The deadly heatwaves that have fuelled blazes and caused transport disruptions in Europe, the US, and China this month have one thing in common: a peculiar shape in the jet stream dubbed “wave number 5”.

Scientists are racing to understand whether the band of fast-moving air that controls the weather in the mid-latitudes is changing in a way that makes heatwaves more frequent and persistent.

“The jet stream is the leading driver of our weather,” explains Paul Williams, professor of atmospheric science at the University of Reading. “The jet stream is like a conveyor belt, delivering storms to us one after the other.”

When the jet stream bends into five waves, known as the ‘wavenumber 5 pattern’, it leads to heatwaves across the northern hemisphere. Week of July 18 2022 shown here

It can also generate heat waves when it forms into a U-bend shape, called an “omega block” because it resembles the shape of the Greek letter omega.

Right now, a global pattern of five big waves is circling the world, leading to simultaneous heatwaves across continents. This pattern, known as wavenumber 5, can persist for weeks, causing hot areas to stay hot for a long time.

In China, more than 900m people are experiencing heat waves, and more than 70 weather stations have broken records this month. In the US, Texas and Oklahoma are experiencing record-high daily temperatures, and more than 20 states have issued heat warnings.