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- Weather, Snow, and Snowpack layers
Weather creates avalanche conditions, primarily through a combination of Rain or Snowfall, Wind and Temperature.
Snow – it comes in different varieties, and can be described by many words. Powdery, chunky, buttery, chalky, funky, crusty, light/heavy, dry/wet, and even the oxymoron 'packed powder'?(according to the ski resort marketing depts. – haha)
The list is long and seldom scientific. Alot 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.)
Snowpack layers - 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 potential slab avalanche.)Here is an example snow pack test that highlights where weak layers can be located.
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, and humidity.
Exposure to Wind
In the mountains, especially close to ridge lines, 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 retain more or less snow depending on:
ASPECT- which direction a slope faces. Some aspects will be sheltered from the wind and collect more snow (sometimes called Lee areas), and other aspects will be exposed to the wind and have much of the snow stripped away.ALTITUDE - Generally the higher on the mountain the stronger the wind will be.
Lee Slopes –When the wind blows, snow is taken from upwind areas and gets redeposited in downwind, sheltered areas. We call the sheltered areas lee slopes (from the sailing term leeward). You can see this alot around terrain which changes abruptly like ridge lines. Signs of lee slope snow loading include smooth, pillow shaped snowdrifts and overhanging cornices.
In the picture below you can see that the wind has been from left to right. Left of the ridge it is back to bare rock and tussock, but on the right side of the ridge (the lee side), cornices have formed and there are large deposits of snow.
Lee slopes are not always at the top of a slope. In the picture above you can see where the wind has swept across the mountain face from left to right, overloading the bright white side of the vertical gullies. We call this ‘cross loading’.
Mountains act like large sails sticking up into the sky. They bend and deflect the speed and direction of the wind as it flows over the terrain. We call this ‘local wind effect’, and it is always present. Example: Even if you know it’s been blowing Northwest all night, and expect to see wind drifted snow piled up on Southeast aspects, there will be variation due to ‘local wind effect’.
Exposure to Sun
Over time the heat from the sun can help to stabilise the snowpack, but if we get too much too quickly it can do the opposite. If a slope gets little or no sun, it can also prolong unstable conditions.
SHADY SLOPES - slopes facing south (southerly aspects) will remain colder as they receive little or no sun. New snow will take longer to stabilise, and weaknesses in the snowpack can persist.
SUNNY SLOPES - slopes facing North (northerly aspects) receive the most heat from the sun. When this heating is intense or rapid it can quickly weaken the snow on these aspects. It is common in springtime to see naturally triggered loose snow avalanches in the middle of the day.
As we are surrounded by ocean in NZ, 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, especially when warmer air is coming down from the North or Northwest. These changes in temperature and humidity can warm and melt the surface snows, and then have it refreeze into crusts. You can find crusts on the snow's surface or it may be buried by the next storm or wind event.
Once snow has settled to the ground the story does not end. Layers of the snowpack continue 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)
Below: ski tourers boot packing on a firm morning crust
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.