Mystery solved: we finally know why snow shines

Understanding the optical properties of snow can unlock progress ranging from improved snow removal techniques to increased avalanche safety. One might think that snow is easy to describe: it is cold, white, and covers the landscape like a blanket. What else can you say about it?

Many, according to Mathieu Nguyen. He has just defended his doctoral dissertation on the optical properties of snow at the Norwegian University of Science and Technology (NTNU) in Gjøvik.

“Snow reflects all wavelengths of light and can have very different colors depending on the conditions and the angle at which light hits it. The age and density of the snow and air pollution also affect how it looks. The appearance of snow is very complex,” says Nguyen.

He analyzed more than a thousand images of snow. “This type of method can be used in many sensor technologies, which include everything from giving us a better basis for deciding when to clear roads to more closely monitoring the risk of avalanches in the mountains.”

Landscape of mirrors

Among other things, Nguyen studied how snow absorbs and reflects light, and how the sun makes snow crystals sparkle was of particular interest. He believes that the beautiful appearance of winter landscapes may hold the key to answering a number of questions that have puzzled researchers for many years.

But first, what exactly makes snow sparkle on bright sunny days?

“Snow is an accumulation of ice crystals. When the conditions are perfect, they act like tiny mirrors. If they are positioned at the right angle, they reflect the sunlight directly onto you and shine like ‘sparks’ in the landscape,” says Nguyen.

Huge potential

A lot of research has been done on how different metals sparkle in this way, but the luster of snow is still poorly understood.

“If we want to have fully autonomous cars here in Norway, this technology will also contribute to safer travel on winter roads,” says Nguyen.

Therefore, Nguyen tried to find out how these sparkles change in contrast and density in images of snow in different conditions. He hopes that this will provide an analysis method that will allow us to classify different types of snow from images. Now it is impossible.

“This type of method can be used in many sensor technologies, which include everything from giving us a better basis for making decisions about when roads need to be cleared to more closely monitoring the risk of avalanches in the mountains. If we want to have fully autonomous cars here in Norway, this technology will also contribute to safer driving on winter roads,” says Nguyen.

We need images from all over the world

So far, the researchers have collected data only from various locations in eastern Norway. The results are promising and show that glitter can be used to classify the size of snow grains. However, a more accurate classification of snow type requires a much larger amount of data than they have been working with so far. Preferably with images from around the world.

“It will be important to get images from other places where the environment is different. Understanding how different levels of pollution play a role in the appearance and properties of snow will be crucial,” says Nguyen.

Experience for the future

In addition to the fact that snow is difficult to interpret from images, it has long been discovered that snow is surprisingly difficult to reproduce digitally.

“The artificial images of snow we have today in computer games and simulations are not much better than white surfaces,” Nguyen says.

His discoveries have also shown promising results in this area. He believes that his work will give people who don’t have access to snow a good winter experience – also in the future, when there may be much less snow.

According to the Norwegian Meteorological Institute, more than one million Norwegians in 2050 will live in places with less than one month of winter. In addition, a recent study in the journal Nature confirmed that the entire Northern Hemisphere is facing a future with less snow as a result of anthropogenic climate change.

“If we want to teach someone who may have never seen snow before what it is, we need to be able to recreate it in all its complexity,” Nguyen says.