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.
What a nice post (and I linked to it). One thing I’ve been wondering recently, and perhaps you’ll have some thoughts on it, is why snow crunches when it’s really cold? You know that sharp squeak that you hear when it’s around 20 degrees, or the crunch under your tires… I’ve been wondering this recently but haven’t spent the time to google around or think about it in depth.
Why, thank you!
Links and praise are always appreciated… and this comes with a neat question too! Awesome!
I did a bit of googling. From the few (sketchy) sources I saw, it looks like the crunch sound comes from the sudden release of air from the air pockets in a pile of snow. That I can believe, however, one explanation includes the following:
As elegant as that explanation sounds, I suspect it won’t really add up… (literally even). It sounds a lot like the physicist’s myth of ice skating being explained by a similar process (Pressure from skate -> melting ice -> sliding). In one of my classes we did this calculation and it turned out that the freezing point of ice under a skate would only change by about 1 degree maximum. I suspect something similar for the preceding claim about crunching snow.
Going on physics intuition alone… I’d probably say the temperature dependence of the squeakyness of snow has more to do with the temperature dependence of the structure of the snowflakes. Maybe the shapes that snow crystals take on at low temperatures are better at making noisy air pockets… * shrug *
I’d love to hear what you (or anyone else) thinks about this sqeaky snow puzzle! If I find a good answer, perhaps I’ll blog it… unless you beat me to it
I see why you’re skeptical of that explanation, given the small effect of pressure on melting temperature. I’m inclined to wonder if it is something about the *rubbing* of ice crystals together (rather than the *crushing* of them). It seems implausible that crushing would create that sound, but maybe my experience misleads me. It just seems like most of the sound is coming from the sides of my shoes in the snow, creating friction, rather than from my shoe coming down. If I step straight down, rather than grinding my foot sideways into the snow, it is quieter.
But I also don’t think that a “different shaped snow crystal” explanation works for me, since the snow has already fallen to the ground and thus its crystal shape is already determined. Once it’s on the ground, it crunches when you walk on it if it’s really cold, and doesn’t if it’s not.
After I wrote all that, I found a good link that seems to support what I just wrote (don’t you love it when that happens), at
http://everything2.com/e2node/Crunching%2520snow. And it supports your ice crystal shape theory, though not in a temperature dependent way.
They write:
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Three physical factors
There are two — no, actually three — physical factors affecting the crunching / noncrunching of trodden snow. The mechanism behind all three is the same — lubrication, good or bad. When snow does NOT crunch, then the grains / crystals in the snow are well lubricated. When snow DOES crunch, then lubrication is poor. The lubricant is of course water in all cases, coming from two sources, both of which are temperature-dependent:
(1) Ice crystals are always surrounded by a very thin layer of water (a phenomenon already observed by Michael Faraday). The thickness of this layer varies with temperature, ranging from a one molecule thick layer at about -10 oC, to hundreds of monomolecular layers at -1 oC.
(2) Pressure lowers the melting point of water. If you step on snow, then the crystals are pressed against each other. The ice at the contact points may melt and create a thin lubricating layer of water. Unfortunately, the pressure from the soles of your shoes is far to small to melt snow at any temperature, so this factor, interesting as it my seem in itself, is rather irrelevant in this connection.
(3) The third factor is the shape of the ice / snow crystals: crystals with a greater number of pointed edges crunches more readily. An extremely pointed structure of the snow crystals can sometimes offset the other factors, making snow crunch even when it is warmer than -10 oC.
It is difficult to say how these phenomena interact in order to lubricate (or not lubricate) the snow crystals, but in any case something seems to be happening at around -10 oC, enough to make a sharply noticeable difference: if it is colder than about -10 oC, then snow crunches, if it is warmer, then it usually doesn’t.
Ten-degree rule of thumb
These factors, taken together, determine the precise temperature at which snow starts crunching. But the -10 oC rule is a surprisingly good rule of thumb, if you want to predict whether or not you will experience the nice crunching sound of snow when you take a walk at Christmastime.
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Thanks for the blog-discussion! I may cross-post this.
Stephanie
I cannot comment on the snow crunch but I am still curious to know how the brilliant points of color are produced that I see on the surface of the snow when I am facing the sun and it is about 20 degrees above the horizon. On close inspection the color seems to come off of the whole surface of the plate. But if the light enters, is reflected off of the back surface, and then exits at the same angle, the color dispersion should cancel. Or is this really a plate edge prism effect similar to the sun dogs we see around the sun.
And when conditions are right I can see that the collection of these colored points form a parabola on the surface of the snow opening away from me toward the sun. What is that?
Of course its all just physics and geometry but I would like to read something more about it.
Could you point me towards some information on these little ‘diamonds in the snow’?
I have spent some time searching on the internet without much satisfaction.
@Bob
Hi.
Unfortunately I don’t have any enlightening links for you. Nothing I found using google gave any glimour of insight. (puns intended).
I am not exactly sure what your first question is referring to actually. Your second question about the parabola sounds very neat. I don’t have an answer, but I’ll think about it…
If you find any answers on your own, feel free to report back!