Building the perfect snowman can be a serious challenge. Often people's efforts turn out a little lumpy or lopsided and quickly melt into unrecognizable shapes. When you decide to build one, it helps to have science on your side.
Understanding the physical properties of snow can help you figure out whether a snowman is even feasible. Scientists classify snow based on its moisture content, or the amount of free water relative to ice crystals. Snow comes in five categories. It can be dry (zero percent water), moist (less than 3 percent), wet (3 to 8 percent), very wet (8 to 15 percent) and slush (more than 15 percent).
Moist to wet snow is ideal for snowman building, according to Jordy Hendrikx, a snow scientist at Montana State University. Dry snow is like a loose powder with particles that don't stick together well. Slush is too fluid to hold a shape. "You can think of the free water as the glue.' You need enough to stick the crystals together, but not too much. Otherwise it won't form a solid snowman," says Hendrikx.
The air temperature during a snowstorm determines the amount of water in snow. Wet and moist snows fall at around 32 degrees Fahrenheit. Temperatures that are far below-freezing make for drier snow. More water particles freeze into crystals. "Years of experimentation and research with my kids reveal a snow-to-water equivalence of about 5:1 yields the snow ideal for building the perfect snowman," says Dan Snowman, a physicist at Rhode Island College in Providence.
This special blend of moist or wet snow falls when it is just a degree or two above or below freezing. On the U.S. East Coast, a cool, wet Nor'easter would do the trick. The blizzards that have hit the Northeast this winter are examples.
Dry powder is more likely during frigid conditions. Powder offers a soft, smooth ride that is ideal for skiing, but not for building snowmen.
Once the snow is on the ground, it's time to build your snowman. Spheres are the best building blocks, because forming snowballs and packing the snow together exerts pressure on the ice crystals. The pressure makes some crystals melt during construction. "After melting, the water will crystallize once again, binding together the snowball," Snowman notes.
Stacking the spheres in the usual large to medium to small structure is the way to avoid toppling. "Keeping the snowman's center of mass low is paramount in the construction of any snowman," says Snowman. The center of mass refers to the point in any object where its mass is concentrated. The closer that point is to the ground, the less likely a vertical object is to fall over.
Students at Bluefield State College in West Virginia see snowman-building as a way to teach basic engineering principles. They say the optimal diameter ratio for the snowballs is 3:2:1 from bottom to top. This ratio keeps the base at a sufficient size to support the combined weight of the top two snowballs. And don't build your snowman too big. There may be an upper limit to the size of the spheres related to the water content of the snow.
Building your snowman in spheres can also help it achieve longevity. Because the shape minimizes the surface area exposed to rising temperatures, it slows down melting. Unfortunately, melting is inevitable as temperatures rise. Thanks to Frozen, we all know what happens to snowmen in summer.
Critical thinking challenge: Explain how moisture in snow acts like "glue," and explain why too much or too little doesn't work.