Jet Streams - Climatology

What are jet streams?

Jet streams are powerful, fast moving ribbons of wind that circle the Earth from west to east. This west to east flow is called a westerly circulation. They sit just above the top of the troposphere, in a narrow band only a few hundred kilometres wide, at a height of around 12,000 metres.

Jet streams form at the margins of the three large atmospheric circulation cells, because those margins are the boundary between hot and cold air masses. These boundaries keep shifting north and south through the year, because the Earth is tilted and different parts of its surface receive different amounts of solar radiation across the seasons.

The most familiar use of jet streams is in air travel, where flying with the flow or against it can change flight time a great deal. To save both time and fuel, airlines often try to fly with the jet stream.

What causes jet streams?

Jet streams on Earth are the result of two forces working together:

1. Differential solar heating: the Sun heats the atmosphere unevenly, and this sets up the large scale Polar, Ferrel and Hadley circulation cells.
2. The Coriolis force: as the planet spins on its axis, the Coriolis force bends the moving air and fixes the direction in which it flows.

On other planets, jet streams are driven mainly by internal heat rather than by solar heating.

How jet streams were discovered

In the 19th century, weather observers tracked volcanic smoke and readings from pilot balloons to work out the wind patterns high in the atmosphere. Jet streams were understood far more clearly during the Second World War, when American bomber crews flying west towards Japan met a strong airflow pushing against them from the opposite direction, flowing from west to east.

Properties of a jet stream

• Tropospheric westerly flow: they flow from west to east at a height of about 7.5 to 14 kilometres, in a narrow strip only a few hundred kilometres wide.
• Thousands of kilometres long: jet streams can be thousands of kilometres long and hundreds of kilometres wide, but only a few kilometres deep, roughly 2 to 4 kilometres.
• Found around both poles: because they wind around the poles in both hemispheres, jet streams are sometimes called circumpolar whirls.
• Wavy, meandering path: their path is wavy rather than straight, partly because the ITCZ shifts over time.
• Changing speed: their wind speed changes with the seasons, and they become especially strong in winter, when the pressure difference is sharp.

Why are jet streams faster in winter?

Jet streams sit at the boundary between hot and cold air masses. The temperature difference between those masses is greater in winter than in summer, and a bigger temperature gap means a stronger, faster jet stream. Their top speed ranges from about 110 kilometres per hour in summer to about 184 kilometres per hour in winter.

Types of jet streams

Jet streams are grouped by where they form:

• Polar front jet stream: forms over the sub polar low pressure belt, where cold polar air meets warmer sub tropical air. It is a strong westerly jet, flowing from west to east.
• Sub tropical westerly jet stream: found above about 30 to 35 degrees latitude, on the poleward side of the sub tropical high pressure belt. It flows steadily from west to east.
• Sub tropical easterly jet stream: develops in summer in the upper troposphere over the Indian subcontinent. It flows from east to west and helps bring the monsoon winds to India.
• Polar night jet stream: also called the sub polar stratospheric jet. It appears mainly in winter, driven by a steep temperature gradient in the stratosphere near the poles, at a height of about 30 kilometres.

Jet streams and the Indian monsoon

The sub tropical jet stream (STJ) matters a great deal for India, because it can both hold back the monsoon and help it arrive. The polar jet has little effect here, since it shapes weather close to the poles rather than in the tropics.

The upper air over India is dominated by the sub tropical westerly jet streams, which blow near the northern edge of the subcontinent at about 27 to 30 degrees latitude.

In summer, the northern hemisphere receives more solar energy, and these sub tropical westerly jet streams shift north of the Himalayas. At the same time, a sub tropical easterly jet stream flows over peninsular India, roughly north of 14 degrees North.

For the rest of the year, and especially in winter, the sub tropical westerly jet stream shifts back to the south of the Himalayas. It carries western cyclonic disturbances in from the Mediterranean, and these storms deliver winter rain to the north and north west of India.

Tropical cyclones, by contrast, form during the monsoon season and again in October and November, as part of the easterly flow. They strike the eastern coastal regions, including West Bengal, Odisha, Andhra Pradesh and Tamil Nadu.

Why it still matters

Jet streams are not just a topic for the exam hall. They are one of the reasons the weather has felt so unusual in the last few years, and scientists are watching them closely right now.

Take flying. A team led by researchers at the University of Reading found that severe clear air turbulence over the North Atlantic, one of the busiest flight corridors in the world, rose by about 55 per cent between 1979 and 2020, from roughly 17.7 hours a year to 27.4 hours. Their explanation points straight at the jet stream: as the planet warms, the wind shear inside the jet stream is growing stronger, and that instability is what shakes an aircraft in otherwise clear skies.

The jet stream also steers our day to day weather. NOAA notes that a jet stream travels from west to east at about 180 kilometres per hour on average, and can race past 400 kilometres per hour when the temperature contrast is sharp. When a weather system drifts away from the jet and gets parked in one place, the result can be a heatwave that will not break or rain that will not stop. A wavier, more sluggish jet stream can make this stuck weather more common, which is one reason researchers are studying the link between jet stream behaviour and the run of extreme summers and cold snaps in recent years.

So the same band of wind that decides whether your flight lands early also helps decide whether a monsoon arrives on time or a European city bakes under a heat dome. If you want to see how this one idea connects to climate, oceans and the atmosphere, explore the wider Learnacy Hub, read our other geography study notes, or browse all of our study notes.

Sources

1. University of Reading, Aviation turbulence strengthened as the world warmed (2023), reporting Prosser, Williams and colleagues in Geophysical Research Letters.
2. NOAA NESDIS, What is the jet stream?

Key takeaways

• Jet streams are narrow, fast-moving bands of wind that flow from west to east high in the atmosphere, driven by uneven solar heating and the Earth's rotation.
• They develop at the borders of major atmospheric circulation cells where warm and cold air masses collide, and their positions shift as the seasons change.
• The speed of a jet stream increases significantly during the winter because the temperature contrast between the colliding air masses is much sharper.
• Different types of jet streams exist based on their location and altitude, including polar front, sub-tropical, and stratospheric polar night jet streams.
• Sub-tropical jet streams play a major role in India's climate by influencing the arrival of the summer monsoon and delivering winter rainfall to the north.

Test yourself

What two main forces cause jet streams to form on Earth?

Differential solar heating and the Coriolis force.

Why do jet streams reach higher speeds during the winter?

The temperature difference between hot and cold air masses is much greater in winter, which creates a stronger flow.

How do commercial airlines take advantage of jet streams?

Airlines often plan flight paths to fly with the jet stream, which saves both travel time and fuel.

Frequently asked questions

What exactly is a jet stream and where are they located?

Jet streams are powerful, fast-moving ribbons of wind that circle the Earth from west to east, located just above the troposphere at a height of around 12,000 metres. They form in a narrow band only a few hundred kilometres wide.

How do jet streams affect air travel?

Flying with the jet stream can significantly reduce flight time and fuel consumption, while flying against it can increase both. Airlines often plan routes to take advantage of these winds.

Why do jet streams become stronger in winter?

Jet streams are faster in winter because the temperature difference between hot and cold air masses is greater during this season, leading to stronger winds. Their top speed ranges from about 110 km/h in summer to 184 km/h in winter.

What is the difference between the Polar front jet stream and the Sub-tropical easterly jet stream?

The Polar front jet stream forms over the sub-polar low-pressure belt and flows west to east, while the Sub-tropical easterly jet stream develops in summer over the Indian subcontinent and flows east to west, aiding the monsoon winds.