On 19th July 2022, in Coningsby, Lincolnshire, the UK reached a record-breaking daily maximum temperature of 40.3C. As many of us would have felt, night-time minimum temperature records were set too, with 25.8C being recorded at Kenley Airfield, Greater London. These temperatures were part of a short but unprecedented heatwave that lasted from 16th-19th July.
So what exactly is a heatwave? According to the Met Office, “A heatwave is an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity.”
Heatwaves across the entire world are becoming more intense and frequent, and lasting longer. But, this is especially true in Western Europe where temperatures are rising three to four times faster than the global average.
Why is Europe becoming such a heatwave hotspot?
Experts have largely attributed the sweltering temperatures to climate change – quite simply because heatwaves start from a higher than average baseline temperature.
However, the jet stream can also have a significant impact on heatwaves.
What is the jet stream?
Jet streams are bands of strong wind encircling the Earth that influence our daily weather. They flow five to seven miles above the Earth’s surface from west to east because of how the Earth rotates on its axis.
The atmospheric boundary at which jet streams exist is known as the ‘tropopause’ and is also the region where aeroplanes fly. Jet streams are the reason that aeroplanes may have shorter flight times when travelling from east to west – because they’re boosted by the winds. However, pilots never attempt to fly against jet streams when travelling from west to east as they’re too strong. Instead, they fly above them.
The Earth has four key jet streams: two subtropical jets close to the equator and two polar ones near the north and south poles.
Here in the UK, we’re closest to the north polar jet, which is largely responsible for our weather.
What causes jet streams?
Jet streams exist because the Earth is heated unevenly by the Sun – with colder areas near the poles and warmer areas near the equator. Jet streams are formed when cold and warm air masses meet.
Warm air rises up in the atmosphere and cold air sinks down to replace it. This process creates an air current which travels at an average speed of around 110mph.
Though the more dramatic the temperature differences between warm and cold air, the faster the jets will move. During winter, speeds can get up to 250mph or faster.
“The freezing, powerful winds that whip the top of Mt. Everest, the world's tallest mountain, are actually jet streams. Jet streams can be so cold, and so strong, that climbers cannot leave the shelter of their tents”
How do jet streams affect weather?
Jet streams are like invisible directors of our weather. They act as conveyor belts, delivering weather systems – for example, storms – to one place after another. Without jet streams, our weather would change very little day to day and countries may experience ongoing droughts, heatwaves, floods, and other extreme weather events.
Rarely travelling in a straight line, jet streams have peaks and dips. And because they separate warm and cold air, their peaks often determine which areas have rain or shine and for how long. Areas caught in between these peaks may have prolonged periods of the same weather, such as rain or heat.
If a jet stream bends too much, part of it can break off from the main branch and start spinning counterclockwise in a circular motion. When it does this, it moves air masses around and creates shifts in global weather patterns.
As well as being affected by the shape of the jet stream, our weather is also influenced by its position and speed – which change depending on the season. In winter, the north polar jet moves further south over the UK and speeds up (due to a more dramatic contrast in hot and cold air temperatures). This brings with it colder temperatures and lower pressure weather systems like rain, wind, and stormy weather.
In summer, on the other hand, the polar jet moves further north and winds aren’t as strong (as there’s less of a contrast between hot and cold temperatures). This causes the UK to experience higher pressure weather, which is why we tend to have warmer, drier conditions during these months.
To see a short animation about what jet streams are and how they affect our weather, check out the video below.
What’s the link between jet streams and heatwaves?
Research shows that the average length of heatwaves in the UK has more than doubled in recent years – increasing from 5.3 days in 1961-90 to over 13 days in the decade 2008-2017.
As mentioned earlier, it’s believed that this is partly due to the fact that the world is heating up, so heatwaves are starting from a higher baseline temperature. However, it’s also linked to jet stream activity.
Science suggests that heatwaves are caused by ‘sticky’ wave patterns that remain in the same place for long periods of time after they form. An example of this is the ‘wavenumber 5’ jet stream that was seen during the heatwaves in Europe, the US, and China this summer. This jet stream pattern has five big waves, which cause simultaneous heatwaves across the continents.
Wavenumber 5 can cause heatwaves when it forms a U-bend shape, called an ‘omega block’ (because it looks like the Greek omega symbol Ω), which acts like a clog in a drain. An omega block causes a high-pressure weather system (hot, dry conditions) to become stuck between two lower-pressure weather systems (wet, stormy conditions), meaning that it’s unable to move on.
A 2021 study also revealed how rising global temperatures may be changing jet stream activity. It suggests that climate change is causing the north polar jet to move further north. This is because the Arctic is melting and isn’t as cold as it once was – which is disrupting the delicate balance of warm and cold air that keeps the jet stream in place.
The study predicts that the jet stream will break out of its normal range by 2060 if greenhouse-gas emissions aren’t significantly reduced – bringing with it more extreme, longer-lasting weather events.
Finally, as we’ve already seen, less dramatic temperature differences between areas of warm and cold air can also affect the jet stream’s speed and strength, causing it to slow down.
A weaker jet stream can cause high-pressure weather systems (hot, dry ones) to linger and move on more slowly. So, as the Arctic continues to warm up, it will cause jet streams to slow further during summer, making omega blocks (and therefore, heatwaves) more likely.
Final thoughts…
Hopefully, this short article has given you more insight into how heatwaves work and why they’re becoming more frequent.
If you’re concerned about the impact of climate change on our weather systems and you want to take action, there are plenty of ideas for things you can do to help protect and preserve our planet in our article; 13 tips for sustainable living.
For tips on how to keep cool during a heatwave, you might also want to read our list of 17 ways to stay cool during a UK heatwave.