`Let it snow, let it snow, let it snow.''
You know that song, right?
Well, it was playing the other day at the mall. And as I was mouthing the words, I started thinking about that fluffy white stuff that goes crunch-crunch under your feet.
Then I realized that I didn't know a whole lot about snow, except that it makes pine trees look pretty, ski conditions better, snowmen possible, and driving a bit tricky.
So, I made a phone call to a ''snow expert,'' Pao Wang.
Dr. Wang is a professor of atmospheric science at the University of Wisconsin. In other words, he researches, studies, and teaches people about what goes on in the clouds.
Snow just happens to be his specialty.
My burning question for Dr. Wang was this: People like to say that no two snowflakes are the same. Is that true?
Dr. Wang was sorry to disappoint me, but he says that idea is based more on folklore than on fact.
''Actually, we're talking about a wide variety [of snowflakes], when you look under a microscope. But snowflakes are very simple ice crystals. Most of them are six-sided, or hexagonal. Many of them look exactly alike,'' he says.
Some freak snowflakes can be three-sided or 12-sided, but in general they are six-sided and come in different formations. These shapes are called ''habits,'' he explains.
Common habits include:
Columnar. This type of snowflake grows in columns.
Planar. This type grows to look like a flat plate.
Dendrite. This type is much more complicated, with many branches - or ''dendrites'' - going in different directions, making pretty designs. Most people think of the dendrite type when they think of snowflakes, says Dr. Wang.
How do snowflakes form? ''In order to have snow, you must have water (in the clouds), cold temperatures, and certain particles, such as dust particles,'' Dr. Wang says.
Water can stay ''supercool'' and still not freeze in the clouds, he explains.
When dust particles collide with the supercool water droplets, they freeze and crystallize together.
A simple crystal is called a pristine crystal. When a few pristine crystals lump together, they make a flake.
These flakes then grow to a fairly large size and fall as snow. (The lighter ones just kind of hang around in the clouds.)
In fact, many times when it rains, even in the summer, the raindrops are actually snowflakes that have melted, Dr. Wang says.
The average size of a snowflake varies, but most are just a few millimeters (about 1/8 to 1/4 of an inch).
Snow is actually very dry, adds Dr. Wang, even though it's crystallized water.
A fun experiment is to pack a large container, such as a bucket, with snow and see how much of it melts into water. (You'll find the water to be only a fraction of the snow's density.)
As for ski resorts, they sometimes use snow-making machines. These ''snowblowers'' spray supercold water along with a ''particle'' (some of them use silver iodide or a special bacteria) into the cold winter air.
But what you end up with are not really snow ''flakes,'' Dr. Wang says.
The particles and water droplets are frozen too quickly to form crystals. Instead, they become miniature ice balls.
Some people call it ''granular'' snow - kind of like granular sugar - because the particles are round, not flaky.
With natural snow, water crystallizes in the clouds ''in a more leisurely way,'' Dr. Wang explains. That's why snowflakes turn out prettier and softer than ''man-made'' snow.