Soaring temperatures broke records in Germany, France, Britain and the Netherlands on Thursday as a heat wave gripped Europe for the second time in a month, in what scientists said were becoming more frequent events as the planet heats up.

As a cauldron of hot air from the Sahara desert moved across the continent, drawn northwards by high pressure, Paris recorded its highest temperature since records began and Britain reported its hottest weather for the month of July.

An all-time high was measured in Germany for a second day running, at 41.5 C in the northwestern town of Lingen — similar temperatures to those in some Gulf Arab capitals on Thursday.

The unusual conditions brought a reduction in French and German nuclear power output, disrupted rail travel in parts of Britain and sent some Europeans, not habitual users of air conditioning in their homes, out to the shops in search of fans.

Health authorities issued warnings to the elderly, especially vulnerable to spikes in temperature. In cities,

children splashed about in water fountains to cool off.

"It's very hot at the moment. I saw 42 degrees [Celsius] is forecast for today," 19-year-old French tourist Ombeline Massot said in the capital's Montmartre district, where visitors drank chilled bottles of water and fanned themselves.

The mercury in Paris touched 42.6 C in mid-afternoon, above the previous Paris record of 40.4 C recorded in July 1947.

In Britain, the temperature reached its highest for July, hitting 36.9 C, said the Met Office, the national weather service. The temperature, recorded at Heathrow Airport, London, beat the previously July record of 36.7 C.

A police officer cools down with a drink carton on her neck as temperatures rise far above 30 C in London on Thursday. (Frank Augstein/The Associated Press)

In the southern Netherlands, the temperature peaked at 40.4 C, topping 40 C for the first time on record, Dutch meteorology institute KNMI said. That broke the national record of 39.3 C set the previous day. Before this week, the national heat record of 38.6 C had stood for 75 years.

The heat is expected to persist until Friday.

Global warming

Climate specialists said such heat waves are becoming more frequent as a result of global warming from greenhouse gas emissions.

Britons were facing travel disruption, with trains being forced to slow down to prevent tracks buckling in the heat. Several train operators asked commuters not to travel or set off very early.

A Met Office study found that a heat wave like one that broke records last year was 30 times more likely to occur than in 1750 because of the high amount of carbon dioxide in the atmosphere. Since the pre-industrial period, the Earth's surface temperature has risen by 1 C.

People walk through a park fountain in Antwerp, Belgium, on Thursday. Belgium braced for a code red extreme heat warning on Thursday as temperatures soared during the second heat wave of the summer. (Virginia Mayo/The Associated Press)

"There is a 40-50 per cent chance that this will be the warmest July on record. This heat wave is exactly in line with climate change predictions," said Karsten Haustein at the Environmental Change Institute at the University of Oxford.

Peter Inness, senior research fellow at the University of Reading, said: "The fact that so many recent years have had very high summer temperatures both globally and across Europe is very much in line with what we expect from man-made global warming."

The heatwave in Britain is expected to come to an abrupt end on Friday with thunderstorms forecast for several parts of the country, the Met Office said.

Very high temperatures across Europe coupled with prolonged dry weather have reduced French nuclear power generation by around 5.2 gigawatts (GW) or eight per cent, French power grid operator RTE's data showed.

Electricity output was curtailed at six reactors on Thursday, while two other reactors were offline, data showed. High water temperatures and sluggish flows limit the ability to use river water to cool reactors.