Another heatwave is building across the UK this weekend, with forecasters warning that temperatures could reach 34C in southern England and a week-long yellow heat health alert has been issued. For some areas, this will be the third heatwave of the year.
In the UK, a heatwave is officially defined as a period of at least three consecutive days where daily maximum temperatures meet or exceed regional thresholds, which vary from 25C to 28C across the country. The current hot spell is being driven by a ridge of high pressure linked to the Azores High, a large semi-permanent centre of high atmospheric pressure, extending north-eastwards across the UK while the jet stream shifts north, blocking Atlantic low-pressure systems and bringing settled weather and plenty of sunshine.
“What a UK heatwave is, how it affects health and driving, and what to expect.”
The UK Health Security Agency has issued yellow heat health alerts for the East Midlands, West Midlands, east of England, London, south-east England and south-west England, valid from 12:00 BST on Saturday until 20:00 on Saturday 11 July. These alerts warn of an increased risk to health, especially for vulnerable people such as the elderly and those with underlying health conditions. During heatwaves, there is an increased risk of heat-related illness.
This year has already seen record-breaking heat in June, and while the current heatwave may not be as intense, it is expected to last significantly longer, potentially into the middle of July. The highest temperatures will be focused across southern and eastern England, where values could widely exceed 30C. Meanwhile, areas on the western side of the UK – such as Lancashire, the Lake District, western Scotland and Northern Ireland – will be exposed to an Atlantic breeze, limiting temperatures to the low to mid-20s.
For UK readers, the practical impacts are significant. Drivers are being urged to fill up their tanks before the heatwave, as hot weather can cause cars to burn fuel faster due to increased use of air conditioning. According to the US Department of Energy, under very hot conditions, A/C use can reduce a conventional vehicle's fuel economy by more than 25%, particularly on short trips. Fuel gauges can also become inaccurate, and running out of fuel in traffic in extreme heat is a real risk.
Q: What is a heatwave in the UK? A heatwave is defined as at least three consecutive days where the daily maximum temperature meets or exceeds a regional threshold, which ranges from 25C to 28C depending on the area.
Q: How do heatwaves affect health? During heatwaves, there is an increased risk of heat-related illness, particularly for older people and those with underlying health conditions. The UK Health Security Agency issues colour-coded health alerts – yellow, amber, or red – to warn the public and healthcare services.
Q: Does hot weather affect car fuel economy? Yes. Using air conditioning in very hot conditions can reduce a conventional vehicle's fuel economy by more than 25%, especially on short trips. The effect can be even larger for hybrids, plug-in hybrids, and electric vehicles.
What happens next? Temperatures are expected to peak during the second half of the week, with 34C possible in the South East on Thursday or Friday. The hot spell could persist into the middle of July, though there is some uncertainty. The yellow heat health alert remains in force until 20:00 on 11 July.