Every summer, temperatures rise faster in the city center than in the suburbs. This isn’t just a feeling—it’s the result of a well-documented physical phenomenon known asthe urban heat island effect (UHI). With heat waves becoming increasingly frequent and intense, understanding this mechanism has become a priority for local governments, developers, and property managers.
An urban heat island refers to the temperature difference observed between a densely built-up urban area and the surrounding rural or suburban areas. In France, this difference regularly exceeds 3 to 5°C on average and can reach 10°C or more during heatwave nights.
The phenomenon is not new—it was first described as early as the 19th century—but its severity has increased significantly due to urbanization and climate change.
There are two types of ICU:
Several physical mechanisms interact to create and amplify ICU:
Asphalt and concrete absorb a large amount of solar radiation (low albedo) and release it as heat. In contrast, moist natural soil regulates temperature through evapotranspiration.
Trees and plants play a crucial role as natural air conditioners. Through evapotranspiration, a mature tree can evaporate up to 400 liters of water per day, cooling its immediate surroundings by several degrees.
Narrow streets and tall buildings trap solar radiation (the "urban canyon" effect) and reduce air circulation, limiting nighttime cooling.
Air conditioners, vehicles, industrial activities: cities continuously generate heat, which contributes to the heat balance.
On health
The 2003 heat wave caused nearly 15,000 excess deaths in France, mainly in densely populated urban areas. Extreme heat leads to dehydration, heatstroke, and excess mortality among vulnerable populations.
On comfort and productivity
High nighttime temperatures disrupt sleep and reduce quality of life. When nighttime temperatures exceed 25°C, the effects on health and productivity become significant.
On energy
Air conditioning already accounts for 15 to 20% of global electricity consumption in buildings. A 3°C increase in the indoor temperature can lead to a 5 to 10% rise in air conditioning energy consumption.
On property value
Exposure to climate-related risks is becoming an increasingly significant concern for investors and buyers. The European Taxonomy now requires real estate stakeholders to assess and reduce their assets’ exposure to physical climate-related risks—including urban heat.
To effectively combat heat waves, they must first be accurately measured. For a long time, forecasts relied on isolated weather sensors or satellite imagery measuring land surface temperature (LST). While this data is useful, it is incomplete.
Surface temperature does not tell the whole story when it comes to residents' comfort. What matters for health and quality of life is the perceived temperature —the temperature the human body senses by taking into account the combined effects of radiation, wind, and humidity.
This is precisely whatthe UTCI (Universal Thermal Climate Index)—the benchmark indicator for outdoor thermal comfort used by Netcarbon—measures to assess and simulate the impact of development projects.
The levers for action are well known, but their actual effectiveness depends on their scale and location:
Urban heat islands are no longer a marginal phenomenon: they pose a major health, energy, and economic risk, one that is set to worsen with climate change. In addressing this challenge, data and simulation are essential tools for turning intentions into measurable action.
Would you like to map the heat islands in your area or on your property and simulate the impact of your greening projects? Request a Netcarbon demo.
