Snow often blankets the ground in white. Yet, water, the substance from which snow forms, is typically clear and colorless. This raises a fundamental question: why does snow appear white? The answer lies not in a pigment, but in the intricate way light interacts with its unique structure.
How Light Interacts with Snow
Snow’s white appearance stems from how sunlight interacts with its complex structure. A single snowflake is a collection of numerous tiny ice crystals. When sunlight, which contains all colors of the visible spectrum, strikes snow, it encounters countless irregular surfaces and air pockets within and between these ice crystals.
Light does not pass directly through snow. Instead, it is continuously refracted, meaning its path is bent as it moves from air to ice and back again, and reflected multiple times off the many facets of the crystals. This constant bending and bouncing scatters all wavelengths of visible light equally in every direction. Because all colors are scattered back to our eyes in roughly equal measure, the snow appears white, as white light is a combination of all visible colors.
Snow Versus Ice: A Comparison
While snow appears white, a solid block of ice is typically clear or may exhibit a blue tint. Ice forms a denser, more uniform structure with fewer internal air pockets compared to snow. This allows light to pass through more directly, with minimal scattering, making it largely transparent.
In contrast, snow’s loose arrangement of ice crystals and abundant trapped air creates numerous surfaces that scatter light diffusely, resulting in its white appearance. When large, dense bodies of ice, like glaciers, or very deep snow are observed, they can appear blue. This blue hue occurs because water molecules preferentially absorb longer wavelengths of light, such as red and yellow, allowing shorter blue wavelengths to penetrate deeper and scatter back to the observer.
When Snow Isn’t White
While typically white, snow can sometimes display other colors due to various environmental factors. “Watermelon snow” appears pink or red. This coloration is caused by Chlamydomonas nivalis, a type of cold-loving algae that contains a red carotenoid pigment.
Snow can also take on brown or black hues due to dirt, dust, soot, or other forms of pollution. Industrial emissions or fine particulate matter from wildfires can be incorporated into snowfall, changing its color. Yellow snow might result from airborne pollen, other natural particulates, or even animal waste.