A hot day is one thing. A heatwave is something else entirely. The difference isn’t just about temperature — it’s about duration, intensity, and the cumulative effect on the ground, the air, and the people living through it.
What Counts as a Heatwave?
There’s no single universal definition because what counts as a heatwave depends on where you are. A heatwave in Monkton Heathfield isn’t the same as a heatwave in Seville — both places have different baselines for what’s considered normal.
In the UK, the Met Office uses a threshold-based definition: a heatwave occurs when the daily maximum temperature meets or exceeds a specific threshold for at least three consecutive days. That threshold varies by region — in Somerset, it’s around 25°C to 28°C depending on the exact location. In other words, what feels unusually hot in Taunton might be considered a pleasant summer’s day in Madrid.
The key elements are therefore:
- Threshold temperature — set relative to the local climate
- Duration — at least three consecutive days
- Geographic context — what’s extreme in one place may be routine in another
How Heatwaves Form
Heatwaves typically develop when a persistent area of high pressure settles over a region. Sound familiar? That’s because a heatwave and a heat dome are closely connected — a heat dome is often the mechanism that creates and sustains a heatwave.
The high-pressure system pushes air downwards, warming it through compression. Skies clear, allowing the sun to beat down uninterrupted. With each passing day, the ground absorbs more solar energy, heats the air above it, and the cycle intensifies. Overnight, the lack of cloud cover means heat escapes rapidly into the upper atmosphere — but in a strong heatwave, even the nights stay warm, offering no real relief.
Wind patterns play a crucial role too. If the jet stream directs hot air northwards from the continent or North Africa, it feeds additional warmth into the system. When the jet stream stalls, the hot air stays put.
The Cumulative Effect
A single hot day is manageable. Most healthy bodies can cope with a brief spike in temperature by sweating, dilating blood vessels near the skin, and seeking shade. But a heatwave wears the body down over time.
Day after day of heat stresses the cardiovascular system. The heart works harder to pump blood to the skin’s surface for cooling. Fluid and electrolyte loss through sweating accelerates dehydration. Sleep quality deteriorates as nighttime temperatures remain elevated. By the third or fourth day, the body’s cooling mechanisms start to struggle, and the risk of heat-related illness rises sharply.
This cumulative effect is why the third day of a heatwave is often more dangerous than the first — even if the peak temperature is identical.
Measuring the Danger: Wet Bulb Temperature
Air temperature alone doesn’t tell the full story of heat danger. Wet bulb temperature measures the lowest temperature that can be achieved through evaporative cooling — essentially, how effectively the body can cool itself by sweating.
When wet bulb temperatures climb above 21°C, physical labour becomes hazardous. Above 24°C, even resting individuals begin to struggle. At the theoretical limit of 35°C wet bulb, the human body cannot cool itself at all, and survival becomes impossible even in shade.
During the August 2026 heatwave, wet bulb temperatures in southern England reached levels that made national news — a reminder that humidity, not just heat, determines how dangerous conditions really are.
Urban Heat Islands
Cities make heatwaves worse. Concrete, tarmac, and brick absorb solar radiation during the day and release it slowly at night. Air conditioning units pump heat outdoors. Dense buildings block natural ventilation. The result is that urban areas can be several degrees warmer than surrounding countryside — the urban heat island effect. For rural locations like Monkton Heathfield, the effect is less pronounced, but nearby Taunton and Bridgwater still experience elevated nighttime temperatures during heatwave events, particularly in densely built commercial areas.
Health Impacts and Vulnerable Groups
Heatwaves are the deadliest type of weather event in the UK — more so than storms, floods, or cold snaps. The 2022 heatwave was linked to nearly 3,000 excess deaths across England.
Those most at risk include:
- Older adults — reduced ability to regulate body temperature
- Young children — higher surface-area-to-body-mass ratio means faster fluid loss
- People with chronic conditions — particularly heart, lung, and kidney disease
- Those taking certain medications — some drugs interfere with temperature regulation or fluid balance
- Outdoor workers and athletes — prolonged exposure with physical exertion
Practical steps during a heatwave include staying hydrated, avoiding the sun between 11am and 3pm, keeping curtains closed on sun-facing windows, and checking on neighbours who may be vulnerable.
Are Heatwaves Becoming More Frequent?
The scientific consensus is clear: climate change is increasing the frequency, intensity, and duration of heatwaves. The UK’s top temperature record has been broken repeatedly in recent years. What was once considered a once-in-a-century event is becoming a once-in-a-decade occurrence — and by mid-century, could happen even more regularly.
This doesn’t mean every summer will be scorching, but it does mean that when the right atmospheric conditions align, the resulting heatwaves will be hotter than they would have been in a cooler climate.
Living With Heat
For a country historically designed to retain heat rather than shed it — insulated homes, limited air conditioning, architecture built for cool summers — adapting to more frequent heatwaves presents a genuine challenge. Understanding what a heatwave is, how it forms, and why it’s dangerous is the first step toward being prepared for them.