Snow is white, but there are several shades of white, and sometimes it even takes on other, more original hues. A dive between physics and weather with the director of the Snow Study Center based in Grenoble.
Immaculate white that sparkles in the sun, the snow when it covers the ground has the power to transform landscapes into something magical. But why is she white? It is made up of ice and air and the ice cubes that come out of our freezer are not white, they are 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 that make it up are very unlikely to be absorbed as they pass through the ice.
Snow is in fact a kind of foam of ice and air: the light passing through it will have very little chance of being absorbed by passing through the ice or air, both of which are transparent.
On the other hand, at each air-ice interface, the light will be reflected (like a mirror) or refracted (changed direction inside the ice), and will end up emerging from the snow cover, because it has very little chances of being absorbed.
So most of the visible light that enters the snow comes out upwards, making the snow white.
This white color of snow is very important for our planet. In fact, this means that when snow covers the ground, most of the sunlight will be reflected towards the atmosphere, unlike bare ground or covered with vegetation, which is darker and which absorbs more light. The white color of snow therefore limits the absorption of solar energy, and thus warming. However, the more the temperature increases, the less snow there is on the ground, therefore the darker the color of the planet and the warmer it becomes. It is a phenomenon of “runaway”, which we also call “positive feedback”, linked to the albedo (that is to say the fraction of solar radiation reflected by a medium) of the snow and which is very important for our climate.
50 shades of snow
Snow is not just white, it can take on different shades of white.
This comes from the interaction of light with the structure of the snow. The structure of snow, that is to say, the three-dimensional arrangement of air and ice on the scale of a micrometer (one millionth of a meter, or approximately fifty times less than the thickness of a hair), varies greatly depending on the state of the snow.
The finer the snow has a structure, as is the case for example with fresh snow, the greater the surface area of the air-ice interface in relation to the volume of ice contained in the snow. To make the analogy with a ball pool, for fresh snow, we would then have a large quantity of very small balls, i.e. a large surface area of plastic in contact with the air. Later, the snow evolves and our pool would contain larger balls, in smaller quantities, which results in less contact surface between air and plastic.
The quantity of light absorbed is proportional to the volume of ice while the quantity of light scattered is proportional to the surface area of the air-ice interface. So the greater the ratio between the surface of the interface and the volume of ice, that is to say the finer the structure, the whiter the snow will be. Fresh snow will therefore appear whiter than snow with a coarser structure, for example which has already melted and refrozen.
This shade of white, which comes from the interaction between light and the structure of snow, is also the source of an important positive feedback for our climate. Indeed, when the temperature increases, the structure of the snow tends to increase, the snow becomes less white, absorbs more solar energy and therefore can melt more quickly.
Snow in color
But snow is not only white, you can find orange, red, black, purple or even green snow. When such colors appear, it is because the snow contains colored particles which can be of different origins.
We often find soot carbon there from the combustion of fossil fuels and which turns the snow gray.
In the French mountain ranges, it is common to find orange or even red snow after episodes of mineral dust deposits from the Sahara.
Finally, snow contains living organisms, and in particular algae which produce pigments which can be of different colors. In the Alps, the most common species of snow algae is called Sanguina Nivaloides and dyes the snow a blood-red color, which you may have already observed during a walk in the mountains in late spring.
By changing the color of the snow, all these colored particles cause an increase in the amount of sunlight absorbed by it, and accelerate its melting.
The whiteness of the snow and its subtle nuances are therefore very important for the evolution of the snow cover and for the climate of our planet.
The EBONI project and the ALPALGA project were supported by the National Research Agency (ANR), which funds project-based research in France. Its mission is to support and promote the development of fundamental and finalized research in all disciplines, and to strengthen the dialogue between science and society. To find out more, consult the ANR website.
