Four Seasons in One Day
Ancient wisdom says: if you don’t like the weather in Melbourne, just wait five minutes.
Winter, 2016, early afternoon.
I meandered out of my geography tutorial wearing jeans and a t-shirt when a blast of wind whipped my hair across my face and the cold bit at my exposed arms. Students trudged passed rugged up in coloured knitwear and black Northface jackets leaping over puddles and gushing gutters. I felt like the biggest idiot wearing a t-shirt in that weather. But, it had been sunny when I arrived for class an hour ago. I was a meteorology student; I should be the last person not dressing properly for the weather!
I threw on my jacket and ambled along Swanston Street to my next class. It is really amazing how quickly your day can turn. I was nearly at the traffic lights to cross over to the Earth Sciences building. Traffic lights went orange. The driver of a white van floored the accelerator attempting to prevent two minutes of his life being wasted at traffic lights (I’m not holding a grudge at all).
Accelerating van. Puddle. Corny movie trope. Me on the footpath next to the traffic lights. You can imagine what happened next…if you can’t, here is a clue.
Melbourne. It’s the city of foodies, footy, fine coffee and four seasons in one day.
But why is our city’s weather so changeable? How can the weather be a sweltering forty degrees one day, followed by a drizzly low twenties maximum the very next?
Two words: Cold fronts. The solid blue lines with spikes that appear on weather maps.
Because of the rotation and tilt of the Earth, different locations receive different amounts of sun (e.g. Townsville receives much more than the South Pole). Air over Melbourne will be warm or cold depending on where it travelled from. For example, in summer, wind from the south sends cold Antarctic air to our shores, while wind from the north carries warm desert air to Melbourne.
Cold fronts mark the boundary between warm and cold air. This boundary is not a peaceful divide, but a battle ground. Warm air soars above the cold like an army of dragons bellowing, not fire but, lightning at the White Walkers below… (Okay, this analogy isn’t working)…but the warm air does rise above the cold causing strong winds and thunderstorms. And rain. Lots of rain.
Between 30-40% of all of Victoria’s rainfall and up to half of our really heavy rainfall is estimated to be from cold fronts.
We need cold fronts to fill our water storages.
But, their numbers are in decline.
Higher or lower than usual sea level pressure (called ‘pressure anomalies’) in the Southern Ocean steers cold fronts away or towards Australia respectively. You would expect to see a roughly even amount of events when the sea level pressure is higher or lower than usual.
Since the 1950s, the high pressure anomalies have occurred more often than low pressure anomalies and the difference in occurrence is getting bigger every year. This means that cold fronts are shifting south – away from Australia. Experts have shown that a combination of high altitude ozone and low altitude greenhouse gases are probably to blame. And with climate change this trend is likely to continue.
What does this mean for Melbourne?
Fewer cold fronts are likely to collide with us in the future. Before you warm weather lovers start rejoicing, remember: we need cold fronts to help fill Victoria’s dams.
What does this mean for our future water supply?
Drought will be more likely. But research into exciting technology to extract water from our surrounds has been ongoing. Victoria’s desalination plant has the capacity to supply one third of our water needs.Towns in NSW, Queensland and Western Australia are already including recycled water into their water supplies.
Next time you’re out and about on a hot day and the wind starts to pick up, be prepared for the possibility of rain and storms later on.