How jet streams work

In this new lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more aboutjet stream and how it works.

How rivers come out of the air

Any wind, i.e. a rapid movement of air, appears between air volumes with different temperatures and density, as cold air always pursues warm air. But at the boundary of air masses - especially large uniform air volumes with big temperature differences - particularly strong winds appear, capable of reaching hundreds of kilometers per hour.

These winds are called jet streams. Jet - because they are relatively narrow: with a length of thousands of kilometers, the width of a 'jet' is a couple of hundred kilometers, and its height is about 5 kilometers.

Jet streams are tracked from space through clouds - in this NASA photo, the stream curves off the east coast of the United States.

Jet streams are very high, constantly branching like rivers and changing in height and speed. The wind bends because of the Earth's movement: instead of simply moving from cold to warm areas, it is deflected by the Coriolis force generated by the rotation of the planet and moves along the boundaries of air masses.

The main jet streams

When jet streams are mentioned, it's usually a reference to polar and subtropical streams that surround the entire globe and always move from west to east.

The polar jet streams are located at an altitude of 9-12 km in a temperate climate zone, at the junction between polar and temperate air masses.
The subtropical jet streams are substantially higher, 10-16 kilometres above the ground, but it is weaker and slower than the polar one.

There are also jet streams of low altitudes — for example, along mountain ranges and at the transition of the mountains to the valley, but they are incomparably weaker than polar and subtropical ones and appear due the terrain.

Polar and subtropical jet streams on Earth. Illustration: Valerya Milovanova / Windy.app

How do people use jet streams

If flying along a jet stream, the aircraft can significantly reduce flight time and save fuel. But if you fly against the jet stream, you can lose speed — that’s why pilots who fly east to west — for example, from Europe to the United States — have to make a detour. This effect is more significant for the polar stream as it is stronger.

Jet streams, however, generate turbulence in a clear sky — when a plane can “shake” heavily without any visible pointers to a turbulent area. Jet streams of low altitudes also cause turbulence, which can be dangerous for small aircraft.

Jet streams are also used to predict the weather, as they help the movement of fronts and cyclones. A strong bending of a polar stream may indicate the formation of a cyclone within a few hours or days, while a smooth bending may indicate warm weather to the south (or north for the Southern Hemisphere).

The Aqua satellite data shows clearly a polar jet stream (blue winding line) and how different the temperature of the Earth or the clouds covering it is in degrees Celsius to the north and south.

Some experts also suggest using jet streams to generate electricity, but so far there is no technology (wind generators are not so easy to locate at an altitude of 10 kilometers), or enough research to prove that it will be profitable and will not harm the climate.

Text: Windy.app team

Illustration: Valerya Milovanova, an illustrator with a degree from the British Higher School of Art an Design (BHSAD) of Universal University

Cover photo: Unsplash