Plain English Snow Science: for off-piste skiers and snowboarders

 10 minute read. Written by George Treble. Photos by yours truly unless credit otherwise stated. Learn more about Off Piste snow & avalanche safety here.

The idea that no two snowflakes are the same is down to the fact that there’s about a quintillion water molecules in a typical crystal.

That’s a billion billion.

The number of possible combinations and arrangements of these quintillion H₂O molecules is so vast that it is generally accepted that all snowflakes are unique.

While it is theoretically possible for two snowflakes to be identical, it would be somewhat of a massive hassle to try check them all.

During snowstorms I sometimes find myself drooling in admiration at the intricate beauty of nature as I try and fathom this complexity, but then I normally go back to catching flakes in my mouth and hooning around like a dog on the beach with the zoomies.

Each snowflake starts as a unique work of art – a beautiful hexagonal crystal which appears from the skies and floats down to earth.

This magical substance is the stuff of fairytales, yet it actually exists in the world that we live in.

Not only that but it serves as an excellent mountain lubricant that us humans can slide around on.

In this blog we're going to cover some interesting stuff about snow which is useful to know about for avalanche averse skiers and snowboarders.

In order to understand some of the key stuff, let’s follow the lifecycle of a snowflake

Snowflakes form when water vapour in the air cools and condenses around a small particle, like dust, for example. The particle acts as a nucleation point – a surface for the water vapour to adhere to.

This is real. life. magic, folks.

Water vapour goes directly from gas to solid, with no liquid phase, and voila: a snow crystal begins!

The technical term for this is 'deposition' - when water vapour goes from gas to solid with no liquid phase.

As more water vapor in the air continues to deposit onto the surface of the crystal, it grows outward, typically forming branches from its six corners.

Snowflakes are typically hexagonal, featuring six arms which radiate from a central point.

(photo by Kenneth Libbrecht, Credit: Barcroft Media)

THE WIND

It’s a tough life, being a snowflake. As soon as you’re born you can expect rapidly changing conditions.

Often, as a snowflake falls it will be subject to the wind.

The effects of the wind on the snow are extremely important for skiers and snowboarders to pay attention to 

The wind bashes the snowflakes around and knocks off their extremities.

It creates smaller, more rounded particles.

This is called ‘fragmentation’.

When it falls on the ground, this type of snow can often become more compacted and cohesive.

This compacted and cohesive snow can form blocks and slabs, which can lead to more dangerous avalanche conditions.

A photo of wind slab, located near a ridge on a leeward slope aspect.

Not only does the wind smash the flakes into smaller particles, but it also blows snow into certain places.

The wind strips snow off aspects which face toward the wind and loads it up on aspects which face away from the wind.

From windward to leeward.

So if it has been windy, leeward aspects, particularly near ridges and passes, are prone to wind slab – which is one of the five key avalanche problems us European skiers and snowboarders need to know about.

Photo: wind slab avalanches on a leeward aspect. Fresh cornices on the ridge above indicate the wind had been blowing toward the camera, from right to left.

OK let's move on from wind slab and talk about what happens to snow once it's on the ground.

 

SNOW METAMORPHISM

Once the snow has fallen on the ground, it is subject to various processes of change.

The fancy word for snow changing is ‘metamorphism’

One of the more common types of metamorphism is 'rounding'.

In the rounding process, the grains become more blob shaped and bond to neighbouring grains.

We often call this rounded snow ‘equilibrium snow’ or simply ‘rounds’.

If all snow were as well behaved as equilibrium snow, we wouldn’t need to worry about most avalanches.

We'll talk about other types of snow crystal metamorphism shortly.

 

TEMPERATURE GRADIENTS

Now pay attention to this peculiar phenomenon.

Ready?

The temperature at the bottom of the snowpack, where the snow meets the ground, is normally at, or close to, zero degrees celsius.

(There are exceptions, of course, such as shallow snowpacks in extremely cold temperatures or in high latitude areas of permafrost - but it's a pretty strong general rule for most snowpacks).

Why?

Because the ground releases heat and the snow insulates the base of the snowpack from temperatures above.

At the top of the snowpack, however, temperatures are all over the place. It could be zero, or it could be minus 20.

When there is a difference between the temperature at the bottom and the top of the snowpack we call this a ‘temperature gradient’.

Photo credit: Amélie Bouan

Let’s say for example that the air temperature is -10 degrees, and the base of the pack is zero, and the snow is 100cm deep.

That’s a temperature gradient of 1 degree every ten centimeters.

The bigger the difference and the smaller the distance, the steeper the temperature gradient.

These temperature gradients set water vapour in motion.

Water can do a really cool trick called ‘sublimation’, where it changes directly from solid ice to gas.

Once it becomes water vapour in the tiny air spaces between snow crystals, it can diffuse from warmer areas to colder areas – (because it goes from high pressure to low pressure)

And that’s not all – it can also go directly from gas to solid ice again! This is called ‘deposition’ (remember what happened when a snowflake first formed?)

 

KINETIC GROWTH AND FACETED GRAINS

When the vapour deposits onto existing crystals we call this kinetic growth.

This ‘kinetic growth’ creates ‘Faceted Grains’

Faceted grains are sharp edged structures with many sides.

So when water vapour diffuses rapidly, it changes rounded crystals into faceted ones.

The stronger the temperature gradient, the more kinetic growth occurs, the more faceted grains are created.

Photo: Faceted Grains

You might be thinking why is this all important to skiing and snowboarding?

Well, because, the problem with faceted grains is that they are less cohesive and bond poorly with their neighbours.

You know that loose, sugary sort of snow? It can act a bit like a layer of ball bearings in the snow.

They might look beautiful when magnified, but they form a weak layer in the snowpack.

And when you have weak layers, you’re building the recipe for avalanches.

This is why when you have extended periods of cold temperatures, weak layers can grow within the snowpack, even if the snow pack was stable before.

So steep temperature gradients can change rounded crystals to faceted ones - from strong snow into weak snow.

(By the way, weak temperature gradients can turn faceted grains back into rounds, although this process in reverse occurs more slowly). 

1°C per 10cm is approximately the boundary: more than this and we get faceting, less than this and we get rounding (although this depends on the temperature of the snow; in very cold snowpacks, a higher temperature gradient is required for faceting) (Tremper, 2013)

This faceting typically occurs under the snow and out of sight, making it unpredictable to those of us on the surface. We need to dig snow pits and perform stability tests in order to find out about it, which takes a lot of time, effort and skill.

Fortunately - for those of us in populated areas like the European Alps - daily avalanche bulletins will also have information about buried weak layers - be sure to read them before going off-piste.

PERSISTENT WEAK LAYERS

The annoying thing about these weak layers is they can last for extended periods of time.

Once there is a weak layer in the snow, it can ‘persist’ for weeks or even months.

This is why we call them ‘persistent weak layers’ and this is another one of the five key avalanche problems which us European skiers and snowboarders should to pay attention to.

Faceted grains can create persistent weak layers - although they are not the only way these layers are created.

 

Quick recap:

Now you understand fragmentation and the formation of wind slab.

You also know about snow metamorphism – rounding and kinetic growth which creates faceting.

You also know about temperature gradients and persistent weak layers.

SURFACE HOAR

Next up, let’s talk about surface hoar.

Surface hoar is just a fancy name for frost.

This is the feathery stuff that you sometimes see on the surface.

It doesn’t fall from the sky, it grows on the ground, when it’s calm, cold and humid.

These ice feathers are beautiful, and can get pretty big when the cold/calm conditions last a long time.

If it’s windy they can be blown away.

But if it is calm and starts snowing, the surface hoar can become buried, forming a nasty weak layer.

Imagine a load of fragile champagne glasses buried in the snow with weak stems.

The weight of a human above could easily break one of the stems of the glasses, and once one breaks, it's likely it would lead to neighbouring ones fracturing as well.

Add in a steep enough slope and you have an excellent recipe for an avalanche.

So does that mean buried surface hoar is called depth hoar?

Nope, sorry.

We call buried surface hoar, wait for it…… ‘buried surface hoar’!

DEPTH HOAR is a different thing – it is faceted snow near the ground.

It’s a common misconception.

‘Hoar’ comes from an old English word ‘hār’ meaning grey or white.

Surface hoar and depth hoar share the same word, but they originate differently.

 

SPRING SNOW

Next up let’s talk about another common way that snow changes, aside from rounding and kinetic growth.

‘Melt Freeze Metamorphism’ is when melt-freeze cycles change the crystalline structure of the snow.

It can happen any time but is most common in the spring, when the snow thaws on sunny days and then re-freezes at night.

We often call it spring snow, or ‘corn snow’ as the north americans refer to it.

It produces coarse, granular snow crystals.

If there are repeated melt-freeze cycles, this normally stabilises the snowpack, since it can eliminate any pre-existing weak layers.

GRAUPEL

This is that styrofoam ball type of snow that looks and behaves like ball bearings.

It forms from strong convective activity (upward vertical motion) where supercooled water droplets freeze upon contact with snow crystals.

If you have come across graupel, you'll probably remember it stinging your face in the wind.

Once buried, graupel can form weak layers in the snow, although it might not last as long as other weak layers like faceted snow.

 

WEAK INTERFACES

Normally, avalanche fractures occur in a weak layer sandwiched between two harder layers.

However, sometimes there isn’t a weak layer, but a fracture can occur at the bond between two strong layers; i.e. a weak interface.

For example, a wind slab might slide directly on a hard rain crust, without a distinct weak layer being involved.

When the sun melts the snow and it refreezes, this can create a sun crust.

When rain makes the snow wet and then it re-freezes, this can create a rain crust.

Crusts like this can make a good bed surface for avalanches to slide upon.

 

After a period of rain - you can often see 'rain runnels' like this, where liquid water is channeled down the fall-line.

CONCLUSION

OK, so hopefully now you have an understanding of some of the important stuff that happens to snow which leads to stronger and weaker layers within the snowpack.

If you can remember them all, you'll have a lot of valuable new words in your glossary: fragmentation, wind slab, snow metamorphism, rounding, kinetic growth, faceting, persistent weak layers, melt-freeze metamorphism, surface hoar, depth hoar, graupel, weak interfaces, sublimation and depostion. 

You won't truly understand all these concepts until you experience them in the real world.

But I hope this blog helps you to understand some of the key stuff about snow that is important for off-piste skiers and snowboarders to pay attention , since it can help us to score better conditions and avoid being killed by avalanches.

Let me know any questions, thoughts or feedback via [email protected].

Happy sliding folks!

Learn more about Off Piste snow & avalanche safety here.