Monday, February 9, 2015

The Physics of Building a Snowman

Recently, Smithsonian Magazine reported a story about the physics in building a snowman.

Although you can buy a ready-made snowman kit, or follow the instructions on, or even read various patents (posted previously on this blog) we all know, the fun is in the construction with the cold, wet, white stuff - the "free art supplies" that fall from the sky.

Understanding the physical properties of snow can help you figure out whether creating a snowman is possible and the principles of stacking can then guide your efforts.

Firstthe snow. “Snow can either be too wet or too dry,” points out Dan Snowman, a physicist at Rhode Island College in Providence. Scientists actually classify snow based on its moisture content—the amount of free water relative to ice crystals—not to be confused with the amount of water the snow would produce if melted. 

By that scale, 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 very well, while 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. 

Snow crystal shapes change significantly with temperature.
(Kenneth Libbrecht/Based on experiments by U. Nakaya)

Chart from:

The surrounding air temperature mainly determines the amount of water in snow, as well as its crystal structure. Wet and moist snows fall at around 32 degrees Fahrenheit. Far below-freezing temperatures make for drier snow, because more water particles freeze into crystals. Freshly fallen wet or moist snow crystals are usually shaped like classic branching snowflakes, called dendrites, providing lots of surface area for the watery glue to bind with. Colder conditions produce flat plate shapes with less surface area, making it even harder to mold the dry powder into snowballs and snowmen.

“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,” Dan Snowman says.

Selecting your snowman-building surface. Level ground is best, but asphalt absorbs and holds heat from sunlight, so avoid driveways. A flat spot near the bottom of a large hill could provide shade and keep your creation safe from direct warmth from the sun—although it may wind up as a target for sleds.

Spheres are the best building blocks for snowmen. Snowman says, Forming snowballs and packing the snow together exerts pressure on the ice crystals so that some melt during construction. “After melting, the water will crystallize once again, binding together the snowball.” 

Looking at snowman-building as a way to teach basic engineering principles, students at Bluefield State College in West Virginia suggest that 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.

When it comes to stacking, the standard large-medium-small structure is the way to go 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—if you could balance a snowman on your finger, holding it at the center of mass would keep it stable. The closer that point is to the ground, the less likely a vertical object is to fall over.

You can read more about this science of snowman making in the full article at

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