Weather, like nature, is meant to be random.

But according to our research, temperatures in some of Australia's biggest cities follow weekly cycles that appear to match our working habits.

It's new evidence of human influence on the world around us.

Our study, published in the journal Environmental Research Letters, shows that if you're in Melbourne the hottest day of the week will most likely be a Thursday, while Sunday is the coolest. Same goes for Brisbane.

But what exactly does that mean?

Given nothing in nature follows a weekly cycle, it's proof positive of the influence humans have on cities, as well as a fascinating insight into the patterns of our working week.

Scientists have been examining the relationship between human activities and weekly variations in weather for almost a century, since James Reginald Ashworth "accidently" discovered a weekly Sunday minimum in rainfall in Rochdale, England, in the late 1920s.

It's inspired many weekly cycle studies since.

Last year we published a study – along with co-author Professor Nigel Tapper from Monash University – which found a Sunday minimum in global fires – the first time that has been identified.

For this one, we wanted to examine a trend a bit closer to home.

We generate heat by using electricity, using air conditioning or heating and powering motor vehicles. All of this activity has had a definable impact on temperatures.

It's the reason why big cities, such as Melbourne and Sydney are labelled 'urban heat islands', where the buzz of human activity makes them measurably warmer than surrounding areas.

It also deposits waste heat and particulate matter into the atmosphere.

Heat generation tends to build up after several days, which goes to the main finding of our study that most of the hot days are towards the end of the working week.

Getting weekly

The study of weekly cycles in nature is an emerging field of study as we strive to measure our impact on the world we live in.

For our research we looked at basic surface temperatures (two metres above ground) in seven of Australia's cities, including Sydney, Melbourne, Brisbane, Cairns, Perth, Hobart and Adelaide.

We collected temperature data from the Bureau of Meteorology at three-hour intervals dating as far back as 1943 (for some cities).

Nature being random, there should have no statistically significant pattern to when the hottest or coldest days of the week are.

And while that was true for some of the data, we managed to identify some clear and distinctive weekly patterns in the bigger cities, such as Melbourne and Sydney.

The hottest times (compared to the average) in Melbourne and Brisbane were on Thursdays at 9am, which coincides with the usual morning rush on our roads. Sydney's hottest 9am time was on a Friday, while all three cities experienced a temperature low on Sunday morning.

MORNINGS VS NIGHT

Our research found a much stronger weekly cycle in the early mornings compared to the afternoon. This is most likely because in the AM, the cool ambient temperature acts as a surface barrier, trapping the heat.

By contrast, in the afternoon such temperature inversions have usually been eroded and the heat can be carried away to higher levels.

Similarly, there are fewer cars on our roads on the weekends, particularly on Sunday morning compared to weekday mornings. This key difference shows us the impact cars are having on urban temperatures. It is this difference in the morning urban activity that provides a window for us to measure the temperature of the city without cars (Sunday) and with cars (Monday to Friday) at the same time of day and it's significantly cooler on Sundays.

So Melbourne and other major cities are cooler at the weekend during the day because they're less busy, but what about night life? In western cultures, Friday and Saturday evenings are the standard for heading to clubs, restaurants, theatres, etc.

This means that there are enhanced traffic volumes and human activity on these evenings through to the early hours of the following day.

It ties in then, that Melbourne's midnight temperature is warmest on Saturday (Friday night) and Sunday (Saturday night) – the opposite signal to that observed to 9am.

Interestingly though, we weren't able to establish a signal for Sydney, which might come down to the location of the Sydney weather station. It's on relatively high ground and potentially well-ventilated, meaning it could be less affected by heat trapping.

With governments committing to creating cooler, greener and more liveable cities, we hope these results can go some way to informing on how our behaviour affects the urban environment.

It could help policy-makers figure out how traffic amelioration measures might be most effective and how we might one day be empowered to reduce urban temperatures at critical times, such as heat waves.

Banner Image: Sun's reflection in building. Picture: jcwoody/Flickr