In Helena Puumala’s short
story A Christmas Miracle at the Lake, the character Miles comments
about the beauty of the winter landscape:
Miles was full of news when
he came back, and sat down at the brunch table, eyeing hungrily the plenty
which Hannah, with Cara and Tom’s help, had conjured onto it. Hannah, always glad to feed ravenous
offspring, bade him to to dig in, and brought over a platter of potatoes—the
final dish—from the stove.
They all settled down to
eat, and to listen to what Miles had to tell.
“The world out there is
beautiful,” he began, after downing a glass of juice. “We’re going to have a Christmas card
Christmas.”
“That’s what we were wishing
for, coming here,” Tom said. “Though,
I’m hoping that Mother Nature doesn’t go overboard with the white stuff. Life could get a little awkward if the snow
gets too deep.”
…
“I used to get a laugh from
his absurd attitudes,” he said. “But the
truth is that I just don’t get the final stubbornness. Why not reconcile with your son, when you
know that you’re dying, and that the reconciliation would mean a lot to your
wife? I mean, what was there in it for
him?”
“I think that he really did
believe that God—God as he understood the Deity—loathed gays,” said Cara
thoughtfully. “So to stay in that God’s
good books he thought that he had to denounce and disown Jay.”
“That’s utterly insane,”
protested Miles. “The Deity—whatever
such a being amounts to—must surely love all his creations. It’s impossible to look out at the
magnificence of the snow-clad landscape that surrounds us, and not believe
that.”
Snow
can be quite beautiful. Those of us who
live in wintery climates are familiar with the fact that sunlight shining off
freshly fallen snow can be especially beautiful (and dazzling).
On a recent weekend in my northern city, we had a cold dry snowfall of about 5 centimeters. While out walking one Sunday, I noticed that the snow was not only displaying flashes of white light, as the sunshine reflected off the drifts, but also many flashes of coloured light. This effect was very evident (and striking) especially while walking - one generally perceives motion or changes due to motion more readily than static phenomena.
Though
the effect was more striking to the naked eye than the camera, the photos below
still give a pretty good idea of what I saw.
Note that they are the same photo, but one has been cropped and
magnified in GIMP, though no other image manipulation was done. Note also that cameras tend to make snow look darker than it is, because pictures of snow have a lot of contrast, which they try to correct.
As you can see, there are
quite a number of colours evident in this photo – blues, reds, greens, yellows,
oranges, violets. Pretty well all of the
colours in the visual spectrum mnemonic that we learned in high school (ROYGBIV),
in fact. Outnumbering those, of course,
are the white sparkles that we normally associate with fresh snowfalls.
So, what causes this
colourful display? Basically, it is
because the snowflakes in question are hexagonal (6 sided) plates or
columns. This geometry ensures that
light passes through the snowflake, the same way that it does through a prism.
The consequent differential refraction breaks up the white light sunshine
into its constituent colours which we then see as flashes of colour as we walk
past. The light rays have to have the
proper alignment to end up triggering the colour receptors in our retinas, but
that will happen often enough, given the millions of snowflakes and their
random orientations on the ground.
The diagram below gives an
idea of how that works (from “Brilliant colours from a white snow cover”, Michael
Vollmer1 and Joseph A Shaw2, 2013 Phys. Educ. 48 322). That paper has a lot of interesting data
about this phenomenon, along with some very pretty photos.
I haven’t noticed this
effect all that often in my life, even though I have lived in winter
cities all my life. That's because a particular set of circumstances must come together to create the effect.
A combination of cold, dry,
windy weather (temperatures were -20 C or less), producing “platy” snow,
followed by bright sunshine seems to do the trick. As noted, the snow has to have certain
geometric characteristics (hexagonal plates or columns), in order to nicely
produce the diffraction effects that lead to the colours. A bit of wind as the snow falls ensures that
the flakes rest lightly, and in random orientations on the old snow. Then, bright sunshine creates the nearly
parallel rays that can be refracted from the snow flakes. Voila – spectacular colours, glittering off
the snow.
The effect doesn’t seem to
last for long – it was quite striking for a couple of days, but after that it
seemed like I all I could see were the usual while glints off the snow. As the paper noted above says, morphological
changes (caused by snow packing/settling, sublimation, melting, air pollution, etc.) in the
snow eliminate the effect in a short while.
Of course, if it was too common, we wouldn’t consider it to be as
beautiful as we do.
By the way, you may wonder
why I called this an Astrophysics Corner blog – well, the sun is a star and it
plays the key role in this phenomenon.
And, for good measure, here is an XKCS comic about refraction. :).
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