Human activity heats up cities to such an extent that on the weekends when we're having a sleep-in, the centre of town is measurably cooler.
It's a different story at 9am on weekdays when work draws people into the CBD, according to research from Melbourne and Monash universities.
![Researcher Nick Earl in Melbourne at the coolest time of the week - early Sunday morning.](/web/20170212092613im_/http://www.smh.com.au/content/dam/images/g/u/a/w/e/d/image.related.articleLeadwide.620x349.gua658.png/1486880679298.jpg)
"Everyone has a lie-in on Sunday and that is reflected in a city's temperature," said Nick Earl from Melbourne University's school of earth sciences.
The findings revealed that 9am on a Sunday morning was the "coolest" time to be in town, with the city's air temperature up to 0.3 degrees milder than an average Thursday or Friday morning at 9am – which were identified as the warmest days of the week.
![Our collective weekly routine affects city temperatures, as people move in and out of town for work.](/web/20170212092613im_/http://www.smh.com.au/content/dam/images/g/t/z/a/6/9/image.related.articleLeadwide.620x349.gua658.png/1486880679298.jpg)
But not all Australian cities are the same. In Melbourne Thursday was the warmest day while in Sydney, Friday came out on top.
The reason the warmer days occur at the end of the working week is because the heat generated by human activity tends to build up over several days. However the study, outlined in the journal Environmental Research Letters, also revealed a weekend spike in average midnight temperatures as people returned to town for a Saturday night out.
"I wasn't expecting such strong signals when I started looking into it," Dr Earl said. "I was actually surprised."
Other large cities such as Brisbane and Adelaide had similar weekly cycles according to the research, which used 50 years' worth of Bureau of Meteorology air temperature data. For smaller populated cities such as Cairns, Adelaide and Hobart, the temperature spike was not as evident.
![Dr Nick Earl had Melbourne's CBD to himself on Sunday morning.](/web/20170212092613im_/http://www.smh.com.au/content/dam/images/g/u/a/w/m/c/image.related.articleLeadwide.620x349.gua658.png/1486880679298.jpg)
Dr Earl said "waste heat" generated by human activities such as driving cars and using public transport, heaters and air-conditioners was reflected in the weekday warmth cycle.
Given nothing in nature conforms to a weekly routine, the fluctuating temperatures identified in the study provide clear evidence of human influence on the environment, he said.
![Traffic is the main contributor to higher city temperatures on week days.](/web/20170212092613im_/http://www.smh.com.au/content/dam/images/g/t/k/o/8/d/image.related.articleLeadNarrow.300x0.gua658.png/1486880679298.jpg)
"It highlights that we can affect the temperature immediately, whereas people often think it's all due to with the concrete and the land use in cities," Dr Earl said.
"But while that has an affect, on a hot day you could ban cars from the city centre and that would make it cooler ... I'd say by more than a degree on a non-windy hot day."
Dr Earl said the findings had other practical applications including encouraging people to work from home which would potentially enable air-conditioning to be turned off and see a reduction in cars on the road.
Previous studies on city temperature cycles have been confined to maximum, minimum or average temperatures.
"This study is different as we have used three-hourly snapshots of the air temperature," Dr Earl said.