Thermometers in parts of Croatia, France, Germany, Spain and Portugal hit the symbolic 40-degree Celsius mark (104 degrees Fahrenheit), with no sign of relief until at least the start of next week.

"The current heat is totally exceptional -- even for August," said Dominique Escale of France's national weather service Meteo France, which said the summer was the hottest on record in the country since 1949.

The searing heat has sparked devastating forest fires in Croatia, France, Spain, Italy and Portugal. Eleven have been killed in Portugal, 12 more in Spain and five others in France, four of them foreign holidaymakers.

With hundreds of firefighters struggling to control the blazes raging in central and southern Portugal, the government in Lisbon asked NATO to provide water-carrying aircraft and appealed to the European Union for financial aid.

The hot and dry weather stood in sharp contrast to last summer, when at least 100 people were killed and hundreds of thousands forced to flee their homes in floods that stretched from the Balkans to the Baltics.

The dog days are caused by an anti-cyclone which has anchored itself firmly over the west European land mass, holding off rain-bearing depressions over the Atlantic and funnelling hot air north from Africa.

The effects of the weather pattern are aggravated by the long dry period that preceded it, meteorologists said. Because the landscape contains little moisture, less of the sun's energy is taken up in creating evaporation.

Though there was no clear evidence putting the blame on global warming and greenhouse gas production, scientists at the World Meteorological Office point out that the world's 10 hottest recorded years have all taken place since 1987.

"It doesn't contradict climate change, but it's certainly not the proof of it," said Martin Beniston, a researcher working with the UN's Intergovernmental Panel on Climate Change.

Farmers in many countries are suffering the effects of severe drought and ozone concentrations have reached dangerous levels around some cities. In Paris, police were enforcing speed restrictions to cut the pollution.

Officials in Croatia said it was suffering the worst drought in 50 years, with the country's largest river, the Sava, at its lowest level in 160 years. Experts said there was a risk some popular tourist areas such as the Adriatic islands could see shortages of drinking water.

Electricity production in France has been hit because rivers used to cool down nuclear powers plants are slow-moving and warm. Workers at Fessenheim near the German border doused the outside of the reactor with water to keep the temperature down.

In Britain, many trains were running at half speed because of fears the rails could buckle in the heat. The Czech Republic also ordered speed restrictions after some track was seen to have twisted out of shape.

The temperature in northern Italian city of Milan rose to 38.5 degrees Celsius, breaking the previous record of 38.0 set in 1902.

In Germany state workers in the capital Berlin were given the right to leave their desks once temperatures hit 29 degrees -- which they have -- on the grounds that working conditions are too difficult. It was unclear how many were taking advantage of the rule.

But not everyone was complaining about the heat: in Denmark, balmy nights have sent the libidos of Copenhagen residents into overdrive, with couples engaging in public displays of affection across the city, the tabloid daily BT reported.

Police said they will only intervene if someone complains. "Everything is not allowed, and we'll act if a couple is found having sex in a park, for example, where children can see them. We'll ask them to get dressed," one officer said.

European zoos were adapting novel ways of keeping animals cool. To the well-worn shower technique, a park outside Paris added a new treat for polar bears: mackerel ice-lollies.

SPACE.WIRE