When a heatwave hits, attention fixes on the daytime peak, the dramatic number that breaks records and leads the news. But meteorologists and public-health officials watch a quieter figure just as closely: the overnight low. It is often the night, not the afternoon, that decides whether a stretch of hot weather is merely uncomfortable or genuinely dangerous.
The reason lies in how the human body copes with heat. Through a hot day the body works hard to shed warmth, mainly by sweating. That system can keep up for a while, but it needs a recovery window, and that window normally comes at night when temperatures fall and the body can cool down and reset. When the night stays hot, that relief never arrives. The strain accumulates hour after hour, and by the second or third hot night the cardiovascular system of vulnerable people is under sustained pressure with no chance to recover. This is why heatwave death tolls often climb most steeply during multi-night events rather than after a single scorching afternoon.
Forecasters have a term for the threshold that matters here: a tropical night, defined as one where the temperature never drops below 20 degrees Celsius. During Europe's June 2026 heatwave, numerous regions recorded their warmest June nights on record, with cities staying above that threshold night after night. In urban areas the effect is amplified by the urban heat island, where concrete, asphalt and brick absorb heat all day and release it slowly after dark, keeping city centres several degrees warmer than the surrounding countryside well into the small hours. A city dweller can face a meaningfully hotter night than someone a few miles away in open country.
Humidity raises the stakes further. Sweating only cools you if the sweat can evaporate, and evaporation slows dramatically when the air is already moist. This is why a humid 32-degree day can be more dangerous than a dry 36-degree one, and why the apparent or feels-like temperature, which combines heat and humidity, is frequently the more honest measure of risk. A warm, humid night is the worst combination, because the body is denied both the cooler temperature and the evaporative cooling it relies on.
The groups most at risk are predictable but worth restating: older adults, very young children, people with heart or respiratory conditions, and anyone without access to cooling. The danger is magnified across much of Europe because home air conditioning remains relatively uncommon, so when the indoor temperature of a poorly ventilated flat fails to fall overnight, there is often no easy escape. Emergency departments brace for exactly this during prolonged heat.
The practical response follows directly from the science. Through the day, the priority is to keep indoor spaces as cool as possible: close blinds and windows against the sun during the hottest hours, then open up to ventilate once the outside air finally drops below the inside temperature, which during a heat dome may not be until the early hours. Staying hydrated, avoiding exertion during peak heat, and checking on vulnerable neighbours all matter more during a multi-night event than during a brief hot spell. And because heat stress depends on humidity as much as raw temperature, watching the feels-like figure rather than the thermometer alone gives a truer sense of the danger.
The headline number will always be the daytime high, but the smarter way to gauge a heatwave is to ask what the nights are doing. A run of tropical nights, especially humid ones in an urban area, is the signal that a hot spell has crossed from inconvenient into hazardous.