Jet stream
The jet stream refers to narrow bands of strong winds in the upper troposphere, typically flowing from west to east at altitudes around 30,000 feet (9,100 meters), driven by sharp temperature contrasts between polar and equatorial air masses.[1][2] Earth features four primary jet streams: polar jets near the poles and subtropical jets closer to the equator in both hemispheres, with the polar jets generally stronger and more variable due to greater seasonal temperature gradients.[2][3] These winds average about 110 miles per hour (177 km/h) but can exceed 200 mph (320 km/h) under extreme conditions, meandering in wavy patterns influenced by Rossby waves that steer mid-latitude weather systems and storms.[2][4] Jet streams play a critical role in global atmospheric circulation, separating cold polar air from warmer subtropical air, modulating precipitation patterns, and enabling efficient high-altitude flight routes for aviation while posing hazards like turbulence.[1][3] Variations in jet stream position and intensity, often linked to phenomena like El Niño, have been associated with extreme weather events, including heatwaves and cold outbreaks, though causal attributions require careful empirical scrutiny beyond media narratives.[5]Definition and Basic Characteristics
Physical Description
Jet streams consist of narrow bands of strong winds concentrated in the upper troposphere near the tropopause, the boundary between the troposphere and stratosphere, at altitudes generally between 9 and 16 kilometers (approximately 30,000 to 52,000 feet) above sea level.[1][6] These winds predominantly flow from west to east, encircling the globe in both hemispheres, with core velocities often reaching or exceeding 100 meters per second (360 kilometers per hour or 223 miles per hour), though seasonal and regional averages are typically around 40 meters per second.[7][4] The structure of a jet stream features a steep vertical shear, with wind speeds increasing rapidly toward the core and decreasing sharply above and below, often confined within a horizontal width of a few hundred kilometers.[8] This concentration arises from large meridional temperature gradients that enhance geostrophic wind speeds via thermal wind balance, resulting in a ribbon-like flow distinct from broader atmospheric circulation.[7] Jet streams exhibit a wavy, meandering path rather than a straight line, influenced by planetary-scale Rossby waves, but maintain a primarily zonal orientation.[4] In the Northern Hemisphere, the primary polar-front jet stream forms at mid-to-high latitudes around 50° to 60° N, while a subtropical jet occurs nearer 30° latitude; analogous streams exist in the Southern Hemisphere, with strengths peaking in winter due to amplified temperature contrasts.[8][9] The polar jet tends to be stronger and lower in altitude compared to the subtropical counterpart, reflecting sharper baroclinicity at higher latitudes.[8] Observations from weather balloons, aircraft, and satellites confirm these features, with wind maxima often aligning closely with tropopause folds where stratospheric air intrudes into the troposphere.[1]Primary Types and Locations
The primary types of jet streams consist of polar jets and subtropical jets, with one of each type occurring in the Northern Hemisphere and the Southern Hemisphere, forming four major circumpolar currents in the upper troposphere.[2] Polar jets develop along the polar front, the boundary separating cold polar air masses from warmer mid-latitude air, typically positioned between 50° and 60° latitude in both hemispheres.[10] [11] These jets flow predominantly from west to east at altitudes of 9 to 12 kilometers, where wind speeds in the core often average around 110 miles per hour (177 km/h) but can exceed 200 miles per hour (322 km/h) during periods of strong temperature contrasts.[2] [12] Subtropical jets form at the poleward boundary of the Hadley circulation cells, near 30° latitude north and south, at similar altitudes of about 12 to 13 kilometers.[9] [13] These jets also exhibit westerly flow, with core speeds typically ranging from 80 to 140 miles per hour (129 to 225 km/h), though they are generally weaker and more stable than polar jets.[12] [3] Both types exhibit seasonal variability, with polar jets intensifying and shifting equatorward in winter due to greater meridional temperature gradients, while subtropical jets may weaken or displace slightly poleward in summer.[14] Jet streams are defined by sustained wind speeds exceeding 60 knots (69 mph or 111 km/h), distinguishing them from broader upper-level winds.[3]Historical Discovery and Early Observations
Initial Identification
Japanese meteorologist Wasaburo Ooishi conducted the first systematic observations of strong upper-level westerly winds, later identified as the jet stream, from his station in Toya, Japan, between 1923 and 1925.[15] Using pilot balloons released twice daily, Ooishi recorded nearly 1,300 measurements revealing wind speeds exceeding 50 meters per second at altitudes around 10-15 kilometers, far stronger than surface winds and directed predominantly westward to eastward.[16] These findings documented a narrow band of accelerated airflow in the upper troposphere, aligning with modern descriptions of the subtropical jet stream over East Asia.[4] Ooishi's research, published primarily in Japanese and Esperanto to reach an international audience, received minimal attention outside Japan due to linguistic barriers, geopolitical isolation preceding World War II, and the nascent state of upper-air meteorology at the time.[15] Despite this, his data provided empirical evidence of the phenomenon's existence, predating Western recognition by two decades; for instance, Ooishi noted balloons drifting eastward at speeds up to 200 km/h, inconsistent with prevailing low-level winds.[16] Earlier indirect hints, such as high-altitude ash dispersion from the 1883 Krakatoa eruption tracked globally, suggested strong upper winds but lacked the targeted piloting and velocity profiling that characterized Ooishi's approach.[17] The initial identification gained traction in the West during World War II, when U.S. Army Air Forces B-29 pilots encountered unexpected headwinds of over 400 km/h at 10 km altitude during missions over Japan starting in late 1944, delaying flights and revealing the jet stream's operational impact.[18] German meteorologist Heinrich Seilkopf independently reported similar strong westerlies over Europe in 1939-1940 using radiosonde data, describing winds up to 140 m/s near the tropopause.[19] These wartime observations, corroborated by balloon and aircraft reconnaissance, confirmed Ooishi's earlier findings and prompted the coining of the term "jet stream" by U.S. meteorologists in 1947 to denote these fast-moving currents.[16]Key Milestones in Research
The first empirical detection of jet streams occurred through observations conducted by Japanese meteorologist Wasaburo Oishi between 1923 and 1925, who launched nearly 1,300 pilot balloons from sites near Mount Fuji and recorded persistent westerly winds exceeding 200 km/h at altitudes around 9-12 km.[16] These findings, detailed in reports published in Japanese, identified the core characteristics of what would later be termed the jet stream but received limited international recognition due to language barriers and Japan's pre-World War II isolation.[20] During World War II, in November 1944, U.S. Army Air Forces B-29 Superfortress pilots encountered unforeseen headwinds reaching 370 km/h (230 mph) at operational altitudes over the Pacific en route to Japan, causing excessive fuel consumption and mission delays that necessitated detailed meteorological analysis.[15] This operational revelation prompted U.S. and Allied meteorologists, including those under Carl-Gustaf Rossby, to initiate targeted high-altitude reconnaissance flights and radiosonde deployments, confirming the existence of narrow, high-speed wind bands in the upper troposphere.[1] In the late 1940s, Rossby advanced theoretical understanding by characterizing the jet stream's association with long-wave undulations in the westerlies—later known as Rossby waves—and integrating it into models of global atmospheric circulation, as outlined in a 1947 collaborative publication by his University of Chicago team.[21] Systematic global monitoring expanded in the 1950s with routine upper-air observations via weather balloons and aircraft, enabling quantification of jet stream positions, speeds (typically 100-300 km/h), and seasonal migrations, which informed early numerical weather prediction efforts.[22]Fundamental Causes and Formation
Atmospheric Dynamics and First-Principles Mechanisms
Jet streams form primarily due to the meridional temperature gradient arising from differential solar heating, with the equator receiving more insolation than the poles, establishing warmer air masses equatorward and cooler ones poleward. This gradient induces baroclinicity, where density contrasts drive pressure differences that intensify with height via the hydrostatic equation, concentrating isobars aloft and amplifying the pressure gradient force (PGF) in the upper troposphere.[23][24] In response to the enhanced PGF, air accelerates poleward, but Earth's rotation introduces the Coriolis force, which deflects motion to the right in the Northern Hemisphere (left in the Southern), establishing geostrophic balance where Coriolis force opposes PGF, yielding strong, narrow westerly winds parallel to isotherms. The balance is expressed as f v = - (1/ρ) ∂p/∂x for zonal flow, with f the Coriolis parameter (2 Ω sin φ, Ω Earth's angular velocity, φ latitude), resulting in winds directed west-to-east due to the geometry of the gradient.[25][26] The thermal wind relation further elucidates the vertical structure: the increase in westerly wind speed with height (∂u_g/∂z) equals (g / f T) (∂T / ∂y), linking shear directly to the meridional temperature gradient (∂T / ∂y < 0 for poleward cooling), such that stronger gradients yield greater acceleration aloft, often peaking near the tropopause at 200-300 hPa where stability inhibits vertical mixing. This mechanism explains typical jet speeds of 50-100 m/s, sustained by angular momentum conservation as air parcels move equatorward in upper branches of circulation cells, gaining eastward velocity.[24][27][28] These dynamics operate within the framework of large-scale atmospheric circulation, where subtropical jets emerge from angular momentum transport in the Hadley cell's return flow, and polar jets from baroclinic instability at the polar front, but both trace causally to the same temperature-driven PGF-Coriolis interplay without requiring external forcings beyond radiative equilibrium and rotation. Empirical validations from radiosonde data confirm jets align with sharp thermal contrasts, with meridional temperature differences of 20-30°C over 1000 km correlating to core winds exceeding 60 m/s.[29][30]Thermal and Coriolis Force Interactions
Horizontal temperature gradients, particularly the meridional contrast between warmer equatorial regions and colder poles, generate baroclinicity in the atmosphere, where isobaric surfaces intersect isotherms. This baroclinicity tilts pressure surfaces, producing a pressure gradient force (PGF) directed from warmer to colder air masses that intensifies with altitude due to greater thermal expansion in warm air.[31][26] The Coriolis force, a deflection arising from Earth's rotation (parameterized as f = 2 \Omega \sin \phi, where \Omega is Earth's angular velocity and \phi latitude), balances this PGF in geostrophic equilibrium, yielding westerly winds perpendicular to the gradient. In the Northern Hemisphere, the Coriolis force deflects poleward air motion to the right (eastward), concentrating flow into narrow, fast west-to-east streams at upper levels where friction is negligible.[32][26][31] The thermal wind relation quantifies the vertical shear induced by temperature gradients: the change in geostrophic wind with height (\mathbf{V}_T) aligns with isotherms (cold air to the left in the Northern Hemisphere) and magnitude |\mathbf{V}_T| = \frac{g}{f} \frac{\partial \ln \theta}{\partial y} \Delta z, where g is gravity, \theta potential temperature, and y meridional direction. This adds westerly shear atop weaker surface winds, peaking jet speeds near the tropopause—typically 25–100 m/s for polar jets at 9–12 km altitude.[31][26] Latitudinal variation in the Coriolis parameter further confines jets, as stronger f at higher latitudes enhances balance against the PGF.[31][32]Distinctions Between Polar and Subtropical Jets
The polar jet stream and subtropical jet stream represent distinct features of the atmospheric circulation, arising from different thermal gradients and circulation cells. The subtropical jet forms near 30° latitude in both hemispheres, linked to the poleward edge of the Hadley cell where air descends after rising in the tropics, conserving angular momentum to produce westerly winds at upper levels.[1] [13] In contrast, the polar jet emerges along the polar front at approximately 50° to 60° latitude, driven by the steeper equator-to-pole temperature contrast in mid-latitudes between the Ferrel and polar cells, enhancing the meridional shear and geostrophic winds.[1] [7] Altitudinally, both jets occur near the tropopause in the upper troposphere, spanning 9 to 12 kilometers (30,000 to 40,000 feet), but the subtropical jet typically positions at higher levels around the 200 hPa pressure surface, reflecting its tropical origins, while the polar jet aligns closer to 250 hPa, influenced by mid-latitude dynamics.[1] [33] Wind speeds differ markedly: the polar jet routinely achieves 50 to over 100 meters per second (110 to 220 knots), with peaks during winter due to amplified baroclinicity, whereas the subtropical jet sustains lower averages of 30 to 50 meters per second (65 to 110 knots), showing less intensity and more stability.[34] [35] Variability further distinguishes them, as the polar jet exhibits pronounced meridional meanders and latitudinal shifts, often spanning 30° to 70° latitude, modulated by Rossby waves and seasonal cooling, leading to dynamic interactions with surface weather.[34] The subtropical jet, by comparison, remains more zonally oriented and positionally fixed, with reduced waviness, persisting year-round but weakening in summer as Hadley cell intensity diminishes.[13] [34] These differences stem from causal mechanisms: the subtropical jet's momentum-driven formation yields consistency, while the polar jet's baroclinic instability fosters variability, as evidenced by observational data from radiosondes and satellite measurements since the mid-20th century.[1] [33]| Characteristic | Polar Jet Stream | Subtropical Jet Stream |
|---|---|---|
| Latitude | 50°–60° | ~30° |
| Primary Driver | Mid-latitude temperature gradient | Hadley cell descent and angular momentum |
| Typical Altitude | ~250 hPa (9–10 km) | ~200 hPa (10–12 km) |
| Wind Speed Range | 50–100+ m/s, highly variable | 30–50 m/s, more constant |
| Variability | High (meanders, seasonal shifts) | Low (zonally stable) |
| Seasonal Strength | Peaks in winter | Year-round, weaker in summer |