50°C in Jacobabad in Pakistan, 49°C in Dadu in India…While the thermometer is panicking in Asia, elected officials in major French cities are worried: are we too heading towards a heat wave? similar to 2019? In our country, temperatures have already exceeded seasonal norms for 38 consecutive days, a record. And the cooling is not for now: the hot winds coming from North Africa will blow on the territory this weekend. Result: some models predict temperatures between 36 and 39 degrees in several departments, this Sunday, May 22. We are still only in the month of May and already, the cities take on the air of a furnace. Something to remind us of the records of three years ago (42.5 degrees in Montpellier and more than 45 degrees in certain areas of Vaucluse and Gard).
“While we put a lot of money on the table for the renovation of buildings, France is still too little concerned about urban heat islands, notes Morgane Nicol, director of the Territories program at the Institute of the economy for the climate.”Cities lack the tools to organize themselves in the face of this threat,” confirms Roland Pellenq, research director at the CNRS. However, we are now able to predict, from meteorological data, the hotspots of a space at the scale of the street corner. How ? Thanks to urban physics, which takes into account the layout of buildings, the height of buildings and the materials used.”
Up to 10 degrees difference within the same city
Within the same city, the temperature difference between the hottest and coldest areas sometimes reaches 5 degrees. In some American cities, differences of 10 degrees have even been noted! The problem comes in particular from the return of heat by materials such as concrete or tar during the night. “We realize that there is a fundamental difference between well-ordered cities and the others. The former, like New York, tend to retain more heat, while others like Paris are less exposed to this effect. radiative”, observes Roland Pellenq.
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On his computer screen, the scientist shows us a temperature map of the French capital on a hot day. While almost the entire city is a bright red, indicating sweltering heat, a few more breathable islands – colored orange or yellow – appear here and there. A large, relatively cool area even emerges above the Bois de Boulogne, to the west of the city.
“With our mathematical equations, we can not only see what the temperature will be like district by district. But we also test hypotheses: for example, what temperature would it be like in Marseille if this city was made up only of pavilions or, at the reverse, only buildings? This digital tool would give local authorities the possibility of warning certain nursing homes by saying: ‘Be careful, it will be particularly hot in your neighborhood'”, explains the researcher. The physics of cities would also make it possible to rethink the construction of new districts.
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“Town planners tend to extend the avenues in a straight line and transplant existing patterns, observes Roland Pellenq. However, by favoring the existing order, they contribute to heat islands. In the city of Marseille where it is already very hot, that can be a problem.” The expert’s recommendation? Vary the heights, break the regularity of the buildings, plant trees… A zest of chaos necessary to find a little air.
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