An unusual weather pattern known as an omega block is at the heart of the extreme heat sweeping across Europe. The phenomenon can trap hot air over the same region for days or even weeks, allowing temperatures to climb to dangerous levels.

What is an omega block?

An omega block gets its name from the Greek letter Ω because of the shape it creates on weather maps.

It forms when a large area of high pressure becomes trapped between two areas of low pressure. Under normal conditions, the jet stream moves weather systems steadily from west to east.

During an omega block, that flow becomes distorted. The high-pressure system becomes stuck in place, preventing cooler air from moving in and pushing the heat away.

As a result, hot and dry conditions can linger over the same areas for an extended period.

Why does it make temperatures soar?

High-pressure systems act like a lid on the atmosphere.

They suppress cloud formation and allow strong sunshine to heat the ground day after day. With little cloud cover and limited wind, temperatures continue to build.

This is what has happened across large parts of France, Spain and other areas of western Europe, where temperatures have climbed above 40°C.

France has recorded some of its highest temperatures in decades, with one town in the south-west reaching 44.3°C.

Meanwhile, regions sitting under the neighbouring low-pressure systems often experience cooler and wetter weather.

Why is France so worried?

The heatwave has stretched public services and raised health concerns across the country.

More than half of mainland France’s departments have been placed under the highest red alert. Schools have closed or shortened lessons, while landmarks including the Louvre Museum and the Eiffel Tower have reduced opening hours because of the extreme heat.

Prime Minister Sébastien Lecornu described the recent drowning deaths as "a sad scourge", saying 40 people had died since 18 June, most of them young people.

Météo-France said the current conditions are comparable to the devastating 2003 heatwave, which lasted more than two weeks and was linked to around 15,000 deaths in France and an estimated 80,000 across Europe.

Is climate change causing omega blocks?

Scientists have not yet reached a clear conclusion on whether climate change is making omega blocks more common.

However, there is broad agreement that climate change is making heatwaves more frequent, longer-lasting and more intense.

The planet has warmed by around 1.3°C since pre-industrial times because of greenhouse gas emissions from fossil fuels such as coal, oil and gas.

That means when a weather pattern such as an omega block develops, temperatures start from a much warmer baseline.

According to Clair Barnes, a researcher in extreme weather and climate at Imperial College London, European heatwaves are now typically between 2°C and 4°C hotter than they would have been without human-driven climate change.

Could these heatwaves become more common?

Scientists say Europe is warming faster than any other continent, with temperatures rising at more than twice the global average.

That does not necessarily mean omega blocks will become more frequent. However, when they do occur, they are increasingly likely to produce dangerous and record-breaking heat.

The current heatwave offers a glimpse of that reality. Across Europe, people are adapting as best they can by buying fans, seeking cooler destinations and changing their daily routines.

For many, however, the heat has become more than an inconvenience. It is a growing public health risk — and one that experts say will become harder to ignore in a warming world.

Read more: 

• What do France's heatwave terms really mean?
• 20 drown in France heatwave, two children die in hot car
• Europe scorched by early summer heatwave as temperatures near 40°C

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