What are snow rollers?

An Ottawa woman captured footage of a rare meteorological phenomenon.

Sheila Nemcsok spotted dozens of snow rollers lying on the lawn outside the Supreme Court of Canada Thursday. She even recorded a rare video of their formation. But what exactly are snow rollers and how do they form?

The National Post spoke to David Phillips, a senior climatologist at Environment Canada, to learn more about “nature’s snowballs.”

Q What are snow rollers?

A A snowball is tightly packed and connected, but snow rollers are feathery light. There’s no pressure put on them, apart from the rolling. There’s no human packing that could make them stronger. They’re almost like blown glass, in the sense that they’re very fragile. I would describe them as discarded, poorly rolled carpet from a family room. There’s something imperfect about them, yet that makes it perfect. That makes it nature’s snowball.

Q How rare are they?

A It’s one of those things that one should feel very lucky to see. It may only be a once-ina-lifetime situation. They’re a little bit more common than crop circles, but they’re still as mysterious as that. It created quite a buzz in Ottawa. My gosh, I’ve been in this business for 50 years and I’ve never seen them in person. I’ve seen them on roofs, but that’s cheating a bit. When you actually see them on the ground, with no footprints around, it’s special.

Q What conditions are needed for them to form?

A You have to have strong winds to push them forward, along with a sloping surface.

There can’t be any blades of grass in the way, because that would stop it.

If there’s sticky snow and warm air, it’s the perfect recipe. But then what you need is the winds at a certain character.

The winds have to almost blast. In Ottawa, the winds were gusting up to 40 km/h, so what that did was get the snowballs rolling.

Q Where are you most likely to see them?

A When I heard about the snow rollers in Ottawa I thought to myself, “That can’t be true. There are no snow rollers in urban areas.” They’re often described being seen in conservation areas, fields and places that are far from civilization.

Q How long do they last?

A They don’t normally survive much beyond hours after occurring.

Q What can cause them to disappear?

A They can be there now, but gone an hour from now. Often, they break ... The wind, temperature, atmosphere can change them and shape them. If the sun comes out, it could melt them. Even temperatures below freezing can still warm the crystals and begin to change the whole morphology.

Q Do they serve any purpose?

A There’s nothing economically important about these. It’s just one of those things that sparks curiosity.

Q Are there any misconceptions about them?

A People often think, “Oh just a bit of wet snow and wind and you get them.” No. No. No. No. It has to be the absolute right amount of everything.