Millions of people in several parts of the world have been experiencing intense heat.
Such periods occur within natural weather patterns, but globally they are becoming more frequent, more intense, and are lasting longer due to global warming.
BBC correspondents are in some of the regions where temperatures have been markedly higher than usual.
Chris Bockman - reporting from Toulouse, France
The region of Toulouse including the nearby town of Albi, a Unesco heritage site popular with tourists, is expected to be the hottest in France on Tuesday with temperatures reaching up to 41 degrees centigrade.
Already the region was the warmest in France last year - enduring the hottest summer ever recorded here.
This year Toulouse city officials went to southern Spain to see what they do to beat the heat and have imported several ideas.
One is gold and silver metal ribbons above the main shopping street. It's an experiment to see if they reflect the sun's rays. Canvas sails common in Seville are being installed above streets in other parts of the city this week.
Some municipal buildings roofs are being painted white and city parks are being kept open until 11 pm and public swimming pools until 10 pm.
French climate change scientists have warned that with Bordeaux and Toulouse experiencing a rapid rise in annual temperatures and suffering from severe drought, they could have the same weather conditions as Algerian coastal cities like Oran and Algiers within 30 years.
Kostas Koukoumakas - reporting from Athens, Greece
Temperatures in Greece are set to climb as high as 41C (105.8F) on Tuesday - and the heatwave and bone-dry conditions have turned forests into a tinderbox.
Residents in the capital, Athens, woke up to learn that three fires were burning for the second day near the city.
The National Observatory of Athens has issued the highest wildfire risk alert on the scale - Level 4.
The biggest blaze appears to be in Dervenohoria, some 45km northwest of Athens, though the fires in the East Attica seaside resort of Saronida and outside Loutraki in Corinth are also active.
The sound of firefighting planes and helicopters operating on the fronts of the fire can be heard over our heads. Traffic is moving normally on most of the road network, except in the areas near the fronts.
Houses and properties are burning, and domestic animals have been killed by the flames or fumes, but no serious human injuries have been reported so far. Three volunteer firefighters had to be treated in a hospital in Saronida after sustaining eye injuries, local media report.
Security authorities are ordering the evacuation of residential areas, and this is causing tensions with residents who do not want to leave their possessions behind.
Justin Rowlatt - BBC Climate editor reporting from Murcia, Spain
It was a long and very hot day here in southern Spain.
I'll be honest, we've retreated to our hotel room and - yes - the air conditioning, to finish our work.
But it has been fascinating to have the chance to talk to people here.
Lots of them say they believe global warming is reshaping the climate in the region dramatically.
They say it is becoming more and more like North Africa.
The Sahara Desert is slowly creeping into Europe, was how one man described what he believes is happening.
Climate Editor Justin Rowlatt has been covering the heatwave in southern Spain
And the changes are raising fundamental questions about the future.
Questions like how viable agriculture will be in the future and whether will tourists still want to come here in summer.
Because, of course, climate change means southern Spain - like the rest of the world - is only going to get hotter.
Sofia Bettiza - reporting from Palermo, Sicily
The Palermo region in Sicily is where, two years ago, the highest temperature in Europe was recorded - this could now be exceeded in the coming days.
Monday was scorching hot and the air felt stifling and oppressive.
Palermo has been placed under a red alert warning, which means the heat poses a threat to everybody - not just to more vulnerable groups such as young children and the elderly.
Getting here was a struggle, as a fire broke out last night and led to one of the island's major airports, Catania, being shut down.
It is still unclear whether the fire is linked to the heatwave.
But in the last few years, Sicily has seen wildfires in the hottest months - which have devastated several areas of the island. Last summer, 55,000 hectares of land were lost.
Samantha Granville - reporting from Las Vegas, Nevada
The usually crowded streets of Las Vegas were considerably emptier than normal on Sunday, with security guards guarding the fountains of upscale casinos and hotels to prevent people from jumping in.
Las Vegas' famous strip was a quiet inferno. Some people walked outside, but mostly just to cross the street to the next casino.
At a taco shop on the strip, the tables were all full of patrons dripping with sweat and looking utterly wiped out from the heat. Workers too were draped in the booths, not speaking to each other, but fanning themselves down.
Inside the casinos though, business continued and as the air conditioning was blasting so high people were wearing jumpers to stay warm.
The heat is set to continue for the foreseeable future, and authorities are warning that vulnerable people - including children, pregnant women, and the elderly - are at serious risk of heat-related illness.
Mobile clinics report treating homeless people suffering from third-degree burns. Public buildings in some parts of California and Nevada have been turned into "cooling centers" where people can take refuge from the heat.
A vast forest fire is raging in Canton Valais in southern Switzerland. It broke out late on Monday afternoon near the village of Bitsch and, fuelled by high winds, spread "explosively" during the night, firefighters said.
Authorities have spoken of a "toxic cocktail" of high winds and extreme dryness. June here was very hot and dry, and so far July has been too. Switzerland recorded its hottest day so far this year on 11 July, with a temperature of 37.6 degrees.
More than 200 residents from nearby villages had to be evacuated for their safety. Local hiking trails have been closed, and cable cars are in use only for firefighters.
The fire is currently raging across one square kilometer, in high altitude areas difficult for vehicles to access. Helicopters, including Super Pumas from the Swiss army, are dropping water on the blaze. But wind and smoke are making the work very difficult.
The aim is to bring the fire under control before it reaches the larger Alpine tourist resort of Riederalp, where many tourists are currently staying.
The Swiss Alps are especially sensitive to climate change, with Alpine glaciers losing more than half their volume in less than 100 years.