Akshay Deoras, University of Reading and Hannah Cloke, University of Reading
Every time the UK experiences a heatwave, many ask the same question: why does the heat in Britain feel so unbearable when people seem to cope with it in Spain, Greece, or India?
Humidity is part of the answer. But Britain’s housing, long summer days, and lack of experience with extreme heat also make hot weather worse.
The geography of the British Isles, surrounded by seas and on the edge of the North Atlantic, frequently exposes the region to moist air, making the weather more humid than many inland European locations. A 35°C day in Madrid might have relative humidity of around 20%, for instance, whereas in London it could easily exceed 40% – meaning roughly twice as much moisture is being held in the air.
This matters because humidity means sweat evaporates more slowly, and sweating is the main way our bodies cool themselves. Sweat does not cool us simply because it is wet. It cools us because it evaporates. Evaporation takes energy, removing heat from the skin and helping keep body temperature within safe limits.
When the air is already humid, the body is forced to work harder to achieve the same cooling effect, which combined with the heat itself increases the risk of heat exhaustion and cardiovascular and respiratory illnesses.
The combined effect of temperature and humidity can be understood through “wet-bulb temperature”, used in the calculation of relative humidity. It measures how far evaporation can cool a surface, given the temperature and moisture of the air. On its own, wet-bulb temperature can be used to assess the risk of humid heat stress.
During the current heatwave, wet-bulb temperatures in southern England might reach around 25°C – that’s well below theoretical survival limits, but high enough to put older people and other vulnerable groups at risk.
For your body to experience similar stress in much drier air, the actual air temperature would need to be considerably higher, typically 40°C or more.
Built to survive winter, not summer
But humidity is not the whole story. There is also the question of acclimatisation.
In hotter countries, daily life is often organised around avoiding the worst afternoon heat. Outdoor activity is reduced. Buildings are designed to minimise heat gain. Shutters, external blinds, thick walls and shaded streets help keep indoor temperatures lower. Air conditioning is also far more common and is treated as a necessity rather than luxury.
Most people in Britain simply have relatively little experience of prolonged extreme heat. For hundreds of years in the past, the UK has been designing homes to retain heat during winter. Many types of buildings, including concrete high-rise flats and typical brick-walled, tile-roofed houses, act as thermal batteries, absorbing heat during the day and releasing it slowly overnight.
When heat arrives suddenly
The seasonal progression of heat in these regions also differs from Britain. Across large parts of India, for example, temperatures typically rise gradually through spring. This gives people, infrastructure, and institutions time to adjust ahead of many consecutive days above 40°C.
In contrast, British heatwaves often arrive abruptly following much cooler weather. May 2026 saw temperatures swing from frosts and chilly evenings to a record-breaking heatwave within a matter of weeks. Such rapid changes leave far less opportunity for acclimatisation.
Long days, sleepless nights
Britain’s long summer days also contribute to the discomfort. During June and July, strong sunshine persists well into the evening, allowing buildings, roads and urban surfaces to continue absorbing heat long after the hottest part of the day has passed. While it can still stay warm in the tropics after sunset, nearly half of each day is consistently spent in the respite of darkness.
Nighttime plays a crucial role in the risks during heatwaves. One of the greatest dangers comes from the so-called “tropical night” where temperatures stay above 20°C. The body needs cooler conditions to recover from daytime heat exposure.
For now, tropical nights remain relatively uncommon in the UK. They are more likely in urban areas due to the “heat island” effect as buildings and roads let out stored heat built up during the day. Humidity often remains high overnight in general, which further impairs the body’s ability to cool itself during tropical nights. These conditions are affecting parts of the UK during the current heatwave.
Temperatures exceeding 40°C and humid tropical nights once seemed almost unimaginable in the UK, until they became reality. But the heat and humidity themselves aren’t the only problem. The country is still adapted – in its routines, its infrastructure, its buildings – to a cooler climate. And as heatwaves become more extreme and humid, that mismatch will continue to make hot weather feel even more uncomfortable and dangerous.
Akshay Deoras, Senior Research Scientist, University of Reading and Hannah Cloke, Regius Professor of Meteorology and Climate Science, University of Reading
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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