Why cities swelter, especially at night, during heat waves

On July ^1st , the Lyon metropolitan area was sweltering under temperatures of over 36°C in the shade. It wasn't until 5:30 a.m. the following morning that the temperature dropped below 25°C... but only in the outlying municipalities. In the center of Lyon, the temperature remained above 27°C all night, complicating the chances of restful sleep for its residents.

The same phenomenon occurred in Marseille. While the day was cooler in the Phocaean city than in other metropolitan areas, due to the sea breeze that cools the city during the day, the thermometer struggled to cool down in the evening. During the coldest hours, the temperature in the city center was still two degrees higher than in other metropolitan areas.

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