France is experiencing a heatwave characterized by "nuit tropicale," or tropical nights, where temperatures remain high after sunset [1].
These conditions prevent the human body and urban infrastructure from cooling down, increasing the risk of heat-related illness and sleep deprivation. This lack of nocturnal relief compounds the physical stress caused by daytime peaks, making prolonged heatwaves more dangerous for vulnerable populations.
A tropical night is officially defined as a period when the night-time temperature does not drop below 20 °C [3]. This phenomenon is appearing across France, with a notable concentration in the Mediterranean coastal regions [2].
Authorities and residents are grappling with the effects of this prolonged heat. The current episode, which occurred in late June 2024 [1], demonstrates a pattern of intense temperatures that persist well into the night [4]. This trend is becoming more frequent as a result of climate change [4].
Beyond the land, the region is facing a marine heatwave. Sea-surface temperatures in the Mediterranean are projected to reach up to 30 °C [2]. This warming of the water can further influence coastal air temperatures, contributing to the persistence of high heat during the night.
The combination of high daytime peaks and tropical nights creates a cycle of heat accumulation. When buildings and pavement do not cool down, the environment retains heat, effectively turning cities into heat islands that exacerbate the impact of the heatwave [4].
“A tropical night is officially defined as a period when the night-time temperature does not drop below 20 °C.”
The increasing frequency of tropical nights in France indicates a shift in climatic patterns where the traditional nocturnal cooling cycle is disrupted. When nighttime temperatures stay above 20 °C, it removes the primary recovery window for the human body and urban environments, potentially leading to higher mortality rates during heatwaves and increasing the energy demand for cooling systems.
