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"Snowball" as a verb, means to increase rapidly in size - a reference to the Snowball Effect:

The common analogy is with the rolling of a snowball down a snow-covered hillside. As it rolls the ball will pick up more snow, gaining more mass and surface area, and picking up even more snow and momentum as it rolls along.

As someone who grew up many hundreds of kilometres from the nearest mountain with snow, I always accepted from the cartoons and popular culture that that was a natural phenomenon, but I've never seen any evidence for it (avalanches not withstanding).

Do rolling snowballs grow in size, as depicted in popular culture?

  • It's not very scientific, but you can see an experiment testing this idea here: youtube.com/watch?v=VILQ1J9d0zQ. (The snowball picks up snow like a disk for about twenty feet, maybe, then falls over.) – 1006a Apr 18 '17 at 19:13
  • “”I've never seen any evidence for it (avalanches not withstanding)” — I’ve never seen any evidence that wind can damage buildings! (Hurricanes notwithstanding.) – Paul D. Waite Apr 19 '17 at 15:54
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    @PaulD.Waite Tornadoes notwithstanding as well. – JAB Apr 19 '17 at 20:54
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    @PaulD.Waite: I was trying to avoid answers that tried to lump avalanches in with rolling snowballs. Yes, they involve snow falling. Yes, they grow quickly. No, they aren't the image conjured when the word "snowballing" is used. – Oddthinking Apr 20 '17 at 12:32
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Only when the snow is "wet", not if the snow is powdery.

As this January 1910 Watson Wagon advertisement says:

The popularity of the Watson continues to grow like a snowball rolling down hill on a wet day.

This can even happen naturally as explained in the US National Weather Service article Snow Rollers Observed Across Central Illinois (Spring 2003)

People in central Illinois observed a strange phenomenon February 11 and 12. Log-shaped "snowballs" showed up on lawns, fields, and other open areas. This is a phenomenon referred to as "snow rollers". These are formed under specific weather conditions:

• The ground surface must have an icy, crusty snow, on which falling snow cannot stick.

• About an inch or so of loose, wet snow must accumulate.

• Gusty and strong winds are needed to scoop out chunks of snow.

Snowfall of 1 to 4 inches occurred across central Illinois the morning of February 11. That evening, as a strong cold front pushed through the area, wind gusts of 40 to 60 mph were noted in many areas.
Once the initial "seed" of the roller is started, it begins to roll. It collects additional snow from the ground as it rolls along, leaving trails behind it. The appearance is similar to building snowmen, except the snowball is more log-shaped rather than spherical, and many times they are hollow. They can be as small as a golf ball, or as large as a 30 gallon drum, but typically they are about 10 to 12 inches in diameter.

enter image description here

The same phenomenon can also occur on hills as explained by Snow Roller Photos by Arnold Brokling:

Alternatively, gravity can move the snow rollers as when a snowball, such as those that will fall from a tree or cliff, lands on a steep hill and begins to roll down the hill.

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    I love both answers, but this is evidence of a natural phenomenon. If it can do it on a flat surface, surely it can do it on a mountain. Bonus points for a coyote with a stick of dynamite tumbling inside it. – corsiKa Apr 19 '17 at 8:11
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    @Sklivvz The whole question/answer is obvious to people in snowy areas. There is a continuum of wet to powdery and the more powdery the more you would have to squeeze the snow with your hands to make a snowball. Sure even little kids know this in places where there is snow, but people in Chennai say might need to know powdery snow doesn't form snowballs unless you really compress it. – DavePhD Apr 19 '17 at 12:36
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    @Phil Frost, it really depends on where you are. I've lived in Finland, generally considered a fairly snowy country, for the last decade and a half, and I have certainly never seen one of these. Even if it's common wherever you live, it can still be "rare" when considered globally. – flith Apr 19 '17 at 16:54
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    @flith Of course it depends on where you are. Earth has diverse climates everywhere. Saying something that isn't found globally is "rare" is pretty absurd. For example american black bears are found "only" on 1 of seven continents, and not anywhere in the oceans which cover "most" of Earth's surface. But no one would call them rare. – Phil Frost Apr 19 '17 at 18:48
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    Clearly I meant "rare" for snowy countries, but well done on taking that extreme and really running with it :) – flith Apr 19 '17 at 18:49
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I would have to say, yes, it does, but as evidence, one looks not at the scenario described of rolling a snowball down a hill.

Instead, in a snowy area, after a significant snowfall and, especially, if it leads to a cancellation of school, go outside and observe children and/or adults building snowmen/women.

How do they build the "snowballs" that are the body parts? There is an initial mounding of snow to create a ball, but to build it into much larger snow boulders, one simply rolls the ball around in the snow, and it increases is size as it picks up more snow from the ground.

YouTube: How We Build A Snowman

The reason why this happens is because snow will stick to other snow because of slight melting. The pressure of hands pushing the ice crystals together or the weight of the snowball pressing down onto snow on the ground causes slight melting at the surface, causing the crystals to stick together. So, if you have really light, fluffy snow, and brutally frigid temperatures, snowballs won't pack very well, and certainly would not accumulate just from rolling onto more snow.

The reason has to do with why snow sticks together in the first place. Snow is basically ice crystals , and when you pack the crystals together, you need to apply enough pressure with your mitten-clad hands to get some of them to melt.

LiveScience: How To Make the Perfect Snowball

This concept obviously applies to larger snowballs as well, which are the foundation of my "snowmen" scenario.

First, let's talk about the 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. Snow comes in five categories: dry (zero percent free water), moist (less than 3 percent), wet (3 to 8 percent), very wet (8 to 15 percent), and slush (more than 15 percent).

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 free water as the 'glue.' You need enough to stick the crystals together, but not too much."

Smithsonian Magazine: Do You Want To Build a Snowman? Physics Can Help

As far as the actual phenomenon of snowballs rolling down a hill, there are probably too many variables in terms of weight, pitch of the hill, initial velicity, moisture content, etc for me to properly find comprehensive analysis that in the space permitted, but most probably the answer is "yes, under the right conditions" for that, as well.

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    My main source of doubt or skepticism is the name of the Rhode Island physicist they chose for the Smithsonian article. :D – PoloHoleSet Apr 18 '17 at 15:23
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    How could you doubt him? I've seen pictures of Snowman — always wearing a smart top hat and contemplatively smoking a pipe. He is clearly a well-educated and refined gent. – Reinstate Monica iamnotmaynard Apr 18 '17 at 15:54
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    The question, though, is about whether a snowball can form under freefall (freeroll?) down a hill being moved only by gravity, rather than being pushed along a flat, as in your answer. The forces and speeds are going to be different, which I think will affect the snowballing. – Reinstate Monica iamnotmaynard Apr 18 '17 at 16:00
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    @iamnotmaynard - no, that's not really the question. "Do rolling snowballs grow in size, as depicted in popular culture?" - basically, do rolling snowballs accumulate more snow and grow. Whether that's someone manually rolling it, or it spontaneously happening in nature (are Shaggy and Scooby falling down a hill really a spontaneous natural event) isn't really germane to the main thrust of the inquiry. That's why I mentioned the freefall aspect more in passing, at the end. At least, that was how I interpreted what was out there. I did consider both aspects. I could certainly be wrong. – PoloHoleSet Apr 18 '17 at 17:10
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    One issue with the pop cultural image is as shown in the video on one of the comments - rolling downhill, they will only grow in one direction, and once the diameter is much larger than the (non-growing) width they are unstable. – Random832 Apr 18 '17 at 20:45

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