Where the jet stream comes from
The polar front jet stream, the one that matters for UK weather, forms along the boundary between cold air over the Arctic and warmer air to the south. The bigger the temperature contrast, the stronger the jet. That is why it is strongest in winter, when the contrast between the Arctic and the subtropics is greatest, with core speeds that can exceed 200 mph.
The jet is not a straight line. It meanders in great waves around the hemisphere, and those waves slowly move, grow and break. Its position over the North Atlantic shifts from week to week and season to season.
How it steers our weather
Depressions form and deepen on the jet stream, and it carries them along. When the jet is strong and points straight at the UK, a conveyor belt of Atlantic lows follows it: mild, wet and windy weather, sometimes with named storms. When it shifts north of the UK, those systems pass by to the north and high pressure can build, bringing settled weather. When it lies to the south, the lows track across or south of us, and the UK can be cool and showery.
- Jet to the north: settled, often warm in summer
- Jet over the UK: wet, windy, mild
- Jet to the south: cool and showery
Simplified: the real jet meanders in larger waves and shifts from week to week.
Summers and winters shaped by the jet
Many of the UK's notably wet summers have come when the jet stream sat further south than usual for weeks on end, steering depression after depression across the country. Hot, dry summers usually come when it lies well to the north and high pressure settles over or near the UK.
In winter, a strong, flat jet aimed at the UK brings stormy, mild weather. Severe cold spells tend to come when the flow becomes blocked: a large area of high pressure to the north or east diverts the jet, and cold air from the continent or the Arctic can reach us on easterly or northerly winds.
Why the jet stream makes long range forecasting hard
The UK sits right at the end of the Atlantic storm track, where small changes in the jet's position make a big difference. A shift of a few hundred kilometres can turn a week of storms into a week of calm. Models can predict the large waves in the jet some days ahead, but where exactly they will sit two or three weeks out is often beyond them.
Sometimes the odds tilt for a reason. A sudden warming high in the stratosphere over the Arctic can weaken the polar vortex and, in some winters, leads to a blocked pattern and cold weather a few weeks later. When long range models pick up on drivers like that, their signals become more meaningful.
Is climate change affecting the jet stream?
Scientists are actively researching how a warming climate is changing the jet stream. The Arctic is warming faster than the rest of the world, which reduces the temperature contrast the jet depends on, and some studies suggest this could make it slower and more meandering. Others find little clear evidence of that yet. It remains an open question, and we will not pretend otherwise.
Questions
How high is the jet stream?
Around 9 to 12 km up, close to the top of the troposphere and about the height at which long-haul jets cruise. Flights from North America to the UK often ride it to save time and fuel.
How fast is the jet stream?
Typically 100 to 200 mph at its core, sometimes faster in winter. Speeds vary a great deal along its length.
Why does the jet stream move?
Because the boundary between polar and warmer air moves with the seasons, and because the jet's own waves grow and break. It generally sits further north in summer and further south in winter.
Does the jet stream cause storms?
It provides the conditions for them. Depressions develop and deepen where the jet is strong and curved, and it then steers them. A powerful jet aimed at the UK is often the setting for winter storms.
More guides: how to read a weather forecast, how accurate are long range forecasts?, weather models explained.