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Jet Stream: Meaning and Definitions
The jet stream refers to continuous, meandering air currents in the upper troposphere, situated between 6,000 meters to 12,000 meters above the Earth’s surface. These air currents flow persistently from west to east throughout the year, in between 20° latitudes and poles in both the hemispheres.
According to the National Oceanic and Atmospheric Administration, ‘Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere, typically occurring around 30,000 feet (9,100 meters) in elevation’.
Kendrew coined the term “jet stream” to describe the powerful air belt of high-level westerly winds located in both hemispheres. Trewartha characterizes the jet stream as a narrow belt of air with an exceptional speed ranging from 320 to 480 kilometers per hour in the upper atmosphere. He views the jet stream as a substantial wind current moving swiftly from west to east.
The World Meteorological Organization provides a comprehensive definition of the jet stream, describing it as an almost parallel, flat, tube-like stream flowing near the tropospheric boundary. The axis of the jet stream aligns with the direction of maximum velocity, characterized not only by tremendous wind speed but also by transverse air deformation. Typically, the jet stream spans thousands of kilometers in length, hundreds of kilometers in width, and several kilometers in height. The minimum wind velocity at any point on its axis is 30 kilometers per second.
How was the Jet Stream Discovered?
The credit for uncovering these high-altitude wind currents goes to the pilots of American bomber aircraft during World War II. When American pilots operated ‘B-29’ bombers at high altitudes towards Japan across the Pacific Ocean, they encountered robust wind currents that impeded the speed of their planes, causing a noticeable slowdown and making it challenging for the planes to advance.
Upon returning towards America, these planes experienced an increase in their speed. Prompted by reports of this unexpected phenomenon, meteorologists initiated investigations into the upper layers of the atmosphere. Consequently, the existence of swift wind currents was revealed, and they were subsequently named the ‘Jet Stream.’ In their fully developed state, these wind currents indeed resemble powerful ‘jets.’ That is why they are known as Jet Stream.
Characteristics of the Jet Stream
- The boundaries of the wind flow under the jet stream are distinct, creating a relatively calm atmosphere on both sides.
- According to Streller, the cross-sectional profile of the jet stream can be likened to a water stream in a hose, with maximum velocity at the center and a gradual decrease away from the center.
- These wind currents flowing from west to east in both hemispheres are located at an altitude of 6000 to 9000 meters from the surface.
- During winter, the jet stream’s position moves closer to the equator, extending up to 20° latitude. The velocity of these wind currents varies seasonally, nearly doubling in winter compared to summer.
- Meteorologists noted that the velocity of these wind currents is more variable than their direction. The average speed in their central part is 65 kilometers per hour in winter and only 24 kilometers per hour in summer. Near the axis of the jet stream, the wind speed reaches more than 480 kilometers per hour.
- The highest average velocity of jet streams occurs over subtropical high-pressure areas on the surface.
- Sometimes, the jet stream crosses the tropopause, entering the lower part of the stratosphere. This intrusion brings water vapor into the typically dry and cloudless stratosphere, resulting in the occasional appearance of cirrus clouds in its lower section.
- In the Northern Hemisphere, when the wind velocity exceeds the air pressure gradient in the jet stream, high-velocity wind is displaced to its right, known as subgradient wind. This accumulation of air on the right bank of the jet stream contributes significantly to the establishment of high air mass areas on the equatorial edge of the westerly wind belt, a process known as anticyclogenesis.
- The jet stream’s flow in the upper part of the troposphere exhibits highly variable air velocity, posing challenges in weather forecasting. Additionally, the wind velocity is uneven along different parts of its axis, with the highest wind speed found in the winter season along the eastern coastal region of Asia, the southeastern part of the United States, North Africa, and the central parts of the Indian Ocean.
- The temperature gradient across the jet stream is steep, resulting in extremely cold air at its polar edge and extremely hot air at the equatorial edge. While the jet stream generally flows parallel to latitude lines, it often generates long waves from north to south, causing serpentine isobar lines.
- As the waves of the jet stream move from west to east, significant changes occur in their shape and extension, with their power gradually weakening.
- Fully developed waves spread over tropical regions, facilitating the exchange of cold and warm air masses. Occasionally, waves spreading towards the equator separate from the mainstream, forming “cold high-level cyclones” from the cold air mass surrounded by the warm air mass. Conversely, waves moving towards the poles and breaking give rise to “high-level anticyclones.”
Index Cycle of the Jet Stream
Irregular changes are often observed in the wind flow system associated with the jet stream, but these changes also exhibit a degree of systematicity. Jet stream, which flows parallel to latitude lines shows weekly variations in the wind velocity. Sometimes, the wind speed may drop to 50 percent of the normal speed, and regional or short-term changes can even triple the wind speed.
The contraction and expansion of the vortices of high-level westerlies lead to alterations in the position and shape of the jet stream. The period encompassing these changes is termed the index cycle, and each cycle typically takes 4 to 6 weeks to complete. The four stages of this index cycle are illustrated below:
First Stage
In this stage, the jet stream is positioned closer to the poles, known as high zonal index (as depicted in figure a). Jet streams typically flow straight from west to east during this phase. In the Northern Hemisphere, the polar cold air mass is to the north of this stream, while the warm air mass is generally found to the south. Westerly winds at the Earth’s surface shift towards the poles, and limited cyclonic activity occurs in high latitudes, facilitating the exchange of air masses between high and low latitudes.
Second Stage
The amplitude of the jet stream waves increases in the second stage (as shown in figure b). Consequently, the jet stream moves towards the equator. This increase in meanders brings cold polar air closer to the equator and tropical air closer to the poles.
Third Stage
Further increasing meanders of waves bring the jet stream even closer to the equator in the third stage (as depicted in figure c). This situation results in a significant displacement of cold and hot air masses. The temperature gradient is no longer in the north-south direction but shifts to the east-west direction.
Fourth Stage
The fourth stage commences with the disintegration of the northward and southward propagating waves of the jet stream (as shown in figure d). Due to the excessive expansion of meanders of waves, they separate from the original stream. This condition, known as low zonal index, features cold polar air in the low latitude upper atmosphere and warm air in high latitudes, surrounded by winds of opposite properties.
In this situation, the temperature gradient in the air masses is observed from east to west. The westerly wind, instead of being tropical, becomes divided into cells. Occasionally, weather patterns become extremely variable, with temperatures in Alaska surpassing those in Florida. This phenomenon is attributed to the presence of hot air masses to the north (in the Northern Hemisphere) and cold air masses to the south in the high-level atmosphere.
