September’s heatwave is likely to peak on Saturday with temperatures rising as high as 33C in London, the Met Office has said.
It is likely to be the hottest day of the year, though further north will be cooler, and would beat the previous one set in June at 32.2C, with the UK Health Security Agency issuing an amber warning for heat until Sunday evening at 9pm.
This means weather impacts are likely to be felt across the health service, with those aged above 65 or those with pre-existing respiratory or cardiovascular disease at greater risk.
Although temperatures have risen this high before in September, it is unusual for the heat to last so long, with the Met Office predicting five to six days above 30C for some areas.
There is also a chance of tropical nights in the south, defined as having temperatures over 20C, with Wednesday and Thursday night threatening to break the September night time record of 21.7C.
The heatwave is being driven by tropical storms pushing a high pressure system over the UK, with the jet stream having moved to the north and bending into what is known as an omega blocking pattern.
Named after the Greek letter omega because of its shape, this system occurs when an area of high pressure gets stuck between two areas of low-pressure to the west and east and also slightly south.
This has brought torrential rain and flooding for Spain and Greece but hot, dry and clear conditions for the UK and central Europe.
Met Office chief meteorologist Neil Armstrong said: “An active tropical cyclone season in the North Atlantic has helped to amplify the pattern across the North Atlantic, pushing the jet stream well to the north of the UK, allowing some very warm air to be drawn north.
“It’s a marked contrast to the much of meteorological summer, when the UK was on the northern side of the jet stream with cooler air and more unsettled weather.”
The Met Office defines a heatwave as three consecutive days of a particular region exceeding a given threshold, which varies around the UK.
For Scotland, Wales, Northern Ireland, Cornwall and northern England, the threshold is 25C; for Somerset, Hampshire and the Welsh Borders, 26C; the south coast, East Anglia and the East Midlands, 27C; and for London and the home counties the threshold is 28C.
As the climate continues warming because of rising greenhouse gases, these hot spells will become more frequent and severe, the Met Office said.
By 2070, 30C for two days or more will become more likely, with southern parts of the UK projected to see this happen 16 times more frequently than today.
US-based, non-profit Climate Central calculated that the UK’s heatwave has been made five times more likely by climate change.
Scientists are seeing heat records broken on a near-continuous scale, with the Earth having just experienced its hottest ever Northern Hemisphere summer, the World Meteorological Organisation announced on Wednesday.
Not only was it the hottest August on record, but it was the almost the hottest month ever measured, second only to July.
The average global temperature last month was 1.5C higher than the pre-industrial average, though this does not mean the target of the Paris Agreement has been breached as that refers to a decades-long average.
Last month also saw the highest global sea temperatures ever recorded, at an average of 21C.
Dr Friederike Otto of Imperial College London, and co-lead of World Weather Attribution, said: “This week’s heat in the UK has been made hotter by climate change that is the result of human activities like burning coal and other fossil fuels.
“This is now the case for every heatwave, everywhere in the world. Until net greenhouse gas emissions end, heatwaves in the UK and elsewhere will continue to become hotter and more dangerous.”