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.

Me, in the bathroom before class, pondering what to do about my saturated clothing. Photo: Author’s own

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.

Synoptic chart showing a cold front over Melbourne. Image: Bureau of Meteorology

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.

Ben Brown style run up required to jump those puddles to get to Swanston St tram stop. Photo: Author’s own

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.

 


9 Responses to “Four Seasons in One Day”

  1. Ruoyu Duan says:

    My hometown is Kunming which is located in southwest of China. People always call it ‘Spring city’ .This city is also famous for rice noodle .Ha ha ha ,welcome to my hometown!

  2. Kimberley Reid says:

    Interesting, it’s not just Melbourne! What is your hometown? If you don’t mind me asking

  3. Ruoyu Duan says:

    Weather of my hometown is similar to Melbourne’s. When I was a high school student, I went home by bike. When I left school, the weather was sunny and comfortable,but when I went through two blocks, it was raining and cold….
    Thanks for your explanation. I can understand why the wired thing always happen in my hometown.

  4. Kimberley Reid says:

    We’ve actually had TWO cold fronts pass over Melbourne in the last 24 hours. Lucky us! While the front passes we tend to get cumulonimbus and cumulus congestus clouds. These produce heavy rain. A key feature of cumulus clouds (unlike stratiform clouds) is they tend to be scattered rather than cover the whole sky, and they are smaller. That’s why sometimes it can be raining in the city while it’s perfectly sunny in other suburbs. Once the cold front passes, we tend to get patchy and higher up clouds. Because of these types of clouds, we get the alternating between sunny and heavy rain….Don’t worry, we can always science Melbourne’s weather!

  5. Debbie says:

    Just as I read this, the weather goes from being beautifully sunny, to bucketing rain, to sunny whilst raining…Sometimes I don’t think we can logic/science Melbourne’s weather – even it doesn’t know what it is doing!

  6. yangx5 says:

    Great! I always feel curious about the changeable weather in Melbourne and now I know the reason!

  7. Kimberley Reid says:

    Yes, talk of further drought is unpleasant.

    Victoria’s rainfall is affected by multiple climate drivers. These are oscillations of pressure, temperature and winds in the atmosphere that occur on large time scales (years to decades) which can impact our local weather. Like the El Nino-Southern Oscillation (ENSO), these other climate drivers have phases when it’s more likely to rain in Victoria (e.g. La Nina) and phases when it is less likely to rain (e.g. El Nino). Another one of these drivers are the Indian Ocean Dipole (IOD) which is a sea surface temperature difference across the Indian Ocean that can cause large bands of cloud to stretch from north west to south east Australia when the IOD is in the negative phase, or it can reduce rain in Australia when the IOD is in the positive phase. The shifting of the cold fronts which I spoke about is part of an oscillation called the Southern Annular Mode (SAM) which has a negative correlation with Victorian rainfall. (Negative phase of SAM brings rain and cold fronts to Victoria).

    The weather we actually receive is influenced by ALL these drivers combined. So, the ‘Millennium Drought’ you refer to was most likely due to a combination of an El Nino event, a positive IOD and fewer cold fronts (and a few other things I won’t go into but I can suggest papers if you’re interested). So, the severity of the next El Nino will depend on what these other drivers are also doing. La Nina will return, but I have no idea when. If you want to dance for rain, I suggest dancing for a negative IOD and SAM as well as a La Nina, but overall, Victorian rainfall is in decline.

  8. Ehlana Tompsett says:

    As a regional Victorian who grew up with buckets in the bottom of my 3 minute max showers, to recycle as much of our restricted water supply as possible, in the 13 years of drought; I’ve effectively been doing rain dances since the government announced the drought’s break in 2010, due to the La Nina activity. Given that my family and I are still struggling to comprehend this strange, magical new color of “green” that is growing on things now whenever I return home, I seriously doubt my emotional readiness for talk of further drought.
    Does this mean the La Nina event isn’t the miraculous rain bringer we have worshiped it to be these past few years? What can we expect when it’s cycle returns to it’s drier less welcome brother El Nino, combined with this continuing trend of fewer cold fronts you speak of? Will these greater effects of climate change prevent the return of La Nina in the future? Should I be prepared to hang up my rain dancing shoes for good?

  9. Alex says:

    Cool, thanks! I already wondered what the cause of the changeable Melbournian weather was, that everyone kept telling me about.