This week, Everyday Einstein looks at some of the science behind snow.
A couple of weeks ago, we had our first snowfall of the season. Our kids eagerly ran outside to build all of the great snow structures they’d been dreaming of. Snow forts, snowmen, snow angels, and of course, snowballs.
Unfortunately, they quickly realized that despite the fact that we had several inches of snow, it was virtually useless for building. Instead of the nice sticky snow that is a little snow sculptor’s dream, they were faced with a fine, white powder. Snow fort walls crumbled, snow balls evaporated into mist as they flew through the air, and Frosty never quite got off the ground.
After coming inside for some conciliatory hot chocolate, they wondered why the snow was so powdery. To answer that question, we have to go deep inside the snow...right down to the molecular level.
Molecules and Movement
Before we look at powdery snow vs sticky snow, we have to take a moment and look at heat. It might seem a bit odd to talk about heat in a discussion about snow, but heat is the secret ingredient to making the perfect snowball.
As I’ve mentioned in the past, heat is a form of energy. When we say that something is hot, what we’re really saying is that it contains heat energy. As heat energy is added to a substance, it causes that substance’s molecules to start to vibrate more...well...energetically. If we take the heat away, those vibrations slow down as the substance cools off.
Now Back to Snow
You might be thinking, that’s nice and all, but what about my snow fort? Well, when snow heats up a bit, not enough to melt, but when it gets closer to the melting point, the molecules inside the snow begin to vibrate.
Now, most people know that snow is really frozen water, (except for yellow snow, which we won’t discuss here). Water molecules are polar, meaning that they have partial charges on each side of the molecule which allow them to stick to one another. This property of water is important for many of water’s super powers, such as cohesion, adhesion, and surface tension.
Now if you have a water molecule that is vibrating around, and it bumps into another water molecule, they will have a tendency to stick together. This is what happens with sticky snow. The snow is heated just enough to allow some of the water molecules to vibrate sufficiently to bump into other water molecules, and they all start to stick together.
This is also why some brave (or crazy) people can make snowballs out of even the most powdery snow, by taking off their gloves and compressing the snow in their bare hands. The heat from their hands is transferred into the snowball, causing the outer layer to become sticky enough to make a decent snowball.
If the weather outside is too cold, the vibrations slow down, and the molecules don’t have much of a chance to bump into their friends. The result is powdery snow. Pretty good for skiing, but pretty lousy for snowball fights.
The Dark Side of Snow
Of course not all snow is fun and games. Every winter snowfall brings with it a need for clearing sidewalks and driveways, as well as spreading salt to keep them clear of ice.
The reason spreading salt on your sidewalk prevents the formation of ice is because adding salt to water lowers the temperature at which it can freeze. In other words, the temperature has to be much colder for salt water to freeze than regular water.
The reason for this is that salt mixes in with the vibrating molecules, preventing the water from forming the nice crystal structures needed for making ice. One interesting fact that you might not know is that salt isn’t the only thing that can do this. Instead of salt, you could spread sugar, or any other substance that mixes with water, and achieve a similar effect. The reason salt is generally used is because it is relatively cheap.
So now you know a little more about the science of snow. Hopefully you’ll get to spend more time playing with snow forts and snowmen this year than you’ll have to spend shoveling the sidewalk.
If you have a question that you’d like to see on a future episode, send me an email at firstname.lastname@example.org. If you liked today’s episode, you can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein.