Let it Snow: The Science of Snow Crystals

By Jeremy Benson, NIU STEAM Educator

As we begin the new year and prepare for the deep of winter, many of us may have mixed feelings about that fluffy white stuff that often covers the ground this time of year. Whether you love the clean beauty of a freshly fallen snow or dread the inevitable shoveling and slush, there aren’t many better examples of STEAM in nature than the humble snowflake. From the science of their formation to the symmetry and mathematics that govern their shapes, there is a lot to discover when examining a snowflake. Snowflakes come in a near infinite variety of shapes and sizes, from tiny specks to big fluffy puffs, and everything in between. Let’s take a closer look at how that diversity happens.

Snowflakes form when the air is cold enough for water to freeze. Now that doesn’t mean that snow is just frozen raindrops, that would be sleet. Snow forms when the water vapor in the air freezes directly to form ice crystals in a process called deposition. This is also how ice crystals form on your windshield overnight. A crystal is a solid material that has an organized structure. For example, because of their atomic arrangement, water molecules usually connect at 60-degree angles, resulting in the six-sided hexagonal shapes we associate with snowflakes. When a snowflake, or snow crystal, first forms, water vapor deposits onto a speck of dust floating in the air. This is called nucleation. At the center, or nucleus, of each snowflake is a perfect hexagonal ring of water molecules, frozen around a single tiny speck of dust.

From there, each flake’s story gets more unique. As the newly formed ice crystal is blown through the air, it gathers more water molecules and continues to grow. While the tiny flake is blown about, the entire crystal experiences the same atmospheric conditions, meaning all six faces of the crystal should form symmetrically. A crystal which encounters more humidity will grow faster than one that has less available humidity. Water molecules will also deposit more readily on the corners and edges of the crystal, causing it to expand outward while staying relatively flat.

As the snow crystal continues to travel and grow, the conditions it encounters along the way will determine how the crystal forms. If it encounters fairly constant conditions as it falls, the resulting snowflake will grow evenly, creating facets shaped like hexagonal plates. If the conditions vary as it falls, the result can be a crystal with longer fern-like offshoots, or dendrites.

Because the snow crystal forms from the center out as it falls, we can think of their structures similar to how we study tree rings. Each “ring” of our snowflake tells us something about the conditions it was experiencing while that portion of the crystal formed. Longer, thicker offshoots on the dendrites tell us the environment was more humid, while thinner, shorter dendrites tell us there was less available water at that point in its formation. Because no two snowflakes take exactly the same path to the ground, each snowflake develops a distinctly unique appearance.

While this process describes most of the snowflakes we might observe, there are other possible shapes as well. When the temperature is just below freezing, or there is little available humidity, crystalline shapes such as needles and columns can also form, though these are much less common, and their formation is not as well understood.

Try this at home!

If you’d like to capture and study your own snow crystals, here’s what you’ll need:

• A nice snowy day
• A piece of black construction paper
• A magnifying glass
• Patience
• Microscope slides (optional)
• Toothpick (optional)
• Hairspray (optional)

Instructions: Use the black paper to catch falling snowflakes, and then observe them using the magnifying glass. See if you can observe the following:

• Are the crystals primarily made of plates or dendrites? Do any show signs of both?
• Do all the snow crystals have six sides?
• Are they all symmetrical?
• What can you tell about the conditions the crystal formed in?
• Do you see any needles or columns?

How to preserve your snowflakes:

• Spray a glass or plastic microscope slide with hairspray.
• Use the sticky side to catch your snow crystal.
• Use a toothpick to carefully move the crystal to the center of the slide if needed.
• Leave the slide somewhere cold and protected where the hairspray can dry. The water will disappear, leaving an imprint of your crystal in the hairspray.