Parts of northern England have seen hail the size of golfballs as the rest of Europe swelters in record-breaking temperatures.
Violent hailstorms in Yorkshire and in County Durham damaged windows, cars and roofs. Residents said it was "unbelievable".
Andrew Fox, who lives in Ravensworth, North Yorkshire, said: "The storm lasted five minutes and came out of nowhere - it was sunny one minute and storm the next, with hailstones the size of golf balls."
He said the hail damaged every car in his street and the windscreen of his own car was smashed.
"I'd hate to think someone could have been caught out in the storm," he added. "It's no exaggeration to say one of those things could have killed someone."
In Swaledale, Yorkshire, Linda Scott described the storm as "carnage". Her home had two windows broken, tiles came off the roof and the satellite dish snapped in half.
"This is the worst hail storm we have ever experienced," she said.
At 11am on Thursday, the Met Office said it had recorded more than 19,525 lightning strikes across the UK in the previous 34 hours.
The storm left 40,000 homes without electricity in parts of North Yorkshire, County Durham, Tyne and Wear and Northumberland.
Northern Powergrid said power had been restored to 32,500 of its customers by 7pm and expected the rest would be connected by this afternoon.
A new UK July temperature record was set on Wednesday, with Heathrow seeing 36.7C.
Elsewhere in Europe it was even hotter - with Paris reaching 39.7C, much of southwest France seeing 42C and Cordoba in southern Spain hitting 44C. Madrid also came just short of 40C.
Much of southern France, from Toulouse to Lyon, faces temperatures of around 41C until the middle of next week but in the UK the weather is expected to cool a little, with the mercury hovering between the mid-20s and 30C over the next few days.
Sky News weather presenter Isobel Lang said parts of England and Wales will see cloud and showers today with some more thunderstorms developing in the North West but any showers should clear during the night.
She added: "Tomorrow will be largely fine and sunny and warmer with highs of 27 to 29C in the southeast.
"Thundery showers are forecast to break out from the south later in the day and will move north overnight.
"It will be a muggy, very warm night."
:: A fire chief has warned about the dangers of swimming in open water after a man died trying to save a girl on Tuesday afternoon.
The 32-year-old man had gone into the River Esk at Longtown in Cumbria after the youngster got into difficulty.
She was rescued but the man, from Carlisle, drowned.
Cumbria's chief fire officer Jim Onions said the tragedy showed the dangers of outdoor swimming.