Jet stream

Jet streams are narrow, fast-moving bands of thermal wind that flow through the upper levels of the Earth’s atmosphere. Typically situated near the altitude of the tropopause, they blow from west to east and exert a significant influence on global weather patterns, aviation, and climate dynamics. Both hemispheres possess distinct polar and subtropical jet streams, each associated with particular atmospheric circulation cells and temperature gradients. Jet streams are also known to occur on other planets, including the gas giants and Venus, indicating their fundamental role in planetary atmospheric behaviour.

Structure and Characteristics

Jet streams form as a result of large-scale temperature contrasts in the atmosphere, combined with the Coriolis effect produced by the Earth’s rotation. The polar jet stream emerges near the boundary between the polar and Ferrel cells, where strong horizontal temperature gradients prevail, while the subtropical jet forms higher up at the interface between the Ferrel and Hadley cells.
The polar jet is typically found at altitudes around the 250 hPa pressure level, roughly 33,700 feet above sea level. The subtropical jet occurs at slightly higher altitudes and is generally weaker. These jets are continuous over long distances yet frequently exhibit discontinuities. Their paths are rarely straight; instead, they follow a meandering pattern shaped by large atmospheric waves known as Rossby waves, which arise from variations in the Coriolis force with latitude. Smaller-scale shortwaves can develop within these patterns, further shaping the trajectory of the flow.
Jet streams commonly have widths of several hundred kilometres and vertical thicknesses of less than a few kilometres. Their speeds vary widely, often reaching well over 100 knots, particularly in regions where temperature gradients between air masses are strongest. The northern polar jet frequently migrates northwards in summer and southwards in winter, consistent with seasonal shifts in solar heating.
The behaviour of jet streams is dynamic: they may split, merge, reverse direction locally, or dissipate altogether. Their variability governs the distribution and intensity of cyclonic systems, making them a critical component of weather forecasting. For example, persistent shifts in the jet stream’s latitude can lead to unusual weather patterns such as extended droughts or severe flooding.

Discovery and Early Theoretical Development

Recognition of high-altitude fast-moving winds predates modern meteorology. In the nineteenth century, the American scientist Elias Loomis hypothesised the existence of a strong upper-air current to explain storm movement across the United States. The eruption of Krakatoa in 1883 offered further indirect evidence: observers noted the rapid global spread of volcanic dust, leading to the identification of the so-called “equatorial smoke stream”.
Throughout the late nineteenth century, researchers such as Sereno E. Bishop, Rollo Russell, and others documented the speed and global extent of high-altitude haze, linking it to upper atmospheric circulation. These studies provided valuable observational groundwork for understanding jet stream behaviour.
In the 1920s, Japanese meteorologist Wasaburō Oishi made the first systematic measurements of what is now recognised as the jet stream. Using pilot balloons near Mount Fuji, Oishi recorded consistent high-speed winds at altitude. His findings, published in Esperanto, remained relatively unknown outside Japan but represent the earliest scientific documentation of the jet stream.
Further anecdotal evidence emerged from high-altitude aviation. American aviator Wiley Post noted unexpectedly high ground speeds during transcontinental flights in the 1930s while wearing an early pressurised suit. Independently, German meteorologist Heinrich Seilkopf coined the term Strahlströmung in 1939, advancing formal recognition of the phenomenon.
A fuller understanding developed during the Second World War. Pilots flying between North America and Europe reported strong westerly tailwinds, and meteorologists in the Pacific observed intense upper-level winds affecting aircraft routes. These extensive wartime observations confirmed the jet stream as a consistent and influential atmospheric feature.

Atmospheric Dynamics and Variability

Jet streams develop predominantly due to two interacting factors: differential atmospheric heating and the Coriolis effect. Solar radiation creates distinct circulation cells—Hadley, Ferrel, and Polar—which divide the atmosphere into latitudinal zones. Where these cells meet, sharp temperature contrasts occur, generating pressure gradients and strong westerly winds.
The steepest gradients occur around the mid-latitudes, leading to the formation of the polar jet stream. This jet plays a major role in steering mid-latitude weather systems, including extratropical cyclones. Its meandering pattern influences storm tracks and regional climate anomalies. A southward displacement of the jet stream can bring cooler, wetter conditions to regions such as the British Isles, as was observed during the notable flooding events of 2007 and 2012.
The subtropical jet forms closer to 30 degrees latitude and is generally found at higher altitudes. Though weaker than its polar counterpart, it remains vital in transporting moisture and influencing subtropical weather patterns.
The El Niño–Southern Oscillation exerts particular influence on jet stream behaviour. During El Niño events, the subtropical jet in the Pacific often strengthens and shifts, affecting rainfall across the tropics and subtropics and altering weather patterns across North America and beyond. Conversely, La Niña events can weaken or shift the jet, producing opposite climatic impacts.
Jet streams also exhibit significant transient features. Embedded shortwaves can trigger or amplify storm systems. Splitting or merging jets can result in complex weather patterns, influencing everything from temperature distribution to precipitation belts.

Applications and Human Interaction

Knowledge of jet streams has practical importance in numerous fields. In meteorology, locating the polar and subtropical jets aids in forecast accuracy, particularly in predicting storm development, temperature changes, and large-scale atmospheric movements.
In aviation, pilots use jet streams to reduce fuel consumption and shorten flight times when travelling eastwards, while westbound flights may seek routes that avoid strong headwinds. These optimised flight paths form an integral part of airline route planning.
Scientific research has also considered the possibility of harnessing jet streams for renewable energy generation, although significant engineering challenges and environmental concerns remain. Historically, during the Second World War, Japan utilised the Pacific jet stream to transport balloon bombs (known as Fu-Go weapons) to North America in an early attempt to exploit atmospheric winds for long-distance delivery.
Jet streams are not unique to Earth. Observations of the atmospheres of Jupiter, Saturn, Uranus, Neptune, and Venus have revealed analogous high-speed bands, demonstrating that such features are common in planetary atmospheres with strong rotation and temperature gradients.