The overlooked summer danger making UK heatwaves feel even worse

Yahoo

a day ago

It's not just the heat that people should be wary of as temperatures soar.
"Have you been one of the many people wondering if the heat this summer feels different? More oppressive, more draining. That's because it's not just hot, it's also humid." That was last month's analysis by the Met Office of yet another sweltering UK summer.
With Britain enduring its fourth heatwave of the summer, many regions are not just basking in the heat but also struggling in the stifling humidity.
The Met Office has forecast a "hot and humid week" for many, with temperatures set to peak on Wednesday (13 August). And while much of the UK's national conversation during the summer is framed around how hot it is, the UK's humidity levels are also increasing.
The UK is surrounded by water, leading to more humid air than in some other countries in Europe.
In many recent summers, warmer weather patterns have created more warm and humid nights, Professor Richard Allan, University of Reading, told Yahoo News.
Prof Allan said: "Certain weather patterns bring hot and humid conditions along with thunderstorms to the UK in summer, such as a southerly airflow sometimes called a 'Spanish plume'.
"These create sweltering, sticky nights because temperatures are high while the air is also loaded with moisture, making it nearly impossible for your body to cool itself through sweating."
What is humidity?
Humidity refers to the amount of water vapour in the air, with higher levels meaning that hot temperatures can feel much more sweltering and oppressive.
There are several different ways to measure humidity, including relative humidity and wet bulb temperatures.
A wet-bulb measurement is taken by covering a thermometer with a water-soaked cloth. The water will then begin to evaporate, lowering the temperature on the thermometer until it reaches a steady level.
The water evaporation is similar to how humans lose heat through sweat, so a wet bulb measurement can determine how people will deal with humid conditions.
High wet-bulb levels are dangerous because humans lose around 80% of heat through sweating, so when both humidity and air temperature are high, it becomes harder to shed excess heat.
Relative humidity is a more direct measurement of water vapour, represented as a percentage.
Many weather forecast apps will show relative humidity alongside the temperature.
It is relative to the temperature, with low temperatures often having higher percentages of humidity but feeling less sticky than summer, which can have a relative humidity of 50%.
How does humidity affect humans?
Humidity can affect human health in various ways, and, in the most extreme cases, hot temperatures paired with high humidity levels can be dangerous.
According to the Met Office, "humidity directly influences our body's ability to regulate temperature. In hot and humid conditions, sweat does not evaporate efficiently, making it harder for the body to cool down.
"This can lead to heat stress, dehydration, and in severe cases, heatstroke or cardiovascular complications. Vulnerable populations, such as the elderly, children, and those with pre-existing health conditions, are particularly at risk."
Humidity also affects mental well-being and even damages productivity.
The Met Office said: "High humidity can reduce physical performance, making it harder to carry out manual labour or exercise.
"This has implications for sectors such as agriculture, construction and manufacturing, where sustained physical effort is required."
A study by The University of Exeter in 2023 found that a wet-bulb temperature over 28C saw average productivity across an entire factory drop by 6%.
The Trade Union Congress in the UK says the comfort range for humidity is between 40% and 70%, but they note it should always be considered when thinking about safe temperatures for people to work in.
Human beings can survive very high temperatures (well over 50C) when humidity is low, but in high humidity, humans cannot survive temperatures of even 35C for long periods, because there is no way to cool down by sweating.
What is considered high humidity?
The most common way to measure it is relative humidity (RH) — which, the Met Office explains, is basically how close the air is to being 'full' of water vapour. When it's 100% RH, the air is saturated.
They don't have official figures, but in summer, generally anything over about 60% RH can start to feel uncomfortable, and once it's up around 80–90%, it can feel downright oppressive – even if the actual temperature isn't that high.
You can find the RH for where you are on apps, such as the BBC Weather app. If it's 100% then it is fully saturated.
Is humidity getting worse in the UK?
As the temperature warms in the UK, the air is able to hold more moisture, Prof Allan explained.
'It's well established that warmer air can hold about 7% more moisture for every degree Celsius of temperature rise," he says. "And this extra moisture capacity drives heavier rainfall while also acting as a greenhouse gas that amplifies warming.
'Rising temperatures mean there's simply more moisture in the atmosphere overall, and this thicker blanket makes it more difficult for the surface to lose heat to space.
But the air is not getting more saturated, accroding to Allan.
He said: "When we talk about humidity making it harder to cool off, we're referring to how full to the brim the air is with invisible water vapour (relative humidity).
"When air is 100% saturated, water droplets can start to form on surfaces, and the air can't hold any more water vapour. There isn't strong evidence that the UK's air is becoming more saturated with moisture."
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