What Are Jet Streams?
The jet streams are narrow bands of strong winds high in the atmosphere, typically flowing from west to east. They form along the boundaries between contrasting air masses, with the polar jet and subtropical jet being the two most relevant to day-to-day weather. Together they act as conveyor belts for weather systems below. When forecasters talk about a storm being “steered” by the jet stream, they usually mean one of these fast-moving rivers of air is guiding where low pressure systems travel and how quickly they intensify.
Jet stream winds are measured at cruising altitude for commercial aircraft, roughly 9–12 km (30,000–39,000 ft) above the surface. Speeds commonly reach 160–320 km/h (100–200 mph), and in powerful setups they can exceed 400 km/h (250 mph). You cannot feel the jet streams on the ground, but their influence reaches all the way down through wind patterns, pressure systems, and the movement of storms.
How Jet Streams Form
Jet streams form because the atmosphere responds to large temperature differences across latitude while the Earth rotates. Those temperature contrasts create pressure gradients high in the atmosphere. Air accelerates along them, and the Coriolis effect turns the flow to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This helps organise the wind into fast currents that run mainly from west to east.
The polar jet is closely tied to the strong temperature contrast between polar and mid-latitude air. The subtropical jet forms differently, near the poleward edge of the tropical circulation, as air moving away from the equator accelerates eastward. This means not every jet stream marks a boundary between two surface air masses.
Both jets shift and change strength with the seasons. In the winter of each hemisphere, stronger temperature gradients generally make the upper-level winds faster and push the main jets closer to the equator.
The Two Main Jet Streams
In each hemisphere, two jet streams are most relevant to weather forecasting:
Jet stream Typical altitude Driven by Polar jet 7–12 km (23,000–39,000 ft) Boundary between polar and mid-latitude air Subtropical jet 10–14 km (33,000–46,000 ft) Boundary between tropical and mid-latitude airThe polar jet is the one most closely tied to day-to-day weather in Europe, North America, and parts of Asia. It meanders north and south, carrying low pressure systems and their associated fronts along with it.
The subtropical jet sits closer to the equator and generally higher in the atmosphere. It is usually strongest and most continuous during the winter of each hemisphere, then weakens or shifts poleward in summer. Its position can affect tropical moisture, monsoon circulations, and mid-latitude storm tracks. At times, the polar and subtropical jets interact or merge.
Meanders, Ridges, and Troughs
The jet streams are not straight lines. They curve in large waves called Rossby waves, named after the meteorologist Carl-Gustaf Rossby. These waves create alternating northward bulges and southward dips along each stream.
A ridge is a northward bulge in the upper-level flow. Sinking air beneath a ridge often favours settled, dry weather. A trough is a southward dip. Rising motion is common ahead of a trough, encouraging cloud formation, rain, and unsettled conditions. The exact surface weather still depends on moisture, temperature, and the position of smaller disturbances within the flow.
The shape of these waves is visible on upper-air charts and is one of the first things forecasters look at when reading a synoptic chart. A deep trough over your region often means cooler, wetter weather. A persistent ridge overhead can mean a prolonged dry or hot spell.
How Jet Streams Steer Storms
Surface weather systems do not move randomly. In the mid-latitudes, their paths are strongly influenced by the upper-level flow. A low pressure system can deepen when it moves beneath a favourable part of a jet streak, where divergence aloft removes mass from the air column. Surface pressure then falls, which can strengthen wind speeds. A low does not intensify merely because a jet stream is overhead.
This is one reason bomb cyclones and other rapidly intensifying storms often develop when a strong jet is in the right position. The upper-level flow can support rising air and rapid pressure falls, but moisture, temperature contrasts, and the structure of the developing low also matter.
When the jet streams bend sharply, split, or are diverted around a blocking high, weather patterns can stall. A trough that stops moving may remain over one region for days, bringing repeated rain. A stationary ridge can prolong heat or drought. Forecast uncertainty rises in these blocked patterns because small shifts in the upper-level flow can have large effects at the surface.
Seasonal Shifts
The jet streams’ average positions change with the seasons. In the Northern Hemisphere winter, the polar jet tends to sit farther south, bringing more frequent storms to the Mediterranean, the southern United States, and southern Japan. In summer, it retreats northward, and settled conditions often dominate regions that were stormy just months earlier.
The strength of the jet streams also varies. A strong jet can support fast-moving weather systems and rapid development, but wind speed alone does not determine the weather below. The jet’s path, curvature, and areas of divergence are just as important. Blocking can make systems linger even when strong winds remain elsewhere in the flow.
Jet Streams and Extreme Weather
A displaced or amplified jet stream is often involved in notable weather events, though it is rarely the sole cause.
When the polar jet dips unusually far south, cold Arctic air can spill into mid-latitudes, producing sharp temperature drops and, in some cases, heavy snow where mild conditions had persisted. When the jet streams retreat far north, warm air can build underneath a ridge, contributing to prolonged heat in regions normally accustomed to cooler summer weather.
Researchers continue to investigate whether rapid Arctic warming changes the waviness or persistence of the jet streams. Results vary by season, region, and method, so the relationship should not be treated as simple cause and effect. The position and shape of the jet streams still provide vital context for whether a forecast looks settled or stormy.
Aviation and Jet Streams
Commercial pilots pay close attention to the jet streams because they directly affect flight times and fuel use. Flying with the jet stream (eastbound) can shave 30–60 minutes off a transatlantic crossing. Flying against it (westbound) adds time and fuel.
Clear-air turbulence, sudden bumps in otherwise smooth skies, often occurs near the edges of a jet stream where wind speed changes sharply over a short distance. This is called wind shear aloft, and it is distinct from the surface wind shear that affects local wind forecasts.
Reading Jet Streams in Forecasts
Most public weather apps do not display the jet streams directly, but their fingerprints are everywhere in the forecast. When you see a string of storm systems arriving every two to three days, a strong jet positioned near your region is often involved. When the forecast shows the same conditions persisting for a week or more, a blocking pattern may be diverting or distorting the upper-level flow.
Numerical weather prediction models simulate the jet streams at multiple levels of the atmosphere. Forecasters compare model runs to see whether the predicted positions are consistent across runs. When models agree, confidence in the surface forecast is higher. When they disagree on the jet streams’ paths, the surface forecast for several days ahead becomes less certain.
How Airpult Shows Jet Stream Effects
Airpult does not display the jet streams as a separate layer, but you can see their influence in the wind speed and conditions on the forecast page. A run of unsettled days with strengthening winds often signals active jet streams overhead. Use the explore page to compare conditions across locations and spot how weather systems are moving through your region.
