Zero-degree line at record height above Switzerland as heat and fire hit Europe

A US weather balloon is prepared. The balloon that recorded the ‘zero-degree line’ at 5,300 metres was flown from western Switzerland on Monday. Photograph: Mark Humphrey/AP

A Swiss weather balloon had to climb to an unprecedented 5,300 metres (17,400ft) before the temperature fell to 0C (32F), meteorologists have said, as a late summer heatwave and wildfires continue to pummel swaths of continental Europe.

A man was found dead in a blaze raging north of Athens on Monday as the Greek government warned of an extreme risk of fire across the country, while more than half of mainland France was placed under an amber extreme heat alert and local records tumbled in the south-east.