How busy is your city? The temperature will tell you

Melbourne’s busiest day is Thursday, Sydneysiders are hardest at it on a Friday and Sunday commonly appears to be a day of rest. That’s according to research led by the University of Melbourne, which investigated temperatures in Australia’s major cities to reveal fascinating insights into how human activity affects the urban environment.

Dr Nick Earl and Professor Ian Simmonds, along with Professor Nigel Tapper from Monash University, analysed surface temperature data dating as far back as 1943 from seven of Australia’s major cities, including Melbourne, Sydney, Brisbane, Perth, Adelaide, Hobart and Cairns. Their results have been published in the journal Environmental Research Letters.

The study showed that weekly cycles in daytime temperatures occur in almost all of the tested cities, with the strongest signals in those with bigger populations, such as Sydney and Melbourne. Early mornings versus the afternoon also gave more statistically significant results.

Some of the study’s findings are as follows:

• Testing temperatures at 9 am delivered the firmest signals, with both Brisbane and Melbourne’s weekly cycle peaking on a Thursday, with a Sunday minimum.
• Sydney peaks on a Friday, also with a Sunday minimum.
• There are two peaks in daytime temperature during the working week, typically from 7–9 am and 4–7 pm.
• Comparing 9 am (daytime) and midnight (night-time) temperatures, weekends were significantly warmer than weekdays.

Professor Simmonds said the study could prove valuable as cities move towards more sustainable living by demonstrating how our behaviour affects the city we live in.

“It’s really a peek into the weekly routines of Australia’s big-city dwellers,” he said.

“Generating electricity, driving cars and using air conditioning and heaters all deposit waste heat into the atmosphere and it shows in the city temperature.”

While a weekly pattern was not evident in all cities or for all tested times, Professor Simmonds said it was possible other factors were clouding the pattern. Sydney, for example, didn’t have warmer weekend nights like Melbourne or Brisbane, which the authors suggest could have to do with its location well above sea level.

Mornings were also generally warmer than the afternoons, with mornings more often associated with atmospheric inversions which can trap waste heat near the surface. By contrast, by the afternoon such inversions have evaporated, thereby carrying heat away to higher levels.

The impact of Melburnians on their city was another focus of the study, with its urban heat island (UHI) effect — ie, the fact that cities having higher temperatures than surrounding areas — analysed for a weekly pattern. Melbourne’s UHI peaks between 9 pm and midnight, highlighting the biggest contrast in activity while the difference is least significant between 9 am and 3 pm.