What is the 'Omega Block' causing Europe's heatwave?

The intense heatwave engulfing Western Europe, resulting in more than 50 deaths in France alone, is being sustained by a weather pattern known as an omega block.

Here is what you need to know about omega ⁠blocks and whether climate change means they could become more frequent in the years ahead.

What is an 'omega block'?

An omega block takes its name from the shape of the Greek letter Ω — with a bulge of warmer, settled high pressure held between two cooler low pressure systems.

The "blocking" element refers to how the high pressure area of warm air gets stuck.

Under normal conditions, the jet stream carries weather systems steadily from west to east. But during an omega block, that flow becomes disrupted and can buckle dramatically north and south, isolating the pressure systems.

Weaker steering winds and temperature contrasts in the atmosphere contribute to these slow-moving, locked patterns. The ‌result is that hot, still air ⁠gets lodged over the same area.

Omega blocks typically last between three and ten days, but can persist for weeks.

What happens during an omega block?

Under the high-pressure area in the centre, conditions become hot and dry. The high pressure also suppresses cloud formation, resulting in clear, sunny skies that allow temperatures to climb.

It is conditions like these that are baking France and Spain, where temperatures have exceeded 40C. Regions in the low-pressure areas flanking the heatwave, meanwhile, are more likely to see cooler, rainy conditions.

Britain lies on the boundary between the high-pressure system and the cooler ⁠air to the northwest - producing intense heat in the south and east, and cooler, wetter conditions in the north ‌and west, according to the UK Met Office.

Is climate change responsible?

Scientists have not yet ⁠agreed upon how ‌climate change is affecting the frequency of blocking events - although some research indicates global warming has increased their frequency in northern and western Europe this century.

However, the global scientific consensus is clear that climate change is increasing the frequency and intensity of heatwaves.

Read more:
Heatwave in Europe is the most severe ever recorded - study
Hospitals overwhelmed as Europe heatwave shifts east

Greenhouse gas emissions, mainly from burning coal, ⁠oil and gas, have heated the planet by about 1.4C since pre-industrial times. That warmer baseline means heatwaves reach higher temperatures.

Scientists said ⁠on 26 June the European heatwave would have been "virtually impossible" without human-caused climate change, which has made this week's stiflingly hot night-time temperatures 100 times more likely than they would have been even two decades ago.

A similar heatwave in the same month 50 years ago would have been around 3.5C cooler than this one, the World Weather Attribution group of climate scientists said in their analysis.

As a result of how climate change has raised global baseline temperatures, when patterns ‌like omega blocks occur, the resulting heat can be significantly more intense.