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 exist because of two forces working together: the temperature difference between the poles and the equator, and the Coriolis effect caused by the Earth’s rotation.
Warm air near the equator rises and moves poleward at high altitude. Cold air near the poles sinks and spreads equatorward nearer the surface. This creates a large-scale circulation. As air moves across latitudes, the Coriolis effect deflects it: to the right in the Northern Hemisphere, to the left in the Southern Hemisphere. The result is a fast west-to-east current concentrated along the boundary between contrasting air masses.
The sharpest temperature contrasts occur in mid-latitudes, which is why the strongest jet streams are found there, not directly over the equator or the poles. Winter strengthens the contrast between cold polar air and milder air to the south, so jet streams tend to be faster and more variable during the colder months.
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 air |
The 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 farther south and higher. It is more prominent in summer and plays a larger role in steering tropical systems and monsoon circulations. In some regions, the two jets merge into a single powerful stream.
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 a jet stream. Air sinks on the eastern side of a ridge, which tends to bring settled, dry weather to the surface below. A trough is a southward dip. Air rises ahead of a trough, encouraging cloud formation, rain, and unsettled conditions.
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. They tend to follow the jet streams, particularly in the mid-latitudes. A low pressure system sitting beneath a jet stream often deepens rapidly as air is evacuated aloft, pulling more air in at the surface and strengthening wind speeds.
This is one reason bomb cyclones and other rapidly intensifying storms often occur where a jet stream is strongest. The upper-level winds provide a powerful exhaust mechanism that helps surface lows spin up quickly.
When the jet streams slow down or split, weather patterns can stall. A trough that stops moving may park over one region for days, bringing repeated rain. A stationary ridge can produce extended heat or drought. Forecast uncertainty rises in these blocked patterns because small shifts in a jet stream’s position have outsized effects on what happens 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. Stronger jet streams generally mean faster-moving weather systems and more changeable conditions. Weaker jet streams allow systems to linger and patterns to become stuck.
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.
Research suggests that a warming Arctic may be linked to more wavy, slower jet streams in some seasons, though the science is still evolving and regional effects vary. What is clear is that the position of the jet streams on any given week is one of the most important factors in determining whether your 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, south-dipping jet stream is likely involved. When the forecast shows the same conditions persisting for a week or more, a blocked pattern with weak or stationary jet streams is a common explanation.
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.
