With much talk of a ‘mini heatwave’ set to grip the UK next week, we’re here to explain how we define what a heatwave is.
A heatwave is an extended period of unusually hot weather relative to the expected conditions of the area at that time of year. These periods are often accompanied by high humidity, making the heat feel even more intense.
Defining a Heatwave in the UK
In the UK, a heatwave is officially defined when a location records at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. This threshold varies by county, reflecting the diverse climate across the UK.
Originally, these thresholds were based on the 1981-2010 climatology of daily maximum temperatures at the mid-point of the meteorological summer (15 July), but were updated ahead of Summer 2022 to use the most recent 1991-2020 climate averaging period as the UK began to see impacts due to climate change.
While temperatures could peak at 27°C next week, with the highest temperatures expected in the south of England and only possibly lasting a couple of days, next week’s conditions are likely to fall just outside of official Met Office heatwave thresholds.
Causes of heatwaves
Heatwaves typically occur in the summer when high-pressure systems develop over an area. These systems are slow-moving and can persist for days or even weeks. In the UK, the position of the jet stream, usually to the north during summer, allows high pressure to settle over the country, resulting in prolonged periods of dry and settled weather.
READ MORE: Understanding weather and climate extremes
Heatwaves and Climate Change
Heatwaves are extreme weather events, and research indicates that climate change is making them more frequent and intense. A study by the Met Office on the Summer 2018 heatwave in the UK found that such events are now 30 times more likely to occur than before the industrial revolution due to higher concentrations of greenhouse gases like carbon dioxide in the atmosphere. As these concentrations continue to rise, heatwaves of similar intensity are projected to become even more common, potentially occurring every other year by the 2050s.
Since the pre-industrial period (1850-1900), the global average temperature has increased by 1.3°C, with UK temperatures rising by a similar amount.
UK Heatwave Records
The UK has experienced several notable heatwaves, with record-breaking temperatures:
- England: 40.3°C on 19 July 2022 at Coningsby, Lincolnshire
- Wales: 37.1°C on 18 July 2022 at Hawarden Airport, Flintshire
- Scotland: 34.8°C on 19 July 2022 at Charterhall, Scottish Borders
- Northern Ireland: 31.3°C on 21 July 2021 at Castlederg, County Tyrone
These records highlight the increasing frequency and intensity of heatwaves in the UK, underscoring the impact of climate change on extreme weather patterns.
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