Today may be the hottest day of the heatwave so far according to weather forecasters.
Scandinavia, Canada, Siberia, Japan and the Caspian Sea have been experiencing unprecedented heat along with the UK.
Some hospitals in England have had record numbers of people attending A&E amid a summer crisis, NHS trusts have said, as the UK experienced its hottest day of the year.
The mercury reached 35C (95F) on Thursday at Heathrow, west London
However, it is forecast to reach 37C on Friday, and could pass the all-time record of 38.5C, set in 2003.
Japan has declared a natural disaster after dozens have died and thousands have been admitted to hospital with heatstroke as temperatures reach above 40C.
In California, the Ferguson Fire has been blazing since July 13 as is now threatening Yosemite National Park which has been forced to close in parts.
BBC's science editor David Shukman said: "What is striking now is that multiple heatwaves are happening at the same time."
"The key is the jet stream. This year it's been meandering in great loops and the UK has ended up to the south of it.
“Add to that sea temperatures similar to previous heatwaves and climate change, the warming of the atmosphere - it all makes heatwaves more likely.
"Climate scientists are not saying we will get heatwaves every year. But they do say the risks of extreme heat are going up."
How long will the heatwave last?
The prolonged wave of hot weather is due to subside on Friday for most, although that will not necessarily mean the end of the hot weather for the coming weeks. The Met Office says that as we move into August, we are likely to see more “very warm” weather in parts.
When did the heatwave start?
It’s difficult to state this accurately as the definition of a heatwave varies from area to area depending on its normal average temperatures. To give a picture of the unusually dry summer so far, East Anglia and the South East have seen no widespread rain since May 29. There were also 16 consecutive days from June 24 where somewhere in the UK reached 28C (82.4F)
How long did the heatwave last in 1976?
In 1976, there were 15 consecutive days when temperatures reached 32C (89.6F) or higher.
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