If you’ve ever gone into the backcountry after a cold clear night, you might have noticed a layer of feather-like crystals sparkling in the chilly winter sun. This is what we know in snow-science-speak as surface hoar. Really it’s frozen dew, and it forms and grows in the shade or overnight in sub-zero temperatures with very light or calm wind and humid air. It may look beautiful, but it has the potential to be a weak layer in the snowpack, on top of which another layer of snow can release as an avalanche.
The winds and damp precip we often get in Australia before a storm will destroy the surface hoar which is why it’s often not a problem here. However if we get a dump of snow directly on top of that surface hoar, without the usual warm front, then we should all be very wary of this red flag during the very cold days after the storm.
Surface hoar as a layer in the snowpack is thin, very weak and commonly forms on hard bed surfaces such as an ice crust, making it slippery. When critically loaded, just one thump will catastrophically collapse all the columns and possibly send the slab on top avalanching down the hill.
In very cold winter climates such as Colorado and Switzerland, the surface hoar can remain as a persistent layer for weeks or months, hidden below multiple layers, ready to release. In Australia, the warmer days we get allow for that buried layer to ‘heal’, bonding to the layers below and above it. So we are not prone to the feared persistent weak layer which is so dangerous in other climates.
Don’t get complacent though – in the right conditions in Australia, surface hoar is a serious factor to consider when planning your travel in the backcountry!