Explainer-What is a 'heat dome' and how does it power heatwaves?

By Kate Abnett

BRUSSELS (Reuters) -Large parts of Western Europe were in the grip of a severe heatwave on Monday, with temperatures breaching 40 degrees Celsius (104 degrees Fahrenheit) in Spain and wildfires breaking out in France.

Scientists said the extreme temperatures in the region - with Britain and the Netherlands among the countries affected - were related to a "heat dome" over continental Europe.

Here's what that means:

WHAT IS IT?

A heat dome is an area of high-pressure air in the atmosphere which gets stuck in place over a region because atmospheric dynamics around it block it from moving. 

It works like putting a lid on a boiling pot. The high-pressure system traps hot air below it, which heats up and compresses to form a "dome". This intensifies heat and prevents the formation of clouds, allowing even more radiation from the sun to reach the ground below.

A heat dome results in clear, sunny days, and still conditions with little cooling wind. 

It builds up over time - the longer the "dome" is stuck over one area, the more that dark surfaces below, such as roads and buildings, absorb and retain heat, and the more that the ground dries out. It also makes wildfires more likely, because the heat dries out vegetation.

Such systems can last for days to weeks. Forecasts suggest this one will dissipate in a few days, which happens when another weather system, such as a storm or a low-pressure system of cooler conditions, pushes the high-pressure system away.

IS IT LINKED TO CLIMATE CHANGE?

Heat domes are not a new type of weather pattern. A specific attribution study would be needed to confirm in what ways the heat dome Western Europe is currently experiencing was specifically affected by climate change.

But scientists said the severity of the temperatures and the early timing of the current heatwave in Western Europe tally with how climate change is known to affect heatwaves.

Scientists have already confirmed that climate change is making heatwave events more intense, more frequent and more widespread. 

The build-up of greenhouse gas emissions in the atmosphere - which mostly come from burning fossil fuels - means the planet's average temperature has increased over time. This increase in baseline temperatures means that when a heatwave comes, temperatures can surge to reach higher peaks.

Spain has likely just had its hottest June on record, according to national meteorological service AEMET, while the southeastern town of Mora in Portugal on Sunday set a new national record-high temperature for June, of 46.6 C.

Today, the planet's long-term global average temperature has risen nearly 1.3 degrees Celsius since the Industrial Revolution in the 19th century, when countries began burning fossil fuels on an industrial scale.

Europe is the world's fastest-warming continent, heating up at twice the global average.

WILL WE GET MORE?

Climate change is making extreme heatwaves like the current one occur earlier in the year, and persist into later months.

Parts of the United States also faced extreme temperatures caused by a heat dome in the last two weeks.

While it is hard to predict specific heatwaves months in advance, current seasonal forecasts for July, August and September indicate Europe is highly likely to experience a warmer than average summer, Dr Samantha Burgess, Strategic Lead for Climate at the EU's Copernicus Climate Change Service, told Reuters.

(Reporting by Kate Abnett; additional reporting by Ali Withers; Editing by Andrew Heavens)