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The Jet Stream is a geostrophic wind that blows horizontally through the upper troposphere from west to east at an altitude of 20,000 to 50,000 feet. Jet streams form when air masses of varying temperatures collide. As a result, where the Jet Stream forms are typically determined by surface temperatures. The speed of the wind inside the jet stream increases with the magnitude of the temperature difference. Jet streams run from 20 degrees latitude to the poles in both hemispheres.
In the 1940s, during the Second World War, high-flying aircraft encountered upper winds of very great velocity. These are known to be concentrated bands of rapid air movement, which are termed Jet Streams.
Read More: Types of Winds
Jet Streams Formation
At high elevations near the top of the troposphere, jet streams are thin bands. In the summer, they travel at a pace of roughly 110 km/h (kmph), whereas in the winter, they go at more than 180 kmph. They have a round form. Radially outward, the speed of the Jet Streams diminishes. They are a few hundred kilometres wide and between two and five kilometres deep. The direction of a jet stream’s flow is not always straight. Their circulation path is wavy and meandering.
These meandering winds are called Rossby waves. Rossby waves dip and rise in altitude/latitude. They split at times and form eddies. They even disappear completely to appear somewhere else.
Jet streams “follow the sun”. This means that as the sun’s elevation increases each day in the spring, the average latitude of the jet stream shifts poleward. The sun’s elevation decreases as autumn approaches and the jet stream’s average latitude moves toward the equator. The tropopause is occasionally broken through by the jet stream. The lower stratosphere is next reached.
Jet streams help a certain quantity of water vapour get to the lower stratosphere, where there are sporadic cirrus clouds. The impact of the jet stream can occasionally be felt as low as 3 km above the earth’s surface. The strength of the jet stream varies along its length in a clearly visible way. The eastern Atlantic and Pacific Oceans experience the lowest jet stream winds and the highest wind speeds throughout the winter. In the summer, the strongest jet is located around the Canadian border and the Mediterranean Sea.
Read More: List of Major Local Winds
Jet Streams Characteristics
- The thermal contrast of air cells like Hadley cells and Ferrel cells is a factor in their origin.
- The ‘Rossby Wave’ refers to the Jet Stream’s meandering or whirl movement.
- Because of the southern shift of the pressure belts, the equatorial extension of the Jet Stream is greater in winter.
- During the winter, thermal contrast increases, as does the intensity of the high-pressure centre at the pole. It accelerates the formation of Jet Streams, as well as their extension and velocity.
Read More: Pressure Belts
Jet Stream Climatic Significance
- Jet Streams occasionally bring moisture to the stratosphere, resulting in the formation of Noctilucent clouds (tenuous cloudlike phenomena in the upper atmosphere which are made of ice crystals visible in a deep twilight.)
- It is important in the onset and withdrawal of monsoon winds.
- It is known to have introduced some ozone-depleting substances into the stratosphere, resulting in ozone layer depletion.
- In its movement, the crust and trough formation intensifies alternative cyclonic and anticyclonic conditions.
- When the air mass moves, it experiences alternate expansion and compression, which causes it to be associated with alternate high and low pressure.
Read More: Atmospheric Circulation
Types of Jet Streams
Each hemisphere has two zones with a persistent jet stream. One is a subtropical jet stream, and another is a polar front jet stream. Another jet stream circulates periodically close to the equator.
1. Polar Front Jet Stream
It originated because of temperature differences. It is associated with the polar front zone in each hemisphere and meanders more than the SubTropical Jet Stream. Further, it extends between 40-degree and 60-degree latitudes in both Hemispheres. It is found at a height between 6 km and 9 km in the atmosphere. It swings towards the poles in summer and towards the equator in winter. When moving to the south, it takes very cold air to subtropical regions.
2. Subtropical Jet Stream
It runs between 25 degrees and 30 degrees latitudes in both hemispheres. It blows constantly. It has a lower speed than polar jet streams. The air currents arising near the equator descend at 30 degrees N and S latitudes. A part of these air currents blows as SubTropical Jet streams. Further, it swings to the north of the Himalayas in summer in North India and plays a significant role in the monsoons.
3. Eastern Tropical Jet Stream
The Tropical Easterly Jet Stream is a seasonal Jet Stream. It blows between the equator and 20 degrees N latitude during the South-West Monsoon in summer over southeast Asia, India and Africa. In comparison to the other two jet streams, its direction is the opposite. It runs in the eastern direction. It is located comparatively at a higher height between 14km and 16km. Its speed is around 180 km per hour.
Read More: Atmospheric Pressure
Jet Streams Diagram
A sustained jet stream exists in two zones in each hemisphere. A polar front jet stream and a subtropical jet stream are two examples. Near the equator, another jet stream recurs periodically. Here is an illustration of the Types of Jet streams.
Read More: Chemical Weathering
Jet Streams Index Cycle
There are four stages of the Index Cycle of Jet Streams.
|Stage 1||A convergence between the cold air in the poles and warm air from subtropics in the subpolar low-pressure belt is formed. The thermal contrast and differences in the physical properties restrict the masses from mixing up. Thus, a zone of stationery is created between these two air masses. The path of jet streams is straight.|
|Stage 2||The cold polar air mass is pushed by the easterlies and warm air is pushed by the westerlies. The straight path gradually gets wavy and meanders, leading to the formation of curvy waves. These curvy waves are known as Rossby waves.|
|Stage 3||The cold and warm air masses further invade each other’s territory and the waves further meander. The jet stream fully meanders and is positioned near the equator. Now the pressure gradient is east to west. There is a displacement of tropical air mass to the poles and the polar air mass to the tropical areas.|
|Stage 4||The cut-off of the meanders of the jet stream from the main path occurs due to very high meandering circulation and gives rise to several cellular circulations of cyclonic and anticyclonic patterns.|
Index Cycle of Jet Streams Diagram
There are four stages of the Index Cycle of Jet Streams. Below is an illustration of the Index Cycle of Jet Streams:
Read More: Biological Weathering
Jet Streams Consequences
Jet streams have a significant impact on the weather because they direct the movement of high and low-pressure systems while also preventing the movement of upper-level moisture and energy. Jet streams do not usually move in straight lines; their paths are typically meandering.
Jet Streams & their Influence on Weather Conditions
Jet streams severely affect weather conditions. They substantially contribute to originating cyclones, anticyclones, storms and depressions and influence their behaviour. The cyclones intensify when the jet streams are positioned above temperate cyclones. These jet streams are still being investigated with respect to their effect on weather conditions.
Jet Streams and Air travel
If the weather is not disturbed the aeroplanes running in parallel directions gain great speed and considerably save fuel. Sometimes aeroplanes cannot be flown in the opposite direction. The violent wind shear within the jet streams poses a major threat to air travel. The east-flying flights take less flying time than the west-flying flights.
Jet Streams & their Influence on the Indian Monsoon
The bursting of the Monsoon in India is closely related to Eastern Tropical Jet streams. In summer the subtropical jet streams tend to deflect northwards and cross the Himalayan Range. The altitude of the mountains hinders the jet streams but once cleared, results in the advent of monsoon. This is explained in detail in the monsoon chapter.
Read More: Mass Movement
Jet Streams Important Facts for UPSC
- The westerly Jet Stream is a cold wind that pushes downwind to the surface, creating high pressure.
- The dry winds from this high-pressure area (northwestern India) begin to blow towards the low-pressure area (Bay of Bengal).
- These winds, in turn, bring cold waves in the northern part of the country, including UP and Bihar, during the winter.
- After reaching the Bay of Bengal, westerly winds become northeast monsoons due to the influence of Ferrel’s cell.
- When this wind reaches the coast of Tamil Nadu, the humidity from the Bay of Bengal causes rain.