Snow lies: it is not white. It’s light playing with our perception.
Despite the popular expression “white as snow”, science explains that snow is not white. What we see as an immaculate surface results from a play of light with millions of microscopic ice crystals.
At , experts in meteorology and ice science describe how the white appearance arises from the structure of the flakes and the way they scatter sunlight. Snow is essentially whether.
Meteorologist Jonathan Belles remembers that precipitation almost always begins as snow in the clouds. This snow often melts as it passes through layers of warmer air before reaching the ground, turning into rain, including summer showers. For it to snow, the column of air between the cloud and the ground must remain cold enough for the crystals to remain frozen until impact.
The formation of snowflakes begins with particles suspended in the atmosphere, such as dust, soot or pollen. As a supercooled water droplet attaches to one of these particles, the water vapor successively freezes around it. The way water molecules are organized when freezing favors a hexagonal geometry, which gives rise to the six-sided shapes typical of flakes. It is precisely this crystalline architecture that explains “white”.
Mark Serreze, director of the US National Snow and Ice Data Center, emphasizes that sunlight contains all the colors of the visible spectrum. When they hit a layer of snow, the multiple faces and edges of the crystals disperse these colors in all directions, relatively uniformly. The result, for the human eye, is the perception of white: a sum of colors distributed across countless “microprisms”.
The difference for an ice cube is mainly geometric: in a compact piece, light can pass through with fewer deviations; In a layer of flakes, there is a “broken mirror” effect, with continuous reflections and scattering on irregular surfaces.
Snow may have other shades. Grains of sand can give a golden-brown tone; rust and particles in the air can introduce reddish tones. A well-known example is (pink or red snow), associated with algae Chlamydomonas nivaliswhich produces a red carotenoid pigment that works as a “sunscreen” that changes the color of snow in mountainous and icy areas.
In Antarctica, penguin droppings can also dye snow pink. The blue of glaciers is explained by another mechanism: very compact ice absorbs more long wavelengths (such as red and yellow) and returns more blue light.
The reflection of light by snow is quantified by “albedo”, a measure of how much solar radiation a surface reflects. Very fresh snow can have high values (around 0.85 or more), which makes it highly reflective. When soot or smoke accumulates, the albedo decreases: the surface absorbs more energy, heats up and melts more quickly. This acceleration of melting can affect water reserves and worsen dynamics associated with global warming.
Atmospheric conditions also change the way snow is perceived. Low clouds and uniform strata can contribute to the whiteoutdrastically reducing visibility and eliminating shadows (“flat light”), with an impact on depth perception. Furthermore, strong scattering of radiation, including ultraviolet, increases the risk of photokeratitis (so-called “snow blindness”) and sunburn during outdoor activities.
