Why is snow white? ⛄

By Marie Dumont - Researcher, Director of the Snow Study Center, National Center for Meteorological Research, Météo France

Snow is white, but there are many shades of white, and it sometimes even takes on other, more unusual hues. A dive into the realms of physics and meteorology with the director of the Snow Study Center based in Grenoble.


With its pristine whiteness sparkling in the sunlight, snow has the power to transform landscapes into something magical when it covers the ground. But why is it white? Snow is made of ice and air, yet the ice cubes that come out of our freezer aren't white—they're transparent! So how is this possible?

Snow is white precisely because ice is transparent. When we say that ice is transparent, it means that visible light and all the different colors it comprises have very little chance of being absorbed when passing through ice.

In fact, snow is a kind of foam made of ice and air: light passing through it has a very low chance of being absorbed as it travels through the ice or air, both of which are transparent.

However, at each air-ice interface, light is either reflected (like a mirror) or refracted (its direction changes inside the ice), eventually exiting the snowpack because the likelihood of it being absorbed is very low.

Thus, most of the visible light entering snow exits upward, making snow appear white.

The white color of snow is very important for our planet. It means that when snow covers the ground, most of the sunlight is reflected back into the atmosphere, unlike bare soil or vegetation, which are darker and absorb more light. Snow's whiteness, therefore, limits the absorption of solar energy and helps reduce warming. However, the higher the temperature, the less snow there is on the ground, which in turn darkens the planet's surface and increases warming. This is a phenomenon of “runaway feedback,” also known as "positive feedback," linked to the albedo (the fraction of solar radiation reflected by a surface) of snow, which plays a crucial role in our climate.

50 shades of snow

Snow isn't simply white—it can display various shades of white.

This comes from the interaction of light with the structure of snow. Snow's structure, meaning the three-dimensional arrangement of air and ice at the micrometer scale (a micrometer being one-millionth of a meter, or roughly fifty times thinner than a human hair), varies significantly depending on its condition.

The finer the structure of the snow—as is the case, for example, with freshly fallen snow—the greater the air-ice interface surface area compared to the ice volume contained within the snow. To use an analogy with a ball pit, freshly fallen snow would resemble having a large number of very small balls, resulting in a large surface area of plastic in contact with the air. As snow evolves over time, the ball pit would contain fewer, larger balls, leading to less surface contact between air and plastic.

The amount of light absorbed is proportional to the volume of ice, whereas the amount of light scattered is proportional to the surface area of the air-ice interface. Thus, the greater the ratio of interface surface area to ice volume—meaning the finer the structure—the whiter the snow will appear. Fresh snow, therefore, looks whiter than older snow with a coarser structure, such as snow that has partially melted and refrozen.

This variation in white shades due to the interaction between light and snow's structure also drives an important positive feedback effect on our climate. As temperatures rise, snow's structure tends to become coarser, reducing its whiteness, increasing its absorption of solar energy, and potentially accelerating its melt.

Snow in color

But snow isn't always white; it can sometimes appear orange, red, black, purple, or even green. When snow changes colors, it is because it contains colored particles that can originate from various sources.

Frequently, soot from fossil fuel combustion is found, which turns snow gray.

In the French mountain ranges, it's common to find orange or even red snow after episodes of mineral dust deposits from the Sahara.

Finally, snow contains living organisms, particularly algae that produce pigments in various colors. In the Alps, the most common snow algae species is called Sanguina Nivaloides, which tinges the snow with a blood-red color, something you may have noticed on a mountain hike in late spring.

By altering snow's color, all these colored particles increase the amount of solar energy absorbed by the snow, thereby speeding up its melting.

The whiteness of snow and its subtle shades are therefore extremely important for the evolution of snow cover and the planet's climate.