Technology

Reason why snow is rare in our major cities

There has been no shortage of snow in eastern Australia over the last week.

Half a metre fell in just a few hours on several ski resorts while the snow made it so far north the flakes were almost blowing into Queensland.

Snow was seen in Orange, Oberon and also in Katoomba, the latter just an hour’s drive from Sydney’s west.

But yet again, Sydney itself was deprived of the cold stuff; as was Melbourne.

Hobart is towered over by an often snowy mountain, yet the actual snow reaches the CBD below perhaps once a decade. Melbourne last saw flurries in 1986, the closest Sydney has got to snow was some sleet the same year.

If sunny Florida can get a smattering from time to time, why are the locals of Hobart not regularly shovelling snow from their driveways?

Much of the reason for this scarcity is due to a striking difference between the northern and southern hemispheres.

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Sydney’s single snowy day

It has actually snowed in Sydney. A mere two centuries or so ago. On June, 28, 1836, almost 4 cm of snow was recorded as falling on Sydney’s Hyde Park. Even the golden sands of Bondi Beach received a sheen of white.

“A razor-keen wind from the west blew pretty strongly at the time and altogether, it was the most English like winter morning … ever experienced,” reported the Sydney Morning Herald.

It was the first time it had snowed in Sydney since colonisation and it hasn’t snowed in the CBD since.

“Australia very rarely sees sea level snow, especially when compared to Europe, Asia and North America,” Sky News Weather meteorologist Rob Sharpe told news.com.au.

But that rare event reveals one fallacy about snow – that is has to be below freezing for it to occur. On that crisp 1836 day in Sydney, the mercury only dipped to 3.3C.

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It doesn’t have to be freezing to snow

Generally snow can easily fall at temperatures as warm as 2C and even as high as 5C. But at that temperature it will often fall as sleet, which is a soggy mixture of rain and snow.

Sydney’s CBD dipped to 5.8C on Friday morning this week, just a touch above 5C. Melbourne, however, regularly gets below that level stalling at 1.7C on May, 30.

The key is for the air temperature within clouds to be at freezing or below. Precipitation will then form into ice crystals. If the ground temperature is close to or below zero the crystals will fall individually and the snow will be powdery. If it’s slightly warmer the crystals will melt a touch and merge with others forming large chunky flakes.

But if there’s no moisture in the air it doesn’t matter how cold it is, there won’t be any snow.

It’s worth noting that hail is very different to snow. It’s formed in the high altitude fury of thunderstorm clouds and can occur in summer.

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Australia straddles a snowy latitude

A lack of snow is nothing to do with being at sea level or living in coastal cities. Sea level snow is quite normal across the globe. However, if you’re at sea level, generally snow only occurs between either pole and the latitudes of 35° north and 35° south.

All of Europe lies above 35° north. Nonetheless, other meteorological conditions can be at play. The seaside cities of New York and Barcelona are both at about 40° north. But while Manhattan plays host to paralysing snow storms, Spain is warmed by the Gulfstream which makes such an event far less likely. Although Barcelona did see lots of snow in 2018.

Sydney is just a touch above of this 35° magic line meaning snow is not a given at sea level.

Snow can fall on the wrong side of the 35th parallel, but that usually occurs on higher ground where the temperature is lower.

Orange, in NSW’s central west, is beyond 35° south but compensates for this by being at an elevation of 900m which means it gets snow most years.

All of Australia’s ski resorts are both below the 35th and are high up – Thredbo’s summit is 2037m. So they have two snowy elements in their favour.

Adelaide sits directly on 35°, Melbourne is well below 35° and Hobart is even further south.

Yet snow remains elusive.

The role of land mass in snow formation

The major reason snow is so rare in our cities, even those below the 35° line, is because of the land surrounding Australia – or lack of it.

There’s just so much more, well, earth in Earth’s north compared to its south.

“In the northern hemisphere there is substantial amounts of land between the poles and these mid latitude cities such as New York, meaning that the air mass heading towards those cities are continental and can remain very cold,” said Sky News Weather’s Mr Sharpe.

“Meanwhile, the air masses that come from Antarctica to Australia travel across vast amounts of ocean, which is warmer than land at the same latitude, therefore warming our air masses considerably by the time they arrive.”

In some cases, water can do the reverse and aid snow production. The “lake snow effect” is when cold continental air rushes across lakes or some smaller seas. It then picks up warmer moisture from the lake, the droplets freeze, and snow is dumped on shore. But there aren’t any lakes big enough to have that effect in Australia.

Hobart and Canberra have seen the most snow out of our capital cities,” said Mr Sharpe.

Canberra ended up just missing out on snow out of this event as there was too much clear sky yesterday, allowing warming before the wet weather kicked in during the afternoon,” said Mr Sharpe.

Canberra is just below the 35° line and relatively high at almost 600 metres increasing its chances of the white stuff.

The scarcity of snow in most of Australia’s major cities comes down to many things: their location at sea level, their latitude and the lack of land mass.

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