Why is crucial tech vulnerable to the heat?

As one of France's hottest days on record unfolded on 23 June, exasperated people painted white chalk on their windows to screen out the sun. Paris's Eiffel Tower closed early.

And in the town of Ergué-Gabéric, in Brittany, the punishing temperatures - around 40C - were too much for one electric transformer.

The chunky metal box malfunctioned, initially leaving more than 100,000 people without power.

It was a "heat related" incident, according to local authorities, external. Videos posted to social media appeared to show a plume of smoke rising from the stricken transformer. A spokeswoman for power company RTE confirmed to the BBC that the video showed one of the firm's facilities.

The day before the accident, RTE had published a statement, external saying there was "no concern" surrounding the availability of electricity across its network this summer.

Just as we all have our own limits in terms of high temperatures, so too does technology. Electrical and telecoms equipment, and railway signalling cabinets sometimes falter during a heatwave. Extreme temperatures can even set off alarm systems.

For instance, six NHS trusts in England declared a critical incident last week after hot weather adversely affected their IT systems, scanners, and cancer and lab equipment.

More frequent and more intense heatwaves triggered by human-caused climate change mean that engineers are increasingly adapting infrastructure to cope.

"Anything to do with the electricity network – the power lines, the interconnectors and transformers – they all struggle to keep themselves cool enough," explains Iain Staffell at Imperial College London. "It reduces the efficiency of everything."

Staffell and colleagues estimate that, in temperatures of 40C, the output of gas-fired power stations drops by roughly 10% versus 20C.

The efficiency of solar panels also falls as temperature rises, though Staffell notes that this effect has become less pronounced with newer generations of panels.

Even so, the impact of high temperatures on solar energy in Great Britain is visible in data he and his colleagues have analysed and shared with the BBC. "Once the UK gets above 27C, our solar output plateaus and starts to slowly fall [as temperatures continue to rise]," says Staffell.