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Snow season forecast to be a bumper after drought factors turn

By Nate Byrne, Wednesday May 20, 2020 - 06:29 EST
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Most people are fascinated by snow, even if they've never seen it. - ABC

It's the final descent into winter, and we're just about to get a decent dump of snow over the mountains in the south-east, with a good 5 centimetres on the forecast over the next few days.

Whether you're rummaging around in the back of your wardrobe for your snow gear or you've never been to the powder, it's hard to find someone that isn't intrigued by the white stuff.

The thing is, snow is even more fascinating than you know. And we're expecting a decent season ahead. Here's why.

OK, first the science

Stay with me, because this bit matters.

Snow is frozen water (so far, so good) but it doesn't freeze from a liquid â€” otherwise we'd all be putting snow in our drinks straight from the freezer.

Instead, snow is formed in a different way that requires some pretty special circumstances.

Here's the step-by-step guide:
Water can stay as a fluid at temperatures below freezing in a super-cooled state
When there is too much water vapour in the air, it gets forced out of its gaseous state
When that happens, the H20 molecules that are whizzing around can attach to something else â€” a bit of pollen, dust, pollution, or something else similarly tiny
Other water vapour molecules then start to attach and it â€¦ahem â€¦ snowballs
If it's cold enough, this forms a tiny ice crystal
The crystal can then start to grow, with more water vapour molecules freezing directly onto the surface without becoming liquid water first
Once it gets large enough, the little chunk of ice starts slowly falling to the ground

Voila â€” it's snowing!

The fact is this is a complicated process that requires particular environmental factors. It's why some places get snow and others never see it.

It's also why, despite their light-hearted attempt, the resolution by the Syracuse Common Council in 1992 that "expressly outlawed" snow in the battered American city was always going to fail.

You can't control the weather.

Why are snowflakes all shaped like that?

It's because those water molecules that attach to the initial ice crystal can only do so in a few specific ways.

It's kind of like Lego â€” there are only a few ways you can position the pieces for them to stick together.

The result is the classic hexagonal shape of every flake of snow.

And yet, you might have heard that every snowflake is unique.

That's because as the growing snowflake falls â€” sometimes over kilometres â€” it encounters different environments with slightly warmer or cooler temperatures, variations in winds, amounts of water vapour etc.

All these factors can influence the way the crystal grows.

But no two flakes of snow travel the exact same path as they fall from a cloud to the ground, hence they are all slightly different while maintaining a similar shape.

Ever heard of summer snow? It's a thing

If there's a particularly strong front, summer snow can happen â€” like in late February when parts of Tasmania saw just enough to settle.

Many clouds can create snow all year, as long as they are high enough, big enough and cold enough.

But the journey to the ground can be a perilous one for a delicate snowflake.

Even if temperatures remain below freezing all the way to the surface, if the flake falls through a particularly dry layer of the atmosphere then ice molecules can be stripped away from the surface.

If the flake falls through a warmer layer, it can end up as a raindrop, or re-freeze into hail.

The upshot is snowflakes are hard to keep around, so sometimes we must look for an alternative.

Snow faking it

We've seen a few dud seasons in Australia in terms of decent snowfalls.

So resorts often, well, resort to having to make the stuff themselves to ensure there's something for the ski bunnies.

The process of human-made snow is very different to the natural process, and you don't end up with the classic snowflake shape.

Instead of a tiny ice crystal at the heart, the machine creates little ice pellets that it blows high into the sky along with a fine mist of water.

The tiny droplets freeze onto the frozen core, creating something that more closely resembles a ball.

Shooting the mix high into the air maximises the chance of the snow freezing completely before it hits the ground and allows any leftover liquid water to evaporate.

In the long run, it all ends up much the same in the snow pack.

But some argue that machine-made snow is easier, or at least more consistent, to groom when preparing the slopes for a day of skiing.

So why are we expecting a good season?

All the main climate drivers that have brought us years of record-breaking drought that culminated in this summer's devastating bushfires have finally started to turn around.

El NiÃ±o is the biggest driver of drought for eastern Australia, but it might surprise you to learn that while we've been close, we haven't actually seen one of those events since March 2016.

Instead, it's been a cruel mix of things to the west and the south that have kept the east so dry.

Waters have finally warmed off the north-west, providing a good source of moisture to be dragged across to the east by cold fronts that are once again skimming through southern Australia.

The increased moisture means that the long-range outlook for winter so far is a lot wetter than we've seen for years.

Add in cold air from Antarctic waters making it up to Australia's south-east, and all of the ingredients are right for bumper snow falls this season.


© ABC 2020

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