What is a heatwave in the UK?

In the UK, a heatwave isn’t defined by a single temperature threshold, but rather by how long temperatures stay above what’s considered ‘normal’ for a given area.

The Met Office has different daily maximum temperature thresholds depending on the region. For example, a heatwave in Cornwall is declared when temperatures hit or exceed 26°C for at least three consecutive days.

In parts of eastern England like Norfolk, the threshold is higher at 27°C, and in London or the Home Counties, it rises to 28°C.

The key factor is not just the temperature itself, but the persistence of heat over several days.

A one-off hot day doesn’t count.

The official definition is tied to health impacts – prolonged periods of high heat can be dangerous, especially for older adults, young children, and people with underlying health conditions.

That’s why the UK Health Security Agency issues heat-health alerts when a heatwave is forecast.

How does a heatwave affect South West England?

In Devon and Cornwall, heatwaves might not sound extreme by international standards – but they can still cause significant disruption. Homes in the South West are typically built to retain heat, not shed it, and many properties lack air conditioning.

When overnight temperatures stay high, it becomes difficult to cool down indoors, which can impact sleep and wellbeing.

Local infrastructure also struggles.

Roads soften, rail tracks buckle, and reservoirs start to drop – especially in rural parts of Exmoor and Dartmoor where water supply is more vulnerable.

Farmers may struggle with dried-out pastures, affecting livestock. If you’re by the coast, beaches like Perranporth or Exmouth become crowded quickly, while inland swimming spots like Roadford Lake or the River Dart draw bigger crowds than usual – sometimes stretching safety and environmental capacity.

How is it different elsewhere?

In places like London, a heatwave often exacerbates the urban heat island effect. Concrete buildings and tarmac trap heat, so city temperatures can remain higher overnight.

In contrast, Scotland and Northern Ireland have lower thresholds – 25°C or even 24°C – because those temperatures are less common there and can have more pronounced impacts on people and nature.

Meanwhile, the Midlands and South East England are often the hottest regions overall.

Cities like Oxford, Reading, and Cambridge frequently record the highest daily maximums during UK heatwaves, sometimes breaching 40°C, as happened during July 2022.

What causes a UK heatwave?

UK heatwaves are typically driven by a blocking high-pressure system over Europe or the North Atlantic.

These systems cause still, dry air and plenty of sunshine, often accompanied by light winds. The lack of air movement means heat builds up day after day. The most intense heatwaves usually come when warm air is drawn up from southern Europe or North Africa, funnelled northwards over Spain and France before reaching the UK.

The South West’s proximity to the Atlantic means it’s often cooler than inland areas — sea breezes can keep temperatures down slightly on the coast – but once the heat sets in and the wind drops, even coastal areas like Falmouth or Brixham can hit the heatwave threshold.

Are heatwaves becoming more common?

Yes. The Met Office has confirmed that climate change is increasing the frequency and intensity of UK heatwaves.

What was once considered unusual — like the 30°C+ days in Devon in 2022 — is now happening more often. In the South West, warmer, drier summers are already affecting agriculture, water use, and the tourism season, while extreme temperatures are becoming part of the new normal for July and August.

In response, local councils and services across the region have begun heat-resilience planning, issuing guidance on staying hydrated, avoiding peak sun hours, and checking on vulnerable neighbours during official alerts.