The physics of a snowball

What does it take to make the per­fect snow­ball? Oh, just a little physics. Photo via Think­stock.

If you grew up in the North­east you know that there are good days for making snow­balls and there are bad days for making snow­balls. You know that some­times, despite your best efforts to squeeze the snow into a per­fectly rounded orb, it just won’t stick.

J. Murray Gibson, founding dean of the Col­lege of Sci­ence, told me why. The problem, he said, is that the tem­per­a­ture of the snow is too cold for you to weld it into a self-​​contained ball.

Welding may seem like a weird term to use here, but Gibson said that’s exactly what you’re doing when you make a snow­ball or even a snowman. As you squeeze the snow into a more com­pact space, it melts from a solid into a liquid. When you release it, the liquid freezes, fusing the loosely con­nected snowflakes in your hand into a larger solid ball.

It turns out water is the only com­monly occur­ring mate­rial in the uni­verse that does this…and it’s there­fore the only mate­rial that could ever be used to make your buddy Frosty, no matter how many corncob pipes you’ve got lying around.

Here’s the deal: water, unlike any other mate­rial out there, is less dense in its solid state than in its liquid state. It’s less dense because the unique shape and prop­er­ties of water mol­e­cules make it very hard to build a dense solid crystal. It’s why ice floats in a cup of water and it’s why trying to com­press solid snow actu­ally causes it to liq­uefy and so take up less space. Most other mate­rials are denser in their solid form, so no amount of pres­sure will get them to liquefy.

When it’s super cold out­side, the amount of pres­sure needed to melt a handful of snow is greater than our hands are able to create. So it’s on those below zero days that you’ve prob­ably been least suc­cessful in your snowman building efforts. Temps right around freezing are your best bet. Also, try not to let it sit for too long, Gibson said. If you do, larger ice crys­tals form which present a whole other set of snow­ball making prob­lems that I won’t go into now.

It’s clearly too cold to be making a snow­ball. This person should just go inside and have some hot choco­late. Photo via Thinkstock.

While snow­balls are of obvious sig­nif­i­cance, espe­cially this time of year, the same phe­nom­enon that makes them pos­sible is also what makes life on earth pos­sible. Yes of course water is impor­tant for biology for a dozen dif­ferent rea­sons, not the least of which is its incred­ible sta­bility and inert­ness. But some­thing I didn’t really think about until our chat is the fact that it’s also impor­tant for cre­ating the geo­log­ical char­acter of our planet.

When water seeps inside the cracks in the ground and then freezes, it expands. This causes the rock to smash apart, Gibson said, and it is sin­gle­hand­edly respon­sible for ero­sion. Without ero­sion, he con­tinued, we’d have no flat plains or soil, which turn out to be really impor­tant for growing food and finding shelter.

But back to the physics of winter fun. Ice skating, Gibson said, is another cold weather activity that we can only enjoy because of water’s unique prop­erty to melt when it’s com­pressed. All of your weight local­izes to the sharp edge of your skate’s blade, putting an incred­ible amount of pres­sure on the ice, causing it to melt. This cre­ates an almost fric­tion­less envi­ron­ment, which, as Gibson put it, “allows you to glide and slide beau­ti­fully for long dis­tances.”  (Assuming of course you have the skill not to fall on your back­side, which he con­fesses to lacking, but that’s another story…)