The jet stream

What it is

Jet streams are relatively narrow bands of strong wind in the upper atmosphere, typically around 30,000 feet (9,100 meters) up.² "Narrow" is relative. A jet stream runs thousands of kilometers long, hundreds of miles wide and thousands of feet deep, with the wind getting faster toward a central core.²⁴

The wind averages about 110 mph. When the temperature contrast is extreme it can pass 250 mph, and the National Weather Service puts the top end at more than 275 mph (442 km/h).¹²

Why it exists

It takes two things: a temperature contrast and a spinning planet.

• Contrast. Jet streams form where warm and cold air masses meet. The bigger the temperature difference between them, the stronger the wind.²¹
• Spin. The ground at the equator moves east at more than 1,000 mph (1,600 km/h) as Earth turns. At the poles it does not move at all. Air heading toward a pole keeps its eastward momentum while the ground beneath it slows down, so the air ends up outrunning the ground. That is a wind from the west.²

Because the boundaries between hot and cold air are sharpest in winter, jet streams are strongest in winter, in each hemisphere's own winter.² They also move with the seasons. By northern summer the polar jet typically sits near the U.S.-Canada border; in winter it shifts south and speeds up.²

Two jets in each hemisphere

Earth has four main jet streams: a polar jet and a subtropical jet on each side of the equator.¹

The subtropical jet is the higher of the two because both jets sit just under the tropopause, the lid of the weather layer, and that lid is higher in the tropics than near the poles.⁴ Heights vary from day to day and from source to source: the National Weather Service gives four to eight miles for jet streams in general, NOAA's satellite service five to nine, and the UK Met Office five to seven.²¹⁵

Waves: ridges and troughs

The jet stream does not circle the globe in a neat ring. It meanders in giant north-south waves called Rossby waves, or planetary waves, named for Carl Rossby, who identified them in the 1930s. There are usually four or five around the hemisphere at once, and each is 3,700 to 5,000 miles (6,000 to 8,000 km) long or more.³

A bulge toward the pole is a ridge. A dip toward the equator is a trough. The waves do real work: they carry heat toward the poles and cold air toward the equator. They also move slowly, and sometimes stall or even drift backward, which is why a weather pattern can sit over you for a week.³

Where you sit in the wave matters. Most stormy weather happens between a trough and the ridge to its east. Fair weather is found between a ridge and the trough to its east.⁶

Blocking: when the pattern jams

Sometimes highs and lows arrange themselves so that nothing can move through. These blocking patterns can stay in place for several days and bring a long run of the same weather.⁶

• Omega block. Two cutoff lows with a blocking high sandwiched between them, shaped like the Greek letter Ω. It is cool and wet under the lows, warm and clear under the high.⁶
• Rex block. A high sitting on the polar side of a low.⁶

Blocking highs are responsible for major heat waves, typically in summer.⁶ See heat domes.

Jet streaks

The wind is not the same speed all along a jet. A jet streak is a pocket of the fastest wind inside it, and jet streaks matter because they help surface storms form and deepen.⁸⁷ The UK Met Office compares the jet stream to a vacuum cleaner: it draws air out of the top of a low-pressure system, which lowers the pressure and makes the storm stronger.⁵

What it does to your week

• It marks the boundary. The jet plays a major role in separating warm and cool air masses, so which side of it you are on goes a long way toward setting your temperature.⁹
• It steers storms. Storms from the Pacific ride it across the United States.⁹
• It changes your flight time. A flight heading east along the jet is generally faster, and uses less fuel, than the same route flown west.¹⁵

The jet stream is not the polar vortex

People often mix the two up, but they are in completely separate layers of the atmosphere. The polar jet stream is in the troposphere, about 5 to 9 miles up. The polar vortex is a band of westerly wind in the stratosphere, about 10 to 30 miles above the pole.¹⁰

They do influence each other. When the vortex is especially strong, the polar jet tends to stay farther north and run straighter. When the vortex weakens, shifts or splits, the jet often becomes extremely wavy, letting warm air flood into the Arctic and polar air sink south.¹⁰ Forecasters track this with the Arctic Oscillation: in its negative phase the jet dips into the mid-latitudes and brings cold-air outbreaks.¹¹

More on the polar vortex

How El Niño and La Niña move it

The tropical Pacific pushes on the jet stream too. During El Niño the Pacific jet moves south and reaches farther east, then runs nearly straight across the southern United States. During La Niña it moves north and weakens over the eastern Pacific, then takes a very wave-like path across North America, with dips that carry cold air into the north.¹²¹³ That change in both position and shape is the main reason El Niño and La Niña winters differ across North America. See the typical winter patterns, and why a jet that moves south warms the north.

Who found it

The Japanese meteorologist Wasaburo Ooishi observed these winds over Japan in the 1920s. His winter wind profile is the first known evidence of the persistent strong westerlies that would later be called the jet stream. He published his observatory's first report in 1926, in Esperanto. The wider research effort came later, after aircraft ran into the same winds during World War II.¹⁴

Last updated November 12, 2026