On a sweltering June day, the radio bulletins are full of talk of heatwave thresholds, amber warnings and tropical nights — but for many in the UK, it can be hard to know exactly what these terms mean and how seriously to take them. The Met Office has issued an amber extreme heat warning for much of southern England and south-east Wales over the coming days, the most extreme such warning in four years. Temperatures are expected to climb to about 30C over the weekend and peak on Monday and Tuesday at 34C, with a chance of reaching 35C in some spots, according to the Met Office. So what actually constitutes a heatwave, and what should you do when a warning is in place?
For a hot spell to officially become a heatwave in the UK, temperatures must exceed a specific threshold — which varies from 25C to 28C in different parts of the country — for three consecutive days. The threshold is lower in cooler areas like northern England and Scotland, and higher in the south-east. The Met Office introduced amber extreme heat warnings in July 2021, and this is only the sixth such warning since then. The warning differs from amber heat health alerts issued by the UK Health Security Agency (UKHSA), which focus on health and social care impacts. The UKHSA issued a separate amber health alert on Thursday, warning that the expected high temperatures could significantly affect the health and wellbeing of people in London, the south-east, the south-west and the east of England.
“What is a heatwave? UK heatwave warnings and thresholds explained.”
The background to these warnings lies in the need to protect public health and infrastructure during increasingly frequent hot spells. The UK has experienced notable heatwaves in recent years, and the Met Office's warning system was developed to give people and organisations time to prepare. The current warning covers much of southern and eastern England and parts of south Wales, and will be in force from 01:00 BST on Monday until 23:59 on Tuesday. There will also be tropical nights in some locations, where overnight temperatures remain above 20C, adding to the strain on the body and infrastructure.
For UK readers, these warnings matter because extreme heat can pose serious health risks, especially for elderly people and those with underlying health conditions. The UKHSA said a rise in deaths is likely in the warned areas, particularly among those aged 65 or over or with health conditions. There may also be more water-related incidents, including risks from cold-water shock and drowning. The RNLI's water safety manager, Ross MacLeod, warned that cold water shock is a real danger — even in hot weather, open water remains cold enough year-round to trigger uncontrollable gasping, increased heart rate, and panic, leading to drowning very quickly. He advised entering the water slowly to acclimatise, and if in danger, to fight the panic instinct and “float to live”: relax and float on your back, with head tilted back, gently moving hands and legs to stay afloat before calling for help. At least 15 people died after getting into trouble in open water during a hot spell in May.
Q: What is the official definition of a heatwave in the UK? A heatwave is defined as when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave threshold. The threshold varies by county, ranging from 25C in some areas to 28C in others, such as London and the south-east.
Q: What is the difference between a Met Office amber extreme heat warning and a UKHSA amber heat health alert? The Met Office amber extreme heat warning focuses on the expected weather conditions and their potential impacts on infrastructure and health. The UKHSA amber heat health alert is issued to health and care services, warning that the high temperatures could significantly affect the health and wellbeing of vulnerable people, and that a rise in deaths is likely.
Q: What should I do to stay safe during an extreme heat warning? Stay hydrated, keep out of the sun during the hottest part of the day (11am to 3pm), and check on elderly or vulnerable neighbours. If you go into open water to cool off, enter slowly to avoid cold-water shock. If you see someone in trouble in the water, call emergency services, tell them to float on their back, and throw them something buoyant — do not enter the water yourself.
What happens next depends on whether temperatures meet the heatwave threshold for three days. Parts of south-east England have already recorded two days above the threshold, so if Saturday stays hot enough, a heatwave will be declared in those areas. The amber warning is in place until Tuesday night, after which temperatures are expected to ease. The Met Office continues to monitor conditions and may update warnings as the situation develops.