As another scorching heatwave settles over much of Europe, climate scientists are drawing attention to a less visible but equally dangerous phenomenon: tropical nights. These are periods when the temperature does not fall below 20°C within a 24-hour cycle, robbing the body of the chance to recover from daytime heat.
France's national weather service, Météo France, has warned that the country is entering a “plateau” of extreme heat that will persist until at least Friday. More than half of France's 96 départements are now under red alert for severe weather, with temperatures expected to reach 40°C. This follows a deadly May heatwave that experts say bore the unmistakable fingerprints of climate change. In Paris, authorities banned alcohol sales during the annual Fête de la Musique and allowed swimming in the Canal Saint-Martin, while parks and gardens remain open around the clock to help residents cool off.
Spain is also bracing for temperatures of 40°C in cities including Bilbao, Zaragoza, Almería, and Madrid. Even the United Kingdom, not known for extreme heat, is preparing for highs of 38°C in southern England. Italy and Greece, by contrast, will experience comparatively milder conditions, with Rome reaching 35°C and Athens staying in the low 30s, according to Ioanna Vergini, founder of wfy24.com.
The Growing Threat of Tropical Nights
Tropical nights are becoming more frequent across Europe, even in traditionally cooler nations. A study by the UK's Met Office found that the likelihood of experiencing three consecutive tropical nights in July has risen dramatically due to climate change. In a pre-industrial climate, such an event had a probability of less than 1% per year in the UK; today, that figure stands at approximately 20%.
The health implications are serious. The human body relies on cooler nighttime temperatures to regulate its core temperature and recover from daytime heat. When that relief is absent, the risk of cardiovascular stress, sleep disturbance, and even mortality increases significantly. “Nighttime heat is often underestimated, but it can be particularly dangerous because people are typically at home and may not have access to cool, air-conditioned spaces,” says Ruth Engel, a data scientist on environmental health and extreme heat at the World Resources Institute (WRI). “In Europe, where air conditioning remains relatively uncommon and temperatures are rising faster than in any other region, tropical nights are becoming a major public health concern.”
Air conditioning use in Europe has more than doubled since 1990, with roughly 110 to 130 million units now installed. Yet that still covers only about 20% of buildings on the continent. While AC can be life-saving during intense heat, it also fuels a dangerous feedback loop. As global warming drives demand for cooling, Europeans become more reliant on air conditioning, which often runs on fossil fuels and releases potent greenhouse gases such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). These chemicals trap thousands of times more heat than CO₂, exacerbating the very problem they are meant to mitigate.
In cities, the situation is compounded by the urban heat island effect. Tall buildings, asphalt, and concrete absorb heat during the day and release it at night, making outdoor temperatures even hotter and driving further demand for air conditioning. This cycle is pushing Europe into what some experts call an endless “cooling loop.”
The cumulative health burden of prolonged heat exposure is particularly acute for vulnerable populations. Studies have linked high nighttime temperatures to increased mortality, especially among older adults and those with pre-existing conditions. Schools are also feeling the impact, with some considering changes to exam schedules to protect students who are not sleeping well due to the heat. “We often talk about the ‘cumulative heat burden’ – the idea that the longer people are exposed to heat, the greater the impact on their health,” Engel explains. “When homes remain hot overnight, people lose the opportunity to recover from daytime temperatures, increasing risks, especially for older adults and those with underlying health conditions.”
As Europe grapples with yet another blistering heatwave, the focus on tropical nights underscores a critical gap in the continent's preparedness. Without widespread access to cooling and a concerted effort to break the feedback loop, the hidden dangers of nighttime heat will only intensify.
