Snow Metamorphism Ask Dave a weather question

Dave Thurlow, Host

 
It's hard to think of anything calmer than a fresh blanket of snow. But there's a lot going on beneath that mantle of white. Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook.

Surface Hoar Layer Photo: Dr. Bruce Jamieson

On mountainsides, snow that's buried far below the surface can change character over a few days or weeks, and this can make an avalanche more likely as time goes by. It's called snow metamorphism. When the first snow falls in the mountains, the ground may not yet be frozen. As winter sets in, the layers pile on top of each other, and each snow tends to fall at a colder temperature.

This temperature difference tends to push water vapor upward through the space between snowflakes. It's a bit like water vapor condensing on the outside of a soda glass on a hot summer day, except here all the action takes place out of sight, within the snow.

As the water vapor flows up and condenses, some of the fallen flakes begin to change. They may lose the classic starry, feathered shape, which has lots of branches that interlock and help keep a snowpack tied together. This makes it harder for them cling to each other. Before long, a stable layer of snow has turned into an avalanche waiting to happen. That's why it's so important for avalanche forecasters to make regular checks on a winter snowpack by digging into the snow. Only then can they tell if the crystals are sticking together or falling apart.

Thanks today to contributing writer Robert Henson. For more information on snow metamorphism, visit our website at weathernotebook.org. Jay Allison is The weather Notebook's series Senior editor. Our show is underwritten by Subaru with major support provided by the National Science Foundation.