After a brief break of cooler weather, Britain is once again bracing for another surge of heat — marking the country’s third heat wave of the year.
While the heat may not reach the extreme highs seen earlier this summer, the latest spell is expected to be more prolonged and widespread.
Jason Kelly, a chief meteorologist at the United Kingdom’s Met Office, warned of temperatures up to 31 degrees Celsius (about 88 degrees Fahrenheit) on Thursday and 32 Celsius (nearly 90 degrees Fahrenheit) on Friday in parts of England and Wales.
According to the Met Office, an official heat wave in Britain is defined as three consecutive days during which temperatures reach or exceed a location-specific threshold. In the north and west, including Scotland and Northern Ireland, the threshold is 25 degrees Celsius (77 degrees Fahrenheit); in London and the nearby surrounding counties it is 28 degrees Celsius (82.4 degrees Fahrenheit).
In the coming days, the Met Office said, most of England, eastern Scotland and eastern Northern Ireland are likely to meet the official heat wave criteria, and regions that fall short of their threshold will still experience above-average temperatures.
The U.K. Health Security Agency issued yellow heat health alerts — its lowest warning level — for all counties in England through Tuesday. The agency warned for the potential of severe impacts across health and social care services, including an increased risk to life across the whole population, especially for older people.
The heat could also affect broadcasters. The BBC issued a broadcast warning on Tuesday, saying the weather system could interfere with television and radio signals, in a phenomenon known as atmospheric ducting. This occurs when a layer of warm air forms above cooler air, creating a temperature inversion that can force signals to bounce between layers and travel much farther than usual. They expected problems with interference through the weekend.
The heat will begin building from Thursday, especially in the south, and spread more widely across Britain by Friday. But the hottest days are expected over the weekend, particularly across central and southern England, where temperatures are forecast to reach or exceed 30 degrees Celsius (86 degrees Fahrenheit). For many, temperatures will be around 10 degrees Celsius (18 degrees Fahrenheit) above seasonal norms.
The heat could also break records in Scotland, where there is a chance of temperatures exceeding 30 degrees Celsius (86 degrees Fahrenheit) in the Moray Firth region on the northeast coast.
Alex Deakin, a meteorologist at the U.K. Met Office, said the Foehn effect — a weather phenomenon in which air becomes warmer and drier after descending from mountain ranges, could play a significant role. The record for July in the Moray Firth region is 31.6 degrees, he said.
“It’s much drier air, and it’s sunnier there, and you’ve had that heating, so the temperatures rise,” he said.
Britain’s spring was the warmest and sunniest on record, according to the Met Office, as well as the driest since 1974. It was particularly dry in England, which saw its driest spring in more than 100 years.
By the end of spring, a drought was declared for parts of northern England, and on Friday a hose pipe ban — a restriction on water usage — was put in effect for more than five million people by Yorkshire Water, a water utility in northern England. The agency said it had received less than half the rainfall that would be expected between February and June.
Dr. Jess Neumann, associate professor of hydrology at the University of Reading, warned that this third heat wave may lead to more water restrictions being imposed across other parts of the country.
It’s not just the land that is heating up. Waters south of Britain, especially in the English Channel, are currently enduring a marine heat wave, with sea surface temperatures between 17 and 20 degrees Celsius, or 63 and 68 degrees Fahrenheit.
While marine heat waves — prolonged periods of unusually high sea surface temperature — are not uncommon in British waters, Oliver Claydon, a spokesman at the Met Office, said they have become more frequent and more intense in recent years because of climate change.
Heat wave conditions may still continue through the beginning of next week, particularly over central and southern parts of England on Monday. Cooler weather is expected on Tuesday with the arrival of clouds and rain, the Met Office said.
Nazaneen Ghaffar is a Times reporter on the Weather team.
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