If the daytime heatwave temperatures aren’t bad enough, Melbourne and Adelaide residents will also have to put up with nightly temperatures significantly higher than surrounding rural areas thanks to amplification caused by the urban heat island effect.

It’s normal for cities to be warmer than surrounding rural areas at night but researchers from the ARC Centre of Excellence for Climate Extremes at Monash University found heatwaves make this difference almost two and a half times greater under some heatwave conditions.

“Our research shows that heatwaves in Melbourne and Adelaide amplify the difference in night time temperatures between urban and rural areas even more than usual,” said Dr Ailie Gallant from the School of Earth, Atmosphere and Environment, who was a co-author on the paper.

“Under usual circumstances, Adelaide and Melbourne are up to 1.4°C warmer than their rural neighbours but when a heatwave kicks in the nightly temperatures in these cities can be on average 1.2°C – 3.3°C warmer.”

The research, lead by PhD student Cassandra Rogers, means it will be more difficult for vulnerable residents to get respite from the hot daytime temperatures that accompany a persistent heatwave.

“This can have significant consequences for those who are vulnerable to heat, particularly the very young and very old. A degree or two can make a big difference to the body’s ability to cool down”,” Dr Gallant said.

The paper, which was published in the journal Theoretical and Applied Climatology looked at three Australian cities, Adelaide, Melbourne and Perth, to see how night-time temperatures changed during heatwaves compared to surrounding areas. Intriguingly, while Melbourne and Adelaide sweltered, Perth was actually cooler than surrounding rural areas.

The reason for this was not immediately apparent with wind speed and direction, sea breezes, and the locations of the weather stations used in the research ruled out as the likely cause of these differences.

“Perth is certainly an anomaly and we are currently investigating the characteristics of the area and large scale weather patterns to see if these play a role, but as the current heatwave hits Adelaide and Melbourne residents need to be aware of night-time conditions,” Dr Gallant said.

Past research has shown the presence of vegetation, large water bodies and fewer concrete surfaces help reduce the urban heat island effect at night and even during the day during our warm summers.

“With increasing numbers of heat records being broken and longer heatwaves, likely the result of background climate change, research by climate scientists is pointing the way for city planners to reduce the impacts of overnight temperatures,” Dr Gallant said.

“It will be fascinating to see how new urban areas are designed and changes to current urban areas will be introduced to make our lived environments safer and cooler places for our residents during these massive heatwave events.”