With Summer 2024 quickly approaching, it's worth reflecting on recent summer trends in the UK.
In the 1980s and 1990s, it was often thought that with climate change, we would end up with warmer, drier, sunnier, more Mediterranean-type summers. This theory was especially supported by many of the 1990s summers, especially high summer (July and August), which saw warmer, drier, and sunnier-than-average weather in most years. The 1995 summer drought was a particularly famous example.
But since the 1990s, shifts in our weather patterns have cast some doubt on this. 2003 and 2006 were famously hot summers, with 2003 having record (at that time) temperatures in early August, and 2006 having a record-breaking hot and sunny July for much of the country. But from 2007 to 2012 we had a run of very wet summers, often characterised by a persistent blocking anticyclone over Greenland, with the jet stream running to the south of its most common position. Some researchers have linked these wet summers with the declining Arctic sea ice.
At present, it is thus unclear how our summers are going to change as far as sunshine and rainfall go, because there are competing feedbacks in the system. A warming climate has potential to push the jet stream northwards, bringing more dry sunny summers, but the declining Arctic sea ice and weakening of the Atlantic meridional overturning circulation have potential to push the jet stream southwards and bring cloudier wetter summers.
From 2013 onwards, our summers have been more of a mixed bag. There have been some notably warm dry sunny summers, but also further wet ones. A common characteristic of recent years has been that the extreme high temperatures have increased at a substantially faster rate than the summer mean temperature. In other words, it is getting somewhat more common to see temperatures in the mid to high 30s Celsius. Although the high daytime temperatures get more publicity, the increase in night time temperatures in these hot spells often has a greater effect on mortality rates, especially in urban areas, making it harder for people to stay cool indoors and overnight.
The UK national record, which stood at 38.5C on 10 August 2003 at Faversham, Kent, for many years, was beaten on 25 July 2019, with 38.7C at Cambridge Botanical Gardens. But this new record did not last long. A record-breaking heatwave on 19 July 2022, particularly in the Midlands and northern England, saw several sites hit 40C for the first time. The national record currently stands at 40.3C at Coningsby, Lincolnshire.
The main culprit, especially since 2019, has often been persistent exceptional heat in north Africa which has periodically established in continental Europe. This results Britain lying very close to some very hot air masses, which typically arrive upon our shores when we get a short bout of southerly winds. The summers of 2019 and 2022, which were characterised by short bursts of exceptional heat in the UK, were often persistently hot in central Europe. While summer 2023 had a relative lack of exceptional heat in north-west Europe, the heat ended up bottled up in the Mediterranean, which frequently saw record breaking temperatures in July.
The most notable warm/hot, dry and sunny summers of this recent period have been 2013, 2018 and 2022. In 2013, July in particular had a prolonged hot dry sunny spell around midmonth, followed by some thundery outbreaks late in the month. In 2018, the period from May to July was particularly sunny for most, and some areas went for over 50 days without any measurable rain. August was generally cloudier, but most regions remained warmer than average. 2022 was characterised by short bursts of exceptional heat, including that 40C July heatwave, but there was also a sustained spell of hot and sunny weather in early to mid-August with temperatures reaching the low to mid-30s Celsius over much of the country.
Summer 2021 was quite unusual in that it was a relatively cool and cloudy summer in the south-east, but a warm and sunny one in many northern and western parts of Britain. Correspondingly, although temperatures never challenged the UK national record, some western areas did see record temperatures in July 2021, with a new national record high of 31.3C at Castlederg, County Tyrone.
We have also had a number of relatively cool, cloudy and/or wet summer months, but which have often seen occasional bursts of exceptional warmth. For example, July 2015 was generally cool and wet, but saw a brief heatwave with temperatures in excess of 35C at the start of the month. Summer 2019 started with a wet June, but there were record breaking temperatures in late July. The generally cool and cloudy July of 2020 ended with a hot day with temperatures exceeding 35C in places, and saw a hot start to August, followed by generally wet weather late to midmonth.
Summer 2023 showed both facets that have recurred in recent years - a record warm June for the UK, followed by an exceptionally wet July, which was also generally cloudy, particularly in the second half. August 2023, although less wet, remained generally changeable.
Taking all of this together, it is hard to say how Summer 2024 will turn out based on recent years, as there has been a lot of variation from one summer to the next. However, it is clear that our chances of seeing exceptional heat at some point are much greater than they would have been a couple of decades ago. 40C is still odds against in the current UK climate, but there is a fair chance of getting into the mid-30s Celsius at some stage.
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