When temperatures soar above 35°C (95°F), the danger seems obvious. But increasingly, health experts are warning that the most dangerous part of a heatwave may not be the scorching afternoon – it may be the night that never cools down.
Across southern Europe, and increasingly in parts of central and western Europe as well as North America, “tropical nights” are becoming more frequent.
Defined as nights when temperatures never fall below 20°C (68°F), they prevent both people and buildings from cooling, placing the human body under prolonged heat stress.
Scientists say that while hot days are uncomfortable, warm nights can be even more dangerous because they deny the body its natural opportunity to recover.
What Exactly Is a Tropical Night?
Meteorologists use the term tropical night when the overnight minimum temperature remains at or above 20°C. In tropical climates this is common. In much of Europe, however, such nights were once relatively rare outside the Mediterranean region.
That is changing. According to the Copernicus Climate Change Service, the number of tropical nights has increased across much of Europe over recent decades, with southeastern Europe, Italy, Greece, the Balkans and parts of the Iberian Peninsula experiencing some of the largest increases.
During recent summers, tropical nights have also become more common in countries such as France, Germany and even the United Kingdom during major heatwaves.
Why Warm Nights Can Be More Dangerous Than Hot Days
The human body depends on cooler nighttime temperatures. During sleep, core body temperature naturally falls, heart rate slows and the cardiovascular system has an opportunity to recover from the day’s heat.
When temperatures remain elevated throughout the night, that recovery becomes much more difficult. Instead, the body continues working to release heat, increasing physiological stress over many consecutive hours.
Medical experts link prolonged nighttime heat with:
- Increased cardiovascular strain.
- Poor sleep quality and sleep deprivation.
- Greater risk of dehydration.
- Higher rates of heat exhaustion and heat-related illness.
- Increased health risks for older adults, infants and people with chronic heart or respiratory conditions.
Researchers often describe this as a “cumulative heat burden” – the idea that the body becomes progressively less able to cope when there is no cool period between hot days.
Cities Feel the Heat the Most
Tropical nights are especially common in cities. Concrete, asphalt and buildings absorb enormous amounts of solar energy during the day before slowly releasing that heat after sunset.
This phenomenon, known as the urban heat island effect, means city centres often remain several degrees warmer than surrounding rural areas throughout the night.
As a result, residents of cities such as Paris, Madrid, Rome, Athens, Barcelona and Milan increasingly experience nights that remain uncomfortably warm until sunrise.
Europe Is Not Alone
Although tropical nights have long been associated with Mediterranean climates, they are becoming more frequent elsewhere.
In North America, large parts of the southern United States – including Florida, Texas, Louisiana and the Gulf Coast – regularly experience tropical nights during summer.
More recently, prolonged heatwaves have brought unusually warm nights to cities farther north, including New York, Washington, D.C., Chicago and even parts of southern Canada.
Scientists note that overnight temperatures are rising in many regions alongside daytime extremes, increasing the overall health burden of heatwaves.
Climate Change Is Making Them More Common
Climate scientists have observed that Europe is warming faster than the global average. One consequence is not only hotter afternoons but also warmer nights.
According to the European Environment Agency, tropical nights have increased across Europe since the 1980s, with the largest projected increases expected in southern Europe.
Under high-emissions scenarios, some Mediterranean regions could experience close to 100 tropical nights each year by the end of the century.
How to Stay Safe During Tropical Nights
Health authorities recommend several practical measures to reduce the risks associated with prolonged nighttime heat:
- Ventilate homes during the coolest hours of the early morning or late evening.
- Keep curtains and blinds closed during the day to limit indoor heat.
- Stay well hydrated throughout the day and evening.
- Avoid strenuous physical activity during the hottest hours.
- Use fans or air conditioning where available, particularly for vulnerable individuals.
- Check regularly on elderly relatives, neighbours and people living alone.
- Seek cooler public spaces if indoor temperatures remain dangerously high.
Even simple measures that improve sleep quality and reduce indoor temperatures by a few degrees can significantly lessen heat stress during prolonged heatwaves.
A Silent Threat After Dark
Heatwaves are often measured by daytime maximum temperatures. Yet many researchers believe that minimum nighttime temperatures tell an equally important story.
A day that reaches 38°C before cooling to 18°C overnight places far less strain on the body than one that reaches 35°C but never drops below 25°C.
That difference may determine whether people recover – or accumulate dangerous levels of heat stress over several consecutive days.
As tropical nights become more frequent across Europe and North America, they are emerging as one of the least visible—but most significant—public health challenges of a warming climate.
Sources:
- Copernicus Climate Change Service (C3S) – What is a tropical night?
- European Environment Agency (EEA) – Heat and cold: Extreme heat (tropical nights and health impacts).
- World Meteorological Organization (WMO) – Guidance on extreme heat and the health risks of high nighttime temperatures.
- Yahoo News Australia – What are tropical nights? Inside the deadly phenomenon hiding behind Europe’s blistering heatwave.
