MAPS: The top five worst French cities to spend a heatwave in

Local France

26-05-2025

Heatwaves in France
are dangerous
, and they are becoming both more common and more severe as the planet warms and they can be especially bad in cities due to the 'urban heat island' effect.
In French, this is referred to as an
îlot de chaleur
urbain
, and it is the phenomenon that makes urban areas hotter than rural ones.
In
îlots de chaleur
, the heat becomes trapped, which can happen for several reasons such as high density, particularly of towers and apartment buildings, a lack of green space and concrete-covered streets which retain heat, combined with heat-producing activities such as traffic and air conditioning units.
As such, temperatures are sometimes more than 10C hotter in cities than in the countryside during the daytime, and temperatures often stay elevated at night due to the heat being trapped in the urban area.
READ MORE:
Trees to trams: How French cities are adapting to summer heatwaves
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Which parts of France are most vulnerable?
However city design is also important, and can make the different between a town becoming an
îlot de chaleur
or not.
Thanks to an interactive map created by the French public administrative body Cerema, which is partnered with the environment ministry and publishes studies on climate issues, there is enough data to compare heat risk by city.
According to Cerema's research, France's 83 largest urban areas (more than 50,000 people) are all vulnerable to extreme heat, with more than five million people living in neighbourhoods considered to be 'highly sensitive to extreme heat'.
In terms of where high temperatures and heatwaves are most dangerous, the data differs slightly based on who you ask. According to the French weather service, Météo France, the five cities most affected by the urban heat island effect are; Paris, Grenoble, Lille, Clermont-Ferrand and Lyon.
However, in an interview with French daily Libération, a representative from Cerema explained that Paris, Lyon, Bordeaux, Marseille and Lille are the worst.
Cerema found that together, these five cities account for almost half of the French population living in 'highly sensitive areas' for heat.
It's important to note that these aren't necessarily the hottest cities in France - southern towns like Nice, Montpellier and Toulouse regularly top 40C in summer, but have urban design better suited to the heat.
Paris, by contrast, suffers from high population density, a lack of green space and design issues such as its famous zinc rooftops which make it especially vulnerable to 'heat islands', even though it's unusual for temperatures to go above 40C.
Paris
Most of the city of Paris is covered in red, with almost a third of the city (32 percent) considered to be LCZ 2, or highly sensitive to the heat island effect.
A study by Lancet Public Health published in 2023 found that Paris is the European city with the
highest risk for heat-related death
.
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The city of Paris has taken
steps to remedy
this issue including a huge tree-planting programme, but in the short term, the city has also
published a list of 'cool islands'
or places people can go to escape the heat, such as parks and swimming areas.
Screenshot of a map of Paris based on heat risk. Credit:
French government, Cerema
Lyon
The third-most populated city in France, Lyon is home to over 520,000 inhabitants. Located in the Rhone valley, temperatures have soared in recent summers.
During a heatwave in August 2023, Lyon beat its previous temperature record from the 2003 heatwave, hitting 41.4C. At the time, the city had also recorded 12 consecutive days of temperatures hitting above 34C, according to
BFMTV
.
Screenshot of a map of Lyon based on heat risk. Credit:
French government, Cerema
Bordeaux
Located in the south-west, along the Garonne river, Bordeaux has been accused of being slower than other French cities when it comes to adding green spaces and planting trees.
Didier Jeanjean, the city councillor in charge of 'nature in the city', told
Franceinfo
in 2023 that he would describe Bordeaux as "a city of stone since the 18th century, and then it adapted to cars."
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Jeanjean added that "at the same temperature, Bordeaux can be 10C warmer than the countryside because of the city's extra heat."
Screenshot of a map of Bordeaux based on heat risk. Credit:
French government, Cerema
Marseille
Despite its location along the Mediterranean, with sea breeze helping to cool off areas directly next to the water, central Marseille suffers from the heat island effect.
As shown in the map, the parts of Marseille at most risk are the dense, built-up areas near the Vieux Port and the train station.
In 2017, the city
recorded
a 15C temperature difference between the Place Castellane traffic circle and the Jardin du Pharo (also known as the Jardin Émile Duclaux - the lawn near the Palais du Pharo with views of the water and city).
Screenshot of a map of Marseille based on heat risk. Credit:
French government, Cerema
Lille
The northern French city may come as some surprise to readers, as this part of France tends to see cooler temperatures during the summer, but the urban heat island effect really comes down to a lack of vegetation and green space.
Lille had, as of 2024, about 15 m2 of green space per inhabitant. In contrast, France's '
most green
' city, Angers, had 102 m2 of green space per inhabitant (as of 2023), the city
said on their website
.
Screenshot of a map of central Lille based on heat risk. Credit:
French government, Cerema
How to use the interactive map
If you are curious about other parts of France, you can check out the map on the
Cerema website,
which even shows heat risk down to individual streets.
Screenshot of Cerema map, showing all French cities at risk of urban heat island effect. Credit:
French government, Cerema
If you add an address or city into the search bar (in the upper right-hand corner), you can see how that specific area is coloured. There are several categories of local climate zones, or LCZs.
Those in dark red and red (LCZs 1, 2, and 3) are for areas that are 'highly or very highly sensitive to the heat island effect'. These are compact areas of towers, buildings and houses.
These areas are classified as 'requiring major adaptation measures' to withstand heat.
READ MORE:
Flooding, wildfire and 50C cities: How climate crisis will impact each region of France
After that, urban planning becomes more spaced out, going medium sensitivity in brown and orange (LCZ 4 and 5), eventually to a peach colour for low or variable sensitivity (LCZ 6-9).