Just a few extra degrees of heat can create major problems for public health, energy supplies and ecosystems. During August 2003 many countries across Europe sweltered under a heatwave, and an estimated 70,000 people are believed to have died as a result of excess temperatures. When the next heatwave takes place, helping city dwellers should be a priority, but which areas of a city are most vulnerable to the urban climate effects, and which people need the most help?

To answer such questions, Jason Schatz and Christopher Kucharik from the University of Wisconsin-Madison, US, carried out a high-resolution study of the urban climate effect in their home city. Madison is small, with a population of 402,000, surrounded by lakes, agriculture, forests, wetlands and grasslands. During the summer of 2012 and the winter of 2013 to 2014 the pair monitored temperature and humidity in and around Madison with an array of 150 sensors. By complete chance their timing was perfect.

"It just so happened that the summer of 2012 was the third hottest since 1869," said Schatz. "And then it just so happened that the winter of 2013–14 was the coldest in 35 years. It's hard to do that kind of study on purpose. You really have to be in the right place at the right time."

During the heatwave, Madison's urban areas experienced twice as many hours with temperatures over 32.2°C as surrounding rural areas, as well as maximum temperatures up to 1.8°C higher and minimum temperatures up to 5.3°C higher. But these averages don't give the entire picture. At the peak of the heatwave some of the densest urban areas in Madison spent over four consecutive nights above the National Weather Service nighttime heat stress threshold of 26.7°C, Schatz and Kucharik also found, whilst rural areas fell below this temperature every night.

"The official airport station was only over 26.7°C on a single night during this period, so a warning was never issued, and overnight cooling shelters were not opened," said Schatz. "Heat stress was underestimated for most of Madison's population." What’s more, the airport station recorded 39 days over 32.2°C, but Schatz and Kucharik estimated that downtown Madison experienced 49 days over this threshold. "The urban heat island can dramatically increase the duration of extreme heat," said Schatz.

The results, published in Environmental Research Letters (ERL), reveal that the urban heat island effect is largely proportional to the density of the built environment, but heat generated from vehicles and air-conditioners also plays a role. The urban heat island effect was also amplified during the heatwave compared to ordinary summer conditions, the team found, and the impact was strongest at night.

The measurements from the severe winter of 2013–14, meanwhile, showed that the urban heat island effect also increases temperatures in winter, but the effect tends to diminish during the most extreme weather. "The coldest conditions often occur on windy days with severe wind chills, and the urban heat island is greatly weakened by high winds," said Schatz.

Like Madison, many other cities are likely to have a complex urban climate. A better understanding of how a city responds to climate extremes will help cities around the world prepare for the next heat wave or cold snap by ensuring that the interventions match the temperatures people actually experience, and that populations are well prepared and resilient.

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