If you look closely at a single snowflake under a microscope, it’s basically clear. Individual ice crystals are transparent, like tiny pieces of clear glass. So why does a field of snow look brilliantly white?
The answer is light scattering. When sunlight hits a pile of snowflakes, it doesn’t pass cleanly through. It bounces and refracts off thousands of tiny surfaces - facets on each crystal - going in all different directions. By the time it comes back out, the light has been scattered so thoroughly that all wavelengths are mixed up evenly, and our eyes perceive that mixture as white.
You can see the same effect with crushed glass: a transparent windowpane looks clear, but smash it into powder and the powder looks white. Snow does sometimes appear other colors. Deep snowpack and ice - like the inside of a glacier crevasse - looks blue, because the snow has compressed and the layers absorb red light, leaving blue. Sometimes snow looks pink or red, when certain algae grow on its surface (“watermelon snow”). But the snow itself is colorless - its color is just light bouncing.
