Perth's cool response to heatwave a mystery

Heatwaves can have detrimental effects on people, particularly in large population centres like cities.

Urban populations are also affected by heat stored in or generated by cities. Heat is stored in building materials, such as concrete and asphalt, while vehicles and heating/cooling systems produce additional heat.

This is why cities tend to be hotter than surrounding rural areas at night. It’s what we call the urban heat island effect.

While we know that both heatwaves and the urban heat island effect independently increase urban temperatures, no previous research had investigated the impact of these two phenomena in combination in Australia. This meant it was unclear whether city populations were more vulnerable to heatwaves than their rural neighbours due to the urban heat island effect.

To understand the relationship between heatwaves and the urban heat island, CLEX researchers examined temperatures during heatwaves in three southern Australian cities – Melbourne, Adelaide, and Perth – and compared these to periods without heatwaves.

The researchers found under non-heatwave conditions all three cities had warmer night-time temperatures than the surrounding rural areas, meaning they all experienced a night-time urban heat island.

However during heatwaves the strength of the urban heat island changed.

During heatwaves in Melbourne and Adelaide the urban heat island became warmer than normal, indicating that there was an amplification of the urban heat island during heatwaves.

By contrast, Perth showed a cooler than normal urban heat island effect during heatwaves. Temperatures in the city during heatwaves were actually cooler than expected.

This research is important as it shows that heatwaves can be hotter and therefore more dangerous in cities. If the urban heat island can be minimised in Melbourne and Adelaide this research suggests that heatwave temperatures may be cooler and less dangerous to human life.

The researchers are currently investigating why the results differed for Melbourne, Adelaide, and Perth.

They have determined that wind speed and direction, sea breezes, and the locations of the weather stations used in the research are unlikely to be the cause of these differences and are currently investigating the effect of large-scale weather patterns.

 

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