How Hot is Too Hot?
by Katie Merriman
This summer, much of Europe experienced extreme heat, with the UK recording its hottest summer on record. With heatwaves lasting for days or even weeks at a time, it became increasingly common to start a conversation with the words: “Isn’t this temperature crazy?”
This isn't exactly uncommon in Britain. Whether it's rain, snow or sunshine, the weather is almost always a topic of conversation. In most cases, we complain about whatever weather we have, only to complain again when it eventually changes.
But this summer, there was something different about the heat. When temperatures climbed towards 35°C, social media was filled with comparisons to countries where temperatures regularly go much higher. If 35°C is a normal summer temperature in parts of the world, why did it feel so unbearable here?
The answer isn't simply the number on the thermometer.
Temperature tells us how hot the air is, but it doesn't tell us how difficult it is for our bodies to get rid of that heat. Humidity plays an important role too. When the air contains more moisture, sweat evaporates less easily, making it harder for the body to cool itself.
So, what if there is another way of measuring heat? One that accounts for something our ordinary thermometer doesn't?
This is where wet-bulb temperature comes in.
What is wet-bulb temperature?
We spend a lot of time talking about the temperature. Whether it's watching a forecast in the morning or reading the daily temperature on an app, the weather has a big impact on our decisions throughout the day. It impacts what we wear, how we travel and our mood for the day.
But wet-bulb temperature is not a part of this conversation or daily routine.
Until this summer, I had not heard of wet-bulb temperature, let alone understood what it meant. However, I suspect I’m not alone in this. In fact, when asking those around me, only a handful knew about the term, with even fewer being able to accurately describe what it meant.
Wet-bulb temperature, however, does exactly this. It is a way of measuring heat whilst also taking humidity into account. Traditionally, it is measured using a thermometer with its bulb covered in a wet material. As water evaporates from the surface, it cools the thermometer. The drier the air, the more evaporation can take place and the further the temperature falls. When the air is already very humid, evaporation becomes more difficult, so the wet-bulb temperature stays closer to the actual air temperature.
The reason why this is so important is that our bodies rely on this same cooling function. When we get hot, we sweat. The function of sweating alone does not cool our bodies down, it is the evaporation of sweat from the surface of the skin that actually lowers our body temperature.
So, the problem comes when the air around us is already carrying a lot of moisture. High humidity makes evaporation less effective, and if the sweat on our skin is unable to evaporate, our bodies have a much harder time cooling themselves.
This is why two days with exactly the same temperature can feel very different. A dry 30°C day can feel very different to a humid 30°C day. The number on the thermometer hasn’t changed but our ability to cope with the temperature has.
Wet-bulb temperature is how we measure this difference.
So, if wet-bulb temperature is so important, why don’t we hear about it more?
When you open a weather app, you see the average temperature, but look a little further, and you also see the high and low temperature for the day, a ‘feels like’ temperature, the precipitation, wind speeds, humidity and more. The reality is, we are already bombarded with so many numbers when looking at weather apps that even if a wet-bulb temperature value were available, it would probably be lost within everything else.
Not to mention that, historically, UK summers have followed a rough cycle of ‘cooler’ summers followed by occasional hot summers, meaning that wet-bulb temperature has been less significant for the UK. However, recent data as well as long-term predictions show that warm summers are rapidly accelerating and becoming more frequent, with the five hottest UK summers occurring since 2000 and the two hottest being 2025 and 2026.
Wet-bulb temperature offers a connection between two already understood values: temperature and humidity. But crucially, as the climate warms, this additional value will become even more important.
An equation called the Clausius-Clapeyron equation, in relation to climate science, dictates the water-holding capacity of the atmosphere. For every 1°C of temperature increase, the atmosphere's capacity to hold water vapour increases by 7%.
This means that as the atmosphere warms as a result of climate change, it can hold more moisture, which increases the humidity and makes it harder for our bodies to cool through sweating.
So the future of extreme heat is not just about high temperatures alone, but how the combination of changing atmospheric conditions can make it harder for us to withstand these temperatures, in a domino-effect of influence.
A changing definition of extreme heat
We are used to hearing that a temperature is “hot”.
30°C is hot.
35°C is very hot.
40°C is extremely hot.
But those numbers don't tell the whole story.
They don't tell us how humid the air is, whether there is a breeze, how much direct sunlight someone is exposed to, whether they are sitting still or exercising, or how long they have been outside.
Rather than just telling us how hot it is, wet-bulb temperature aims to answer a different question:
What does this heat actually mean for us?
As global temperatures continue to rise and extreme weather events become more frequent, the way we think about temperature will have to change. Instead of reading a number and drawing automatic conclusions about the day, we need to start looking more holistically.
So, how hot is too hot?
As I said earlier, until recently, I had never heard of wet-bulb temperatures. I was aware of temperature, and I was aware of humidity, but I never stopped to think about how these two measures interact.
We have all stepped outside on a humid day and felt that the heat is different.
We have all sweated whilst still feeling very hot.
We have all noticed that 30°C by the ocean feels different to 30°C in a city.
Wet-bulb temperature, and the science behind it, gives us a way of explaining why.
The 35° wet-bulb threshold
This brings us to a number often most associated with wet-bulb temperature: 35°C.
Thirty-five degrees is, of course, a number that most of us understand. When the weather forecast shows 35°C, we understand that it is going to be a very hot day. But, in relation to wet-bulb temperature, it takes on a different meaning.
35°C is often described as the theoretical upper limit of wet-bulb temperature and a value which most healthy humans could only survive for up to six hours. This wet-bulb temperature is equivalent to approximately 40°C with a humidity of 75%.
When the body is subjected to these conditions for a prolonged period of time, without external measures to cool itself down, the consequences can be fatal. First, the body will begin to experience hyperthermia (dangerous overheating), which can then lead to organ failure and death.
Currently, instances of 35°C wet-bulb temperatures have only lasted for a few hours at a time. However, the fact that a wet-bulb temperature of 35°C has been reached for any amount of time is incredibly concerning because this is sooner than previously predicted.
The Lancet Countdown for Health and Climate Change 2025 Report says that heat-related deaths have increased by 63% since the 1990s, with average annual deaths now estimated at 546,000. There are often multiple factors that contribute to heat-related deaths, so it can be hard to determine true estimates, but what's certain is that extreme heat is becoming an increasingly serious threat to human health, and now, more than ever, we need to understand how our bodies experience it.
However, there are issues with treating a wet-bulb temperature of 35°C as a simple black-and-white line between safe and unsafe conditions.
Firstly, wet-bulb temperature is also an area of meteorology that needs a lot more study and research, as the 35°C figure comes from a 2010 study, but more recent trials have suggested that the threshold value could be as low as 31°C.
Secondly, no human is identical, and neither are the environments we live in. Factors such as age, physicality, clothing, access to shade and air conditioning can all impact how much we are affected by extreme heat and humidity.
The reality is that there is unlikely to be a universal wet-bulb value that has the same effect on everybody.
So, the question is not just when does heat become impossible to survive?
It is: when does heat become too dangerous for the people experiencing it?
And that answer can arrive a lot sooner than 35°C.
The reality of extreme heat
For someone sitting in an office, a hot day might mean turning up the fan, opening a window or reaching for a cold drink.
But for someone working outside, attending a music festival or playing a football match, the situation is very different.
If your body is producing additional heat through physical activity, it can become even harder for your body to cool down enough to cope with extreme heat.
Outdoor labourers can spend hours of the day in direct sunlight whilst also undertaking physically demanding tasks. Athletes are often pushing their bodies towards their physiological limits, so with extreme temperatures added in, these limits can be exceeded much more easily. Older people and those with certain health vulnerabilities can be automatically more at risk during extreme heat.
As well as physiological characteristics which can make people more vulnerable, there are also a number of socioeconomic and geographical factors which can change the way extreme heat is experienced. For example, people living in densely populated urban areas can face additional exposure to heat due to the urban heat island effect. In cities like Delhi, Manila, Bangkok and Kolkata, the combination of extreme temperatures, high population densities and the urban environment can compound the risks posed by extreme heat.
Wealth also plays a significant role. People who are more financially secure are often able to afford personal air conditioning or adapt their daily routines during periods of extreme heat. Those on lower incomes may have fewer choices, particularly if they work outside or if they have little control over their working conditions.
A lot of times, these socioeconomic and geographical factors overlap, making the effects of extreme heat even harder to escape. Countries such as India and Pakistan experience periods of intense heat and high humidity, resulting in some of the highest wet-bulb temperatures currently recorded. Many of the people in these regions also face barriers to suitable housing, limited access to suitable cooling infrastructure and unsuitable working conditions.
As temperatures rise, the danger is not just simply how hot a place will become, but also if the people living there have the opportunity and ability to protect themselves.
When the heat stops play
Because different activities require different levels of physical effort, and because the people carrying out these activities are likely to have individual physiological limits, it is impossible to set a universal temperature where activities become unsafe.
However, as temperatures globally continue to rise and the frequency of extreme heat increases, we can already see changes being made across a number of different areas.
Sport is one of the clearest examples of this. Athletes are often pushing their bodies to the extreme. Add in equipment, long hours, and direct exposure to sunlight and it can become very dangerous.
Many sports are already creating new rules and regulations in response to rising temperatures. And importantly, these decisions are not being made by just looking at the number on a thermometer but instead taking into account a number of factors, including the combination of heat and humidity.
At the Paris 2024 Olympics, heat risk affected a wide range of sports, with measures including changes to scheduling, additional hydration, cooling facilities and modifications to competition arrangements.
In Formula 1, a race can now be classified as a ‘heat hazard’ with additional cooling systems put in place within the car if conditions become too extreme. This rule was first introduced in 2025, and during the current 2026 season, 4 races have already been classified as ‘heat hazards’.
In tennis, an extreme weather policy has been introduced which indicates certain thresholds which can either trigger additional breaks or, in extreme cases, match suspension.
So, even if you're watching in the comfort of your home, with air conditioning and cold drinks at your disposal, extreme heat still has the potential to impact what you are watching.
But what if you're watching in person?
As well as the athletes being exposed to extreme heat, there are typically thousands of spectators also being exposed to direct sunlight and high temperatures. Across not just sporting events but also concerts and festivals, spectators can be standing or sitting for hours at a time without medical teams on standby to monitor their health or scheduled breaks like most athletes will have.
Already, guidance for attending events during extreme heat is changing. Festivals such as Glastonbury now have dedicated reminders for people to drink water and take time in the shade, as well as increased drinking water facilities throughout the area.
Whilst changes like these might seem small, they represent a wider change in thinking. Extreme heat can no longer be an afterthought; it must now be something that is planned and prepared for. This applies not just to event organisers but also to individual preparations and awareness as well.
When wet-bulb enters the headlines
For a long time, the term wet-bulb temperature has been contained within scientific literature rather than being a part of everyday conversations. Throughout the 2026 heatwaves, we became very used to hearing about the temperature, what it was, if it was breaking records, and how it might affect our day-to-day activities. But as extreme heat continues to persist and becomes more regular, the way we talk about it is changing.
Looking at Google Trends data, it's clear that in the UK, search interest in wet-bulb temperature has increased since 2021, with a noticeable peak in 2026. This shows that the topic may be becoming more mainstream but does not give any indication about the level of understanding that people have of wet-bulb temperature.
In fact, the use of the term wet-bulb temperature has increased in 2026 partially due to the extreme conditions but also due to high-profile events which have been impacted by the heat. Coverage of the 2026 FIFA World Cup often included the term due to the conditions of high heat and humidity experienced by both players and spectators, prompting changes to play and safety guidance.
A change in the language that we use to describe heat is incredibly important. Whilst a dry-bulb temperature of 30°C sounds uncomfortable to many people, it is still considered a manageable condition. However, because this temperature alone gives no indication of humidity, it can be very dangerous as it leaves people vulnerable to not understanding the true risks it may pose.
Whilst it might seem hard to change language that people are used to, we have seen it happen before. Before the COVID-19 pandemic, terms such as ‘immunity’, ‘transmission’, and ‘viral load’ were largely unfamiliar to people outside of science or medical professions. However, the pandemic brought these terms into everyday conversations and news coverage because they were important for understanding the risks to personal and public health.
Wet-bulb may be undergoing a similar transition from a specialist scientific concept to a term that people regularly encounter in the news.
Of course, just hearing the term does not mean that people necessarily understand it, but it is a useful first step. As extreme temperatures continue to increase and become more regular, it is incredibly important that people understand not only the temperature outside, but also how the conditions can affect their body's ability to cope with it. That understanding can help people recognise when the heat is becoming dangerous and make better-informed decisions about how to protect themselves.
With increased coverage of wet-bulb temperatures as a result of continued changes within our climate, perhaps the biggest lesson is that we need to rethink what we mean by ‘too hot’. Because the number on the thermometer only tells us half the story.
Relevant links
https://www.theguardian.com/science/2022/jul/31/why-you-need-to-worry-about-the-wet-bulb-temperature
https://www.lta.org.uk/siteassets/roles/officials/files/regulations/lta-extreme-heat-policy-2022.pdf
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