The Jet Stream

A river of wind in the sky

High above your head, far higher than the tallest storm clouds, narrow ribbons of air race around the planet at astonishing speed. These are the jet streams — fast-flowing currents of wind near the top of the troposphere, roughly 9 to 12 km up, the same height where passenger jets cruise. Their cores can blow at over 200 km/h, and sometimes much faster.

Jet streams were discovered partly by accident. During the Second World War, high-flying bomber crews found themselves mysteriously slowed almost to a standstill when flying west, and pushed along far faster when flying east. They had stumbled into these powerful, invisible rivers of air. Today the jet stream is one of the most important features in weather forecasting.

What creates a jet stream?

Jet streams are born from a simple cause: a large temperature difference. Near the equator, the air is warm; near the poles, it is bitterly cold. Where a mass of warm air meets a mass of cold air, there is a steep change in temperature over a short distance.

That temperature contrast creates a steep difference in air pressure high up in the atmosphere. As you learned in Air Pressure and Wind, air rushes from high pressure toward low pressure, and a steeper difference makes a stronger wind. At the boundary between warm and cold air, this pressure difference is enormous, so the high-altitude wind becomes extremely fast.

Finally, the Earth's rotation bends this flowing air through the Coriolis effect, turning it so that the jet streams sweep mainly from west to east around the globe.

Where the jet streams are

Each hemisphere has two main jet streams:

• The polar jet stream, the stronger one, sits where cold polar air meets milder mid-latitude air. This is the jet that most affects the weather of places like northern Europe, Canada and the northern United States.
• The subtropical jet stream, weaker and higher, flows nearer the tropics.

Because the temperature contrast is greatest in winter, jet streams are strongest and fly furthest toward the equator in the cold months, and weaken and retreat poleward in summer.

How the jet stream rules your weather

The jet stream matters because it acts like a conveyor belt and steering wheel for weather systems. The depressions, fronts and storms you read about in Weather Fronts and Weather Maps form and travel along the jet stream. Its exact position largely decides your weather:

• If the jet stream lies to the north of you, you usually sit in warmer, more settled air.
• If it lies to the south of you, you are in the colder air, and storms tracking along the jet may pass right over you, bringing wind and rain.

A small shift in the jet's path can be the difference between a dry, sunny week and a stormy, soaking one.

Waves and "stuck" weather

The jet stream rarely flows in a straight line. It develops giant north-south meanders called Rossby waves, like the bends of a river. Usually these waves drift eastward, moving weather systems along so no single type of weather lasts too long.

But sometimes the waves grow very large and stall. When the jet stream gets "stuck" in a fixed wavy pattern (a situation called blocking), the same weather can sit over a region for days or weeks:

• A northward loop can trap a dome of hot air, causing a prolonged heatwave and drought.
• A southward loop can drag polar air far south, causing an unusual cold snap.
• A stalled system can dump rain on one area for days, causing flooding.

Many of the most damaging weather extremes are linked to a jammed jet stream. This is why scientists watch its behaviour so closely, especially as a warming Arctic may be changing how the jet behaves.

Real examples you can spot

• Flight times. A flight from New York to London (eastward, with the jet) is often an hour or more shorter than the return trip (westward, against the jet). Airlines deliberately route planes to ride the jet stream.
• Contrails and high cloud. Fast-moving high cirrus clouds streaming across the sky often mark where the jet stream is flowing overhead.

Activity: track the jet stream

Try this weather-watch investigation over two weeks.

1. Each day, find an online jet stream map (many weather services and forecasting sites publish them) and sketch roughly where the polar jet lies relative to your location — to your north or south, straight or wavy.
2. Each day, also record your actual weather: temperature, wind and whether it was wet or dry.
3. After two weeks, compare. Did warmer, calmer days happen when the jet was to your north? Did wet, windy days come when the jet swung overhead or to your south? Did any "stuck" wavy pattern bring a run of similar days?

You will see for yourself how this invisible high-altitude river quietly steers the weather at the ground. To explore another huge driver of global weather patterns, read El Niño and Ocean Currents.