«Depending on the wind direction, the weather can vary a fair bit as you move inland,» says Peter Newham, a duty forecaster at the Bureau of Meteorology.
It all has to do with Melbourne’s weather station. Which, unfortunately, is not really in Melbourne at all.
For 107 years, the Bureau of Meteorology’s weather readings for Melbourne came from a squat little box in La Trobe Street, just opposite Carlton Gardens.
That box was installed back when Melbourne was largely made up of low-rise buildings. But the forest of skyscrapers that grew around it over the last century eventually meant it did not get enough sun, wind or rain to make accurate measurements.
«As the city gets built up more and more, having an observation site that’s surrounded by buildings is not going to be that representative. The wind observation at La Trobe wasn’t very useful because the wind would swirl in different directions, funnel up and down streets,» Mr Newham said.
So in 2015, the Bureau of Meteorology closed that station and opened a new one at Olympic Park, south-east of the city and right on Collingwood’s training oval. The new station is much better at recognising wind speeds and comes with a host of other improvements.
But it’s not really in what we commonly think of as Melbourne. It’s closer to the Nylex Clock in Richmond than it is to Flinders Street Station.
And, crucially, the site is more exposed to winds coming off Port Phillip Bay than the CBD is. That makes it great for measuring wind speed, but not so great at getting the precise temperature right in the CBD.
Wind blowing from the south is cool because it runs over the Bay’s cold waters. That cool air reaches Olympic Park first, cooling the site, but often fails to penetrate far into the city’s skyscraper forest.
That means the site can be showing a reading that is lower – by up to three degrees – than the actual weather in the CBD’s north.
When both the Olympic Park and La Trobe stations were running, the temperatures could be as much as three degrees different, Mr Newham said.
Of course, the CBD is not the only place where the Bureau of Meteorology’s weather reading can be different to what is felt on the ground.
This is a result of a simple fact: the Bureau of Meteorology cannot put a weather station in every single suburb.
The Bureau of Meteorology has four weather stations around Melbourne: Laverton, Essendon Airport, Viewbank and Moorabbin Airport.
The Bureau of Meteorology uses supercomputers to establish a ‘grid’ between those stations. By inputting wind data, they are able to take a best-guess at what the weather will be at each point in between the stations.
For example, says Mr Newham, «Olympic Park might be 23, and Essendon might be 27. So there is some sort of graduation across that area in the temperature.»
The Bureau of Meteorology’s model looks at that gradient and predicts that, say, Flemington will be 25.
It’s a good guess, made by a supercomputer and informed by the best possible science. but it’s only a guess.
So the next time you hear the temperature got to 27 when you know it felt like 30, you might be right.
Liam is The Age and Sydney Morning Herald’s science reporter