snowdrifts and, in some areas, they can grow as high as a double-decker bus!

A Wild Avalanche Ride

Once the wind calms down and the snowflakes settle on the ground, it might seem like not much happens, yet this is not the case. The snowpack is built like a cake, with one layer for each snowfall event. Older layers are at the bottom, and younger layers are at the top. Because the weather controls how snowflakes form, every layer of snow is a little different. Over time, because of changes in temperature, all snowflakes in the layers slowly change their shape. This process is called snow metamorphism, and we call the snowflakes snow grains from this point on.

Most of the time, snow grains get rounder and start sticking to nearby snow grains. But sometimes, under specific temperatures, snow grains become sharp-edged and faceted, so they turn delicate and brittle. Then they form a weak layer within the snowpack: grains that are not well-connected to each other, buried between two more compact layers (Figure 2). You can think about a layer of cookie crumbles between layers of sponge cake. When you push on the top cake layer, a crack might appear in the crumbly layer below and can grow larger and larger. Imagine that the cake is on a steep slope: in this case, the crack moves through the entire weak layer, grains lose contact with neighboring grains, and the cake layer on top starts to slide down. When this happens with snow, it is called an avalanche. A large amount of snow slides down the mountain and can bury animals, humans, and buildings. Avalanches are super dangerous—that is why we are trying to understand how weak layers form and exactly how cracks grow in the snow.

Like Ice in the Sunshine

In a different scenario, when the snow temperature rises above the freezing point, the snow starts to melt. The snow grains change shape once again, becoming round and slushy-wet. If it gets cold again, the snow grains refreeze and form hard layers of snow with large grains. But if high temperatures continue, the snow grains melt completely, turning into meltwater. Meltwater slowly drips through the snow layers below. When the meltwater passes into a cold snow layer, it refreezes, forming solid ice layers in the snowpack which we call melt layers. These melt layers can be as thin as a thread or as thick as several fingers, depending on how much snow has melted. Can you find the melt layer in Figure 2? The warmer it is, the more snow melts, which can create thicker melt layers.

We study these layers by counting them and measuring their thickness, which helps us learn about past melt events and what the temperatures were like in the past. We call this a temperature proxy: by studying something connected to temperature (for example, melt layers in the snowpack), we can learn something about temperature without actually measuring temperature! In Greenland and Antarctica, where the snow never melts away completely, we find melt layers that can tell us about melt events from hundreds and even thousands of years ago. See what else we can learn from layers of ice in this Frontiers for Young Minds article.

When Snowflakes Group Cuddle

In very cold places, where the snow does not melt away, it turns to ice through a process called densification (Figure 2) [2]. When snowflakes fall, they form a light, fluffy layer with lots of air between the snowflakes. It is like a game of Tetris when the blocks fall but have gaps between them. When new snow is added on top, it puts weight on the underlying snow, packing the snow layers tightly together and compressing them. The ice grains get rounded and rearranged, slide along each other, and get squeezed together, which all reduces the air space. Old snow that has been compacted in this way over years is called firn. In the firn, the air between the snow grains forms channels or elongated air bubbles. When more snow falls on top, and the weight increases, the firn layers compress even more. The snow grains bond and fuse together, reducing the air space further. After a long time, all the air is trapped in isolated air bubbles. When this happens, the snow has completely transformed and is now called ice. In mountain glaciers, the snow-ice transition is reached at depths of a few tens of meters, whereas in Antarctica, ice usually forms at depths of more than 100 meters where the layers can be up to 1,000 years old.

Ice Can Move!

Once snow grains have turned into densely packed ice crystals, the direction the crystals face, called their orientation, becomes important. The crystals start facing all in one direction when they are pressed down by a lot of weight. When they all face a similar direction, ice deforms easily if it gets pushed or pulled [3]. This way, ice can move! Think about glaciers that slowly flow down a mountain and look like long tongues of ice (Figure 3A). This ice movement depends on the orientation of all the tiny ice crystals in the big glacier ice mass. If all crystals are neatly organized and face one direction, they can easily glide along each other and the whole mass of ice can flow (Figure 3B). Imagine a deck of playing cards: when all the cards are neatly stacked, they can slide on top of each other easily. Ice movement is similar. We can analyse the crystals’ orientation by looking at paper-thin, polished layers of ice under a microscope with a special light. In this light, the individual crystals have different colors depending on the way they face (Figure 3C). If there are many different colors, the crystals face different directions, which makes ice flow harder. But if all crystals have a similar orientation, and thus a similar color under the microscope, ice in a glacier can easily be deformed and flow downhill.

Closing the Circle

Many glaciers flow toward glacial lakes, rivers, or the ocean, where they break up into icebergs. When it gets warmer, these icebergs melt, and the water, which used to be stored in the form of snow and ice, turns back to its liquid form. When it becomes even warmer, some of the water turns into water vapor, just like boiling water in a pot, but at lower temperatures. From this water vapor, snowflakes can form in a cloud, and the journey of the snowflakes can start again from the beginning.

Now that you have learned about the different forms of snow on Earth, next time you see snow: What form does it have? You can also think about where it comes from, and how it might change its form on its journey on Earth that is yet to come.

Glossary

Snowdrift: ↑ Snow that accumulates in big piles after being blown around by the wind.

Snowpack: ↑ The stack of layers of snowflakes and snow grains with different shapes piled on top of each other. Older snow is at the bottom, and younger snow above.

Snow Metamorphism: ↑ The process in which snowflakes and snow grains change their shape with time, depending on how damp and cold it is.

Snow Grains: ↑ Individual snow crystals that make up the snow layers in a snowpack. They can have different shapes, but they no longer look like star-shaped snowflakes.

Meltwater: ↑ Snow or ice that got so warm that it melted and became water.

Temperature Proxy: ↑ Something scientists measure to estimate past temperatures when they cannot measure the actual temperature directly.

Densification: ↑ The transition from fluffy fresh snow, to firn and finally to very compact ice.

Firn: ↑ Old, tightly packed snow that has not fully turned to ice yet.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

SW acknowledges funding through the Norwegian Research Council (NFR) Grant No. 335140. AM received funding from the Swiss National Science Foundation (SNSF) Swiss-Mobility project number P500PN_217845. MW acknowledges funding from the Swiss National Science Foundation (SNSF), Grant No. 201071. NS acknowledges funding from the German Academic Exchange Service (DAAD) and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 101146092. SZ was supported by the Alfred Wegener Institute’s International Science Program for Integrative Research in Earth Systems in Support of the POF V program “Changing Earth – Sustaining Our Future” (INSPIRES IV).

AI Tool Statement

The author(s) declared that generative AI was not used in the creation of this manuscript. Grammarly in Microsoft Word was used during editing for clarity improvement, spell checking, and grammar corrections (last accessed February 5, 2025, www.grammarly.com).

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