Canadians know snow. Recently, I’ve been around Toronto and Montreal and I’ve been exposed to a great deal of snow. Delayed street cars; slushy, slippery sidewalks; frosty faces; hail and hazard lights on highways. These are just some of the things I’ve dealt with this past week…
But there’s a warmer side to the white stuff which one of my dearest friends reminded me of two nights ago. She drew my attention to the fluffy, freshly fallen snowflakes which sparkled under the illumination of the streetlights. She had heard that there was some interesting physics behind the glisten of snow (of which I had no knowledge). She asked me to, one day, explain it to her. I love questions like this, so I decided to do a bit of googling.
…what makes snow sparkle, you ask?
First, let’s take a look at snowflakes in general. They can come in simple hexagonal shapes or complex tree-like crystals. So much could be said about the anatomy of snowflakes, most of which I couldn’t tell you because I don’t know (but here’s a link that might help). The key point I would like to get across is best illustrated by the graph to the left. The graph shows temperature decreasing to the right and humidity increasing upwards. While the more complex, pretty snowflakes tend to form at high humidity, the type of crystals that sparkle are actually the simple ones near the bottom of the graph. These plates and prisms at low humidity have large flat surfaces which act like mirrors reflecting almost all light that hits them. These snowflakes are randomly scattered on the ground and will reflect the dimmed light reflected from the trees, cars, and whatever else happens to be around into your eyes. The majority of the snowflakes will appear to have some average brightness, however, some snowflakes will happen to be at the correct angle to reflect light emitted directly from a light source (like the sun, or streetlights). It is these snowflakes which will be almost as bright as the light source itself; they will stand out and glisten. As you move your field of view, these snowflakes will no longer be properly oriented to reflect the light source and, instead, other snowflakes will stand out. The sparkling will appear to move as you do, in a sort of spectacular specular reflection.
But wait! There’s more!
These simple snowflakes don’t only need to reflect, they can also refract light. Refraction, in case you’ve forgotten, is the bending of light as it enters or exits a different material. The angle of refraction depends primarily on the materials it is entering and exiting, but it also depends on the frequency (color) of the light. Different colors of light will bend different amounts as they enter and exit a snow crystal. White light (coming from the sun, or a streetlight) consists of many different colors of light, these colors will be separated by the snow crystal as it passes through a prismic snow crystal. This is called dispersion. So if you are far enough away from the sparkling snow that the colors have separated significantly, you may see certain snow crystals as being certain colors! The result is something like this:
So on the next dry winter night after a fresh snowfall, take a look outside and see if you can spot some spectacular specular reflection, or even the colorful dispersion of randomly assorted snowy prisms.
