Due to the “lag effect” of the concrete, the city does not cool down

Even if the hot weather at the start of the week knocks you out, don’t sit on the floor. The temperature is probably even higher there. This Monday and Tuesday, the peak of the heatwave episode is reached in Paris and Ile-de-France with temperatures that can exceed 40 degrees. Floor coverings, which accumulate heat throughout the day, are at risk of heating up. “Bitumen or concrete reflect little light, and potential heat is stored in these materials”, explains Erwan Cordeau, in charge of climate, air and energy studies at the Paris Region Institute.

A phenomenon that is difficult to quantify. “The temperature of concrete varies according to its weight, its porosity, as well as its albedo, a factor which changes according to the color, he explains. If its color is dark and its mass large, the material stores more heat. »

Concrete warms cities at night

The heat contained in the floor coverings is only released into the air 6 to 10 hours later, during the end of the day and then at night. This “delay effect” complicates the cooling of cities and aggravates extreme heat waves. “Unlike the countryside, which gets cold easily at night, the heat is difficult to disperse in the city because of what the materials give off”, specifies Erwan Cordeau.

The risk of excess mortality during a wave of extreme heat is assessed in particular on the basis of nocturnal heat indicators. In the capital, the situation is considered dangerous from three consecutive nights without the temperature dropping below 20°. “Our body needs to cool down at night, reminds the expert. If it is hot day and night, this can be a problem. »

Refresh the floors

White paint, less concrete… Lowering the temperature, “it’s a subtle game, says the research manager. We play on color, we use more or less natural materials”. In this perspective, the town hall of Paris is experimenting with its project Cool & Low Noise designed to reduce heat and noise emissions. She leaves until 2027 to study the properties of new road surfaces on five pilot streets in Paris.

The towns of the inner suburbs are not lacking in imagination either. “Oasis schoolyards, dedicated to revegetating the soil, are spreading in cities,” he explains. In Aubervilliers, the former car park of a hostel for young workers has been transformed into a porous coating capable of allowing vegetation to flourish. For Erwan Cordeau, “we can completely vegetate while responding to traffic constraints, by carefully studying the possibilities of the soil. »

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