Why is snow white?

What do you see when you look out a window after a heavy snowstorm? The world is blanketed in white snow, which is frozen water. But if you turn on a faucet or open the freezer, you can see that liquid water and ice do not look that way. So, why is snow white? Optics, which is the field of science that studies light and how it interacts with matter, can explain why a material may be colorless in one form but white in another.

How Visible Light Strikes Surfaces

When light hits an object, three things can happen. Light can be transmitted or passes through the object. An object can absorb, or take in, the light. Or light can reflect, which means to bounce off an object. Water in a cup has a smooth, flat surface that transmits light through it. Because very little light bounces back to reach our eyes, we don't see any color. From certain angles, so much light passes through water or glass that we might not even see the water or glass. However, shattered glass has many uneven surfaces. When light strikes the many surfaces, it reflects and scatters, bouncing off in all directions. This is also true of snow, which is made up of hundreds of tiny ice flakes that have many different shapes and structures. White light that we see is a combination of all the colors in the rainbow including red, orange, yellow, green, blue, indigo and violet. When an object has a color, say red, only red light reflects off the object. That red light reaches our eyes, and we see the object as red. The same thing happens for any object, the color of light that reflects off an object is the color we see. Because snowflakes have so many surfaces, all light reflects back equally so that a pile of snow looks white.

Why Do Icebergs and Glaciers Appear Blue?

Although the natural color of snow may be white, it can take on other hues. Snowpack, which is snow packed up on the ground, icebergs and glaciers can sometimes appear blue. Light can enter the snow through cracks and crevices (rather than reflecting off of their surfaces). The trapped light travels within the snow and ice, scattering in every direction. The farther the light travels, the more times it scatters. As the light bounces around the ice, the colors separate out from each other. The ice absorbs most colors, but blue light keeps bouncing around. When the light rays finally emerge from the snow layers, it's only the blue light that reaches our eyes. To us, it looks like the snow has a blue tint. Blue snow only happens when snow is deep enough to filter out other colors. According to the National Snow and Ice Data Center, snow has to be at least one meter (3.3 feet) thick to see any blue tint at all.

Particles Can Change the Color of Snow

Snow can appear in different colors because of different particles or living things hiding in the snow. Take watermelon snow, for example. It is a nickname for pink- or red-tinted snow that happens because of a type of freshwater algae. The algae are red and live within the snow pack. Algae are single-celled organisms that, similar to plants, grow in water and depend on sunlight to make their food. Of course, it's no mystery where one color of snow comes from; if you see yellow snow, it's a safe bet animal tracks are nearby.