26 June 2026
As the UK heatwave continues, the University of Reading recorded its highest ever minimum temperature last night, in the 24 hours to 10am Friday, of 21.3 °C. It breaks the previous record of 20.9 °C, set the night before.
This also follows the highest ever June maximum temperature on record, set on Wednesday, 24 June, when the the Reading University Atmospheric Observatory recorded a maximum temperature of 34.9°C, surpassing the previous June record of 34.1°C. This was set only 24 hours previously, on Tuesday, 23 June, 2026.
Overnight temperatures this week have eclipsed the previous 20.8°C record set on 2 August 1995. Wednesday and Thursday nights have been only Reading’s seventh and eighth tropical nights recorded since records began in 1906. A tropical night is defined as an overnight minimum temperature that does not go below 20°C, and can be a key contributor to heat stress.
On Wednesday, Reading also recorded its second hottest-ever average temperature over the entire day at 27.9°C, only beaten by the average 28.85°C recorded on 19 July 2022.
One impact of the UK heatwave many people may not have been aware of is a surge in ozone pollution.
Dr James Weber, Lecturer in the Department of Meteorology at the University of Reading, explains the link between heatwaves and ozone levels.
For interviews, contact the University of Reading Press Office on 0118 378 5757 or pressoffice@reading.ac.uk
Dr James Weber said: “This has been a record-breaking hot June, with temperatures topping 36 degrees this week. That heat has also brought a surge in ozone pollution.
“Ozone is harmful to breathe, particularly for older people, children and those who are medically vulnerable. Some ozone in the UK has been transported from further afield but during heatwaves, high temperatures and strong sunlight increase regional production of ozone. This can exacerbate the other health issues caused by heatwaves. This week ozone breached World Health Organization safety levels at most of Defra air pollution monitoring sites.
"On Tuesday, 60 of the 97 Defra sites measuring ozone went over the limit; 55 did so on Wednesday and 54 on Thursday, primarily in Southern England. Levels are likely to stay high throughout the heatwave, partly because the ozone is remaining relatively high overnight, so each day's build-up starts from an already elevated baseline.
“There is an encouraging message buried in this. For years the UK has been successfully cutting nitrogen oxides (NOx), the pollution that comes from traffic and other forms of burning. On ordinary days this has driven modest increases in ozone, because high levels of NOx typically suppress ozone at ground level. This is because NOx directly reacts with and destroys ozone near the surface and so reducing NOx has let ozone increase. It has still been a good trade overall, as the fall in harmful nitrogen dioxide (NO2) has far outweighed the small rise in ozone, but it is a trade-off nonetheless.
“In a heatwave, factors including high temperatures and strong sunlight mean the key chemistry in some areas, particularly more rural regions, can switch. Instead of suppressing ozone, NOx can start driving it up, producing ozone that is then transported across the UK, elevating surface levels even in urban areas where the local chemistry tends to suppress ozone.
"So, during extreme heat, when ozone is at its highest, the continued efforts to cut NOx bring a double benefit. They reduce NO2 but also ozone by reducing production in the areas when NOx drives it up and lowering the ozone pool available to be transported into cities. The effort the UK is already making pays off most when it matters most. Ozone can also be formed at higher altitudes and brought down to the surface: reducing NOx emissions will help reduce this process too.
“Heat plays more than one role. Ozone needs fuel to form, in the shape of volatile organic chemicals produced by human activity and some plants and trees. The natural emissions of these organic chemicals are much greater when it is hot. Cutting emissions of VOCs from human activity is important but, even with plenty of this fuel about, ozone cannot form without NOx playing its role in the chemistry, which is why cutting NOx remains key, particularly as the frequency and severity of heatwaves is likely to increase.”
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