As the UK moves into autumn, it offers the opportunity to look back at the pronounced, record-breaking heatwave that hit the southeast of the country during August.
Early August saw a long run of days with temperatures somewhere reaching 34C, along with several “tropical nights” where nighttime temperatures did not drop below 20C.
In this article, we look back on the recent heatwave and the factors that contributed to it.
- Temperatures somewhere in the UK reached 34C or more for six consecutive days.
- Five tropical nights were recorded in the UK in August, four of which were consecutive.
- A persistent high-pressure zone over the UK and northwest Europe blocked the arrival of cooler conditions.
- Hot spells are likely to become more common in the future, potentially occurring multiple times per year.
A record-breaking hot spell
After a relatively cool July, which was around 0.8C colder than the long-term (1981-2010) average, the first signs of the heatwave were seen on the last day of the month. This was later confirmed as the third-hottest day on record in the UK, with a temperature of 37.8C at Heathrow.
This isolated hot day turned out to be the prelude to a much longer heatwave, which began the following week on 7 August, during which many areas of the southeast saw six consecutive days of well-above average temperatures.
Heatwave conditions in a particular location are defined by the Met Office as a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. As the right-hand map below shows, this heatwave temperature threshold varies throughout the country and is higher towards the southeast.
The left-hand map below shows that these heatwave conditions were met as far north as Cumbria, but did not extend to Scotland or Northern Ireland.
During August 2020, temperatures exceeding 34C were recorded somewhere in the UK for six consecutive days.
34C has been recorded in the UK during seven out of the last 10 years, compared to seven out of the previous 50 years from 1961 to 2010. This suggests that temperatures of 34C or higher occurring at some point during the summer are becoming a more common occurrence.
The maps below show the daily UK temperatures, compared to the 1981-2010 average, for these six days – 7-12 August. The purple shading in the southeast indicates that this region was the focus of the unusually warm weather.
Many weather stations around the UK also broke or matched their maximum temperature records, many of which were decades old. The highest UK temperatures in August 2020 were around 12C above the long-term August average for 1981-2010, with the peak temperature of 36.4C recorded at Heathrow on 7 August.
Interestingly, the hot spell lasted longer when measured using thresholds for minimum temperature rather than maximum temperature – and hot nights can be particularly significant for health outcomes.
The highest daily minimum temperatures were generally found in London and along southern coasts thanks to the Urban Heat Island effect – where the concentration of buildings, roads and air pollution can magnify warm conditions – and humidity from the sea.
Langdon Bay in Kent recorded the highest daily minimum temperature during the heatwave, seeing 22.3C on 8 August, while central London recorded a minimum of 22C on 12 August.
The maps below show the daily minimum temperatures across the UK, with the dark red and purple shading highlighting the very warmest conditions.
August 2020 also saw five nights during which, somewhere in the UK, the night-time temperature remained above 20C, making them ‘tropical nights’. Four of these five tropical nights were consecutive, breaking records at many monitoring stations.
Hot weather conditions such as these are difficult for elderly and vulnerable people, and this heatwave has been linked by the Office for National Statistics to a rise in registered deaths in England and Wales during the week.
There were 44 tropical nights recorded from 1961 to 1990, but 84 were recorded from 1991 to 2020. Tropical nights are expected to increase in frequency as the climate warms, but they do not yet show a compelling, long-term trend, with fewer occurrences in the 2010s than in the 1990s and 2000s.
The chart below shows the frequency of tropical nights since 1961. Most noticeable are particular exceptional events – such as the summers of 1995, 1997 and 2003 – which indicates a large annual variability inherent in the UK climate.
The impacts of weather events like this can be severe and wide-ranging. A report from the National Farmers’ Union made headlines warning of a rise in the price of bread due to a poor wheat harvest. This was linked to a number of severe and adverse weather events, including those seen in August.
Other reports linked a rise in the UK death rate directly to the hot weather in August, due to the health risks posed by tropical nights and extreme daytime temperatures.
In contrast to the heat seen at the start of the month, the latter part of August saw thunderstorms lashing much of the UK and causing severe flooding in many areas including Wales, central Scotland and southern England.
The August heatwave was not just limited to the UK, with northwest Europe also enduring extreme temperatures.
Most Belgian provinces were placed under a heatwave warning, a measure also taken in the southern Netherlands. The Dutch town of De Bilt saw 30C temperatures for eight consecutive days and the highest weekly average maximum and minimum temperatures since records began in 1901.
In France, Île-de-France, Hauts-de-France and Haute-Normandie were the worst affected areas, with Boulogne-sur-Mer setting a new all-time temperature record of 37.9C and a new night-time record of 25.2C. This all comes months after a major heatwave in Siberia which studies have suggested was made 600 times more likely by climate change. Such studies have yet to be carried out looking at the August heatwave, although a recently published paper suggests similar heatwaves in 2019 would have been very unlikely without a human contribution
Heatwaves are often the result of atmospheric circulation patterns and August 2020 was no different.
For much of the heatwave, a hot, humid air mass sat over southeast England and northwest Europe, separated from cooler conditions in the north by a weather front. This warm air mass was caused by a high-pressure zone across Scandinavia and the North Sea which “blocked” cooler weather systems coming in from the Atlantic, as shown in the chart below.
This pressure system is common for a UK summer heatwave, but climate change is making the record-breaking temperatures recorded far more likely. A recent study by Met Office scientists shows that summers which see days above 40C somewhere in the UK are likely to occur once every 100-300 years at present, but that this could decrease to once every 3.5 years by 2100 if greenhouse gas emissions are not reduced.
As the Earth warms, the climate in the UK is changing. The latest Met Office State of the UK Climate report stated that all the Top 10 warmest years in the UK since 1884 have occurred since 2002, while the Central England Temperature series provides evidence that the 21st century so far has, overall, been warmer than the previous three centuries.
The 2010s were the second warmest (and second wettest) decade on record behind the 2000s.
Future projections indicate a continuation of these trends. A study by Met Office scientists found that the exceptionally warm and dry summer of 2018 was made 30 times more likely as a result of climate change, whilst the projections provided by the UK Climate Projections (UKCP) project suggest such summers could happen every other year by the mid-21st century if warming continues.
These studies all suggest we can expect to see more of these extreme heat events as the climate continues to warm.
Acknowledgements: The authors would like to thank their Met Office colleagues Mike Kendon, Dr Mark McCarthy, Grahame Madge and Dan Williams for their help in producing this article.
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