Jet stream

Jet streams can reach speeds of over 300 km/h and act like rivers of air, guiding weather systems across the globe. They are not continuous bands but rather meandering flows that can change shape, split, or merge. 

Their influence on weather is enormous, affecting storm paths, temperature patterns, and precipitation around the world.

Why jet streams form: The role of temperature and pressure

The root cause of jet streams lies in the uneven heating of Earth’s surface. The equator receives more direct sunlight year-round than the poles, causing warm air to rise near the equator and cold air to sink near the poles. This difference creates strong horizontal temperature gradients, especially in the upper atmosphere. 

As warm air rises and moves toward the poles, it encounters cooler air. The pressure gradient force causes air to move from high to low pressure, while the Coriolis effect—caused by Earth's rotation—deflects that moving air. The result is a high-speed westerly wind current in the upper troposphere: the jet stream.

The three atmospheric circulation cells

To understand where and why jet streams form, it's helpful to look at the structure of Earth's atmospheric circulation, which is divided into three main cells in each hemisphere:

1. Hadley cell (0° to ~30° latitude) Warm, moist air rises at the equator, moves poleward at high altitudes, and descends around 30° latitude. This circulation drives tropical climates and the formation of subtropical high-pressure zones.
   The Subtropical Jet Stream typically forms near the upper boundary of the Hadley cell, around 30° latitude, and can influence weather in the subtropics and southern parts of the mid-latitudes.
2. Ferrel cell (~30° to 60° latitude) This is an indirect circulation zone where surface winds flow poleward and high-altitude winds move equatorward. It acts as a mixing zone between the tropical and polar air masses.
   The Polar Front Jet Stream, often just called the polar jet, forms at the boundary between the Ferrel and Polar cells. It is generally stronger and more variable than the subtropical jet.
3. Polar cell (60° to 90° latitude) Cold air sinks at the poles and moves toward lower latitudes near the surface. Rising air at around 60° completes this loop. While the polar cell plays a smaller role in jet stream formation, its cold air is essential in maintaining the strength of the polar front and jet stream.

The Coriolis effect: Why jet streams flow west to east

The Coriolis effect is caused by Earth’s rotation. As air moves north or south, it is deflected due to the different rotational speeds at different latitudes. In the Northern Hemisphere, this deflection is to the right; in the Southern Hemisphere, it is to the left. 

This deflection turns what would otherwise be a direct north–south flow into a west–east one, creating the westerly flow characteristic of jet streams. Without the Coriolis effect, jet streams wouldn’t curve—they’d simply move from equator to pole.

Rossby waves: The meandering jet stream

Jet streams do not move in straight lines. Instead, they develop large meanders called Rossby waves, which are planetary-scale undulations in the jet stream. These waves are essential for transferring heat from the tropics to the poles and vice versa. 

Rossby waves are responsible for much of the variability in day-to-day weather. A large ridge (northward bulge) can bring warm, dry conditions to one region, while a deep trough (southward dip) can deliver cold, wet weather to another. When these waves become stationary or slow-moving, they can lead to extreme weather patterns.

Jet streams and extreme weather

Jet streams strongly influence local and regional weather patterns by controlling the movement of high- and low-pressure systems, frontal boundaries, and storm tracks. When jet streams shift, intensify, or stall, the consequences can be severe:

• Droughts can develop when a persistent ridge in the jet stream deflects storm systems away from a region, reducing rainfall over weeks or months.
• Flooding may occur when a stationary trough channels moist air over the same area repeatedly, causing prolonged or intense rainfall.
• Heatwaves are often linked to blocked jet stream patterns that trap warm air under a stagnant high-pressure system.
• Cold snaps can happen when a dip in the polar jet brings Arctic air far south into temperate regions.

In mid-latitudes—like much of North America and Europe—the polar jet stream is a major factor in both daily forecasts and long-term weather trends.

Jet streams and climate change

There is growing evidence that climate change is affecting the behavior of jet streams. As the Arctic warms faster than the rest of the planet—a phenomenon known as Arctic amplification—the temperature gradient between the equator and poles weakens. 

This can reduce the strength of the polar jet stream and increase its tendency to meander and slow down. Such changes may be linked to more frequent and persistent extreme weather events. For example, a weakened jet stream may allow cold air to plunge farther south or let heatwaves linger longer than they did in the past.

A key player in a changing climate

Jet streams are far more than just high-speed currents of air; they are critical conductors in the symphony of Earth's atmosphere. Born from the fundamental imbalance between the warm tropics and cold poles, and sculpted by our planet's rotation, these powerful winds act as vital conduits, transferring energy and guiding the weather systems that shape our world. 

From steering everyday storms to influencing severe droughts, floods, and heatwaves, their meandering paths have profound impacts on human societies and ecosystems. As our climate continues to warm, particularly in the Arctic, the delicate balance that drives these currents is being altered. 

Monitoring and understanding the evolving behavior of jet streams is therefore not just a scientific endeavor, but a crucial part of preparing for and adapting to the atmospheric challenges of the future.