Snow’s Properties

“Oh the weather outside is frightful
But the fire is so delightful
And since we’ve no place to go
Let it Snow! Let it Snow! Let it Snow!”
(According to Wikipedia, the song “Let it Snow, Let It Snow, Let it Snow”
was written in July 1945 in Hollywood on one of the hottest days on record.)
Today we’ll answer some questions about snow.
1. Is it true that no two snowflakes are ever the same?
Probably not absolutely true, but it’s easy to understand why we say this.
The diagram below comes from the website Snowcrystals.com. It shows
the life history of a single snow crystal. We are indebted to Kenneth G. Libbrecht,
Caltech, for his extensive research into snowflakes.
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As Libbrecht explains: “The story begins up in a cloud, when a minute
cloud droplet first freezes into a tiny particle of ice. As water vapor starts
condensing on its surface, the ice particle quickly develops facets, thus
becoming a small hexagonal prism. As the crystal becomes larger, branches
begin to sprout from the six corners of the hexagon. Since the atmospheric
conditions (that is, temperature and humidity) are constant across the small
crystal, the six budding arms all grow out at roughly the same rate… While it
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grows, the crystal is blown to and fro inside the clouds, so the temperature it
sees changes randomly with time. Since the six arms see the same conditions at
the same time, they all grow about the same way. The end result is a complex,
branched structure that is also six-folded symmetric.
“The six arms of a snow crystal all grow independently. But since they
grow under the same conditions, all six end up with similar shapes.”
So the answer to the question is that because the growth of a snow crystal
depends so much on the temperature and humidity of the crystal inside the
clouds, and since that may vary dramatically from place to place, the chance of
finding two identical snowflakes is small, but not impossible.
Shown below is a summary of all the shapes possible for a snowflake. This
is also from snowcrystals.com. (The print may be too small to read the name of
each shape, but you can at least see the wide variety of shapes.)
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Aren’t you amazed by the variety of possible snowflake shapes? I certainly am.
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2. Why is snow white?
First we need to talk about color in general. For instance, grass is green
because all the colors of the rainbow found in sunlight are absorbed into the
leaves of the grass except green. Green light, not being absorbed, is eventually
scattered back to our eyes, and so we see green grass. It’s all about what color of
light is not absorbed by the grass.
If you take a piece of clear glass and look at it, it appears to be clear. But if
you grind up the glass and look at the pile, it looks white. Similarly, if you look at
an individual salt or sugar crystal, it appears to be clear. But, as you know, a
spoonful of sugar appears to be white. Where did the white come from?
If you look at an individual snow flake, it will look clear like an icicle does.
But take a pile of snow flakes and it will look white. What’s going on?
Snow and sugar and salt look white because no particular color of sunlight
is absorbed. All the incoming light is reflected back to our eyes.
When you look at your front yard covered with snow, the incident light
bounces around and eventually scatters back to your eyes. All the colors are
scattered equally well, so the snow bank appears white.
3. Why is it so quiet after a snowfall?
According to the book “The Flying Circus of Physics” by Jearl Walker, “The
small spaces in the snow’s surface absorb the sound just as acoustic tile does in
most offices. As the snow becomes more packed, this sound absorption is
reduced.”
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4. Why can’t you make a snowball if the temperature is very low? What holds a
snowball together, anyway? Approximately what is the lowest temperature you
can make a reasonably good snowball?
The Popular Mechanics Magazine web site tells us that ground temperature
has the most to do with making a good snowball. Wet snow packs better than dry
snow. The scientific reason is that melting snow can help to form “ice bridges”
that join two crystals together, while snow at very cold temperatures the crystals
remain separate. The best temperature for a snowball fight or making a snowman
is just below freezing. As many of you have discovered, it’s difficult to make a
snowball when the temperature is 20 degrees below zero or colder.
This is a magnificent season of the year! So if you also have no place to go,
join with all the snow-boarders, skiers, kids with sleds, and snow machine
hobbyists to echo the words, “Let it snow! Let it snow! Let it snow!”