March 9, 2017

What's with all the Snow Mushrooms on the Fenceposts?

If you've driven between Banff and Lake Louise this winter, you've probably noticed the big globes of snow on top of the wildlife fencing posts. Or, if you've been snowshoeing with us, you've seen even bigger piles of snow on stumps or broken tree tops

How do these "snow mushrooms" form? Especially when the surface they're sitting on can be as small as a dessert plate.

To answer, we need to welcome you to the wonderful world of snow science, and specifically the process of equi-temperature or destructive metamorphism. We hope you're ready for a bit of physics!

When new snow falls in the Canadian Rockies through the winter, the cold temperatures almost always guarantee that it will fall as light, low density "champagne powder." But it doesn't stay that way.  If snow lands on something above the ground, like a log suspended in the air, or a stump, it will start to change. The newest snow will always be light and fluffy, but snow that's been on the log or the stump for a few weeks or months will feel like really high density styrofoam. You are hard pressed to push a ski pole into it more than a few inches, and the snow crystals stick to each other tenaciously, even though new snow in the Rockies can't even be formed into a snowball.

Here's what's going on: since the temperature around the suspended snow is the same at any given time (hence "equi-temperature"), the snowflakes start to break down in a surprising way: water in the spiky ends of the flakes sublimates (meaning it evaporates directly from ice into water vapour). The water vapour that is produced through this sublimation moves from the higher vapour pressure at the edges of the snowflakes toward the lower vapour pressure around the interiors of the snow crystals.

Still with us? This small scale water migration means that airy, fluffy snowflakes become denser over time, and closer together. This promotes the stickiness of the snow, meaning it can occupy a bit more space than the surface area of the stump or fencepost. With each subsequent snowfall the surface area for collecting new snow increases, until a giant blob of snow can sit on the tiniest of surfaces.  Big snow mushrooms can measure more than 3 metres across.

photo by Vaughan Cornish, public domain
We are not the first to notice these snow mushrooms.  In 1900, English geographer Vaughan Cornish took a winter trip on the CPR through western Canada.  He photographed giant snow mushrooms in Glacier National Park. Here's one of his archival photos, plus a few more from us.