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!
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.
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photo by Vaughan Cornish, public domain |