Online Course - Basics - Snow and Snowpack Layering
Weather drives avalanche conditions, primarily through a combination of Snowfall, Wind and Temperature.
Snow comes in different varieties, and can be described by many words. Powdery, chunky, buttery, chalky, funky, crusty, light/heavy, dry/wet, packed powder... The list is long and seldom scientific.
A lot of these words imply an indication of strength which is good, as some snow types are relatively strong, and others are relatively weak.
THINK – making snowballs – sometimes it’s easy to press together snow to form a strong hard packed ball, other times it’s tricky because even if you squeeze the snow hard together it doesn’t want to bind and remains loose and weak.
As snow settles on the ground it forms itself into layers of different strengths. These layers start to bind together forming what we call the ‘Snowpack’. Not all of the layers stick to adjacent layers with the same quality. Some will bind together well, and others relatively poorly.
To have a layer of weak snow underneath a stronger layer can be a recipe for a potential slab avalanche.
Watch how in this video a strong dense block of snow (Slab) slides cleanly off the underlying snow (weak layer).
You can imagine if this amount of snow avalanched across a whole slope how devastating it could be.
These stronger and weaker layers are not evenly laid out across the terrain. Some places will have more layers than others, and the depths of the layers can vary from place to place. These variations are heavily influenced and modified by changes in wind, temperature, solar radiation, and humidity.
In NZ it is rare to have a storm without wind, and as it blows, the wind can strip the snow away in places and pile it up in others. Different parts of the mountains will receive more or less snow depending on which direction they face (aspect), and how high up they are (altitude/elevation).
As we are surrounded by Ocean, our temperature and humidity can yoyo up and down. It is not uncommon to receive a mixture of rain and snow even at high elevations during the winter months.
Once snow has settled to the ground the story does not end. The snowpack continues to be under the influence of the weather. Here are two examples you may have experienced:
Winds can shift snow. What might have been fresh loose snow in the morning, can get stripped back to reveal an old hard layer below by the afternoon(Wind transportation)
The sun can heat the snow surface during the day making it damp, only to have a cold overnight temperature refreeze it, turning it into a hard icy crust the next morning.(Melt/Freeze)
The weather is always changing, so it’s reasonable to expect that the snowpack’s layers are also always changing because of the influence from the weather.
These changes can occur quite fast and produce dramatic results that are easy to notice, for example: the wind pushing loose snow to fill in your freshly made tracks. Change can also happen slowly and be more difficult to spot because the clues are hidden down below the snow’s surface.