What is Jet Stream?- UPSC NOTES

What is Jet Stream?

  • The Jet Stream is a geostrophic wind blowing horizontally through the upper layers of the troposphere, generally from west to east, at an altitude of 20,000 – 50,000 feet.
  • Jet Streams develop where air masses of differing temperatures meet. So, usually surface temperatures determine where the Jet Stream will form.
  • Greater the difference in temperature, the faster is the wind velocity inside the jet stream.
  • Jet Streams extend from 20 degrees latitude to the poles in both hemispheres.

Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. The winds blow from west to east in jet streams but the flow often shifts to the north and south. Jet streams are like rivers of wind high above in the atmosphere. These slim strips of strong winds have a huge influence on climate, as they can push air masses around and affect weather patterns.

The jet streams on Earth typically run from west to east, and their width is relatively narrow compared to their length. Jet streams are typically active at 20,000 feet to 50,000 feet, above the surface and travel in what is known as the troposphere of Earth’s multi-layered atmosphere.

What are the causes of Jet Stream?

The jet streams are strongest under the tropopause. If two air masses of different temperatures or densities meet the resulting pressure difference caused by the density difference is highest within the transition zone.

  • Undercutting sub-tropical air masses by polar air masses leads to the formation of polar jet streams.
  • This leads to surface low pressure and altitude high pressure. The absence of friction at high altitude allows free response of air to the low pressure at high altitude.

Discovery of Jet Streams

  • Meteorologists came to know about the existence of jet stream during the second world war when American Fighter jets were obstructed by the flow of the Jetstream during their Bombing mission over Japan. Although previously also, the study of high-speed cirrus cloud (high altitude cloud, found at a height greater than 6000 M) pointed out the existence of high-velocity wind in the upper troposphere in as early as in 1904.
  • They are called Jet streams based upon the analogy of water jet.

Characteristics of the Jet Streams

  • Jet streams follow curved and meandering path and such path is because of Coriolis effect and due to the tendency of large scale system to conserve the angular momentum.
  • The height of Jetstream varies and in general, their height decreases from the equator towards poles since the height of troposphere decreases from the equator towards poles.
  • They are embedded in the upper air westerlies or in the core of westerlies where the maximum velocity is found since away from core there is increased wind shearing.
  • Their length profile varies from 1600-3200KM.
  • The high velocity is due to intense pressure gradient and thermal contrast.
  • Jet streams are characterised by great seasonal variations as during colder part of the year they migrate towards the equator and their velocity also gets increased while during summer the wind speed of the Jetstreams is reduced to approximately half of what it is in the winter season and they reach far into tropics.

Development of the Jet Streams

  • The temperature gradient from the equator towards the poles, the surface high pressure at the poles and genesis of the circumpolar whirl above the poles caused by tropospheric low pressure are the major causes of the origin of the Jet Streams.
  • Due to the subsidence of the cool air over the Arctic region, the surface high pressure is intensified while upper air low pressure develops in the upper troposphere.
  • Because of this phenomenon a cyclonic system of air circulation, whose direction is from west to east in the form of whirl develops around upper tropospheric low pressure and the equatorward meandering part of this upper air circulation is called Jet Stream.

Types of Jet Streams

On the basis of locational aspect jet streams are divided into 5 types

1.Polar Front Jet Stream

2.Subtropical westerly Jet Stream

3.Tropical Easterly Jet Stream

4.Polar Night Jet Stream

5.Findlater/ Cross equatorial Jet Stream

Polar Front Jet Stream

  • This jet stream was discovered by Rossby and also known as the primary jet stream.
  • It has an average speed of 215km/hour and in some cases, the speed may become double of the average speed.
  • Length profile of these Jets is in the range of 1600-3200 km and depth is around 1-2 km.
  • Sometimes, Polar front Jetstream encircles the entire globe and there is diurnal variation in the position and speed of this Jet Stream.
  • Polar front jet streams are associated with Polar Fronts. As Polar cold airmass and warm tropical airmass begin to converge upon the subpolar lows, the warm air is lifted above the cold air and it tires to enter the polar region, while the cold air gradually pushes towards the warm air. The rising warm air due to geostrophic effect becomes westerly and known as Rossby wave. And Polar Front Jet are embedded in the Rossby waves (jet stream lies at the core of westerlies which has the highest speed).

Influence on weather

  • They have an important role in the management of latitudinal heat budget of the earth and it accomplishes it through the creation of various cyclones and anticyclones in the Middle-Latitude.
  • It plays a very important role in the distribution of precipitation in the middle latitude.
  • Cyclogenesis in the Temperate region is associated with the crest of Polar Front jet and maximum cyclonic rainfall occurs ahead of the trough.
  • At the same time, these jets also determine the path and movement of Temperate Cyclone.

Effect of Jet Stream on India

  • It is believed that the southern branch of the jet stream in northern hemisphere exercises an important influence on the winter weather in India.
  • The western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months, originate over the Mediterranean Sea and are brought into India by the westerly jet stream.
  • The Sub-Tropical Jet stream plays a significant role in both hindering the monsoon winds as well as in quick onset of Indian monsoons.
  • The burst of monsoons depends upon the upper air circulation which is dominated by Sub Tropical Jet Streams (STJ).
  •  During summer, there is a presence of easterly jet streams. An easterly jet stream flows over the southern part of the Peninsula in June and has a maximum speed of 90 km per. In August, it is confined to 15oN latitude, and in September up to 22o N latitudes. The easterlies normally do not extend to the north of 30o N latitude in the upper atmosphere.
  • The easterly jet stream steers the tropical depressions into India. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent. The tracks of these depressions are the areas of highest rainfall in India. The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.
  • At times, Jet Streams bring about some moisture to the stratosphere, leading to the formation of Noctilucent clouds (tenuous cloudlike phenomena in the upper atmosphere which are made of ice crystals visible in a deep twilight)
  • Known to have brought some ozone depleting substances to stratosphere which result in ozone layer depletion.

Subtropical westerly Jet Stream

  • They originate at the meeting point of Hadley and Ferrel Cell. The immense temperature contrast between two air masses produce a sudden change in the pressure gradient, thus sudden change in the velocity.
  • On account of permanency of the Hadley cell, this Jetstream remains intense throughout the year although, with the shifting of the pressure belt, its path also shifts North-South.

Influence on the weather

  • It plays an important role in the distribution of precipitation over the subtropical latitude.
  • It has a major role in bringing western disturbances to India during the winter season and thus provides crucial rainfall which is vital for Rabi Crop.
  • The trough of the subtropical jet stream experiences high pressure and thus associated with the Tropical Deserts; and below the crest, the low-pressure system (cyclogenesis) may develop.
  • The contrast between Hadley and Ferrel cell become greater over the continents hence as the jet enters above the western coast of the continents, they suddenly accelerate, and it causes the dynamic sinking of air and due to this reason tropical desert has high pressure.

Tropical Easterly Jet Stream

  • It is located at an altitude of 10-12 km above the Gangetic Plain and roughly parallel to the 250 North Latitude.
  • It originates due to excessive heating of Tibetan Plateau during the summer season due to which low pressure(converging) sets up at the surface and relatively higher pressure(diverging) in the upper troposphere. The part of this southward diverging air comes under the impact of Coriolis effect and it is forced to deflect from east to west leading to the formation of a tropical Jetstream.
  • One of the divergent limbs of Tropical Easterly Jetstream falls over the North -East India and which due to Coriolis deflection becomes Easterly.

Influence on Weather

  • TEJ steers the low-pressure depression found over the Bay of Bengal into the Indian Subcontinent. Due to this, much of the rainfall is received in the Indo-Gangetic Plain and Central India.
  • It has a relationship with oscillation and shifting of ITCZ (Inter-Tropical Convergence Zone) and Monsoon Trough.
  • During the onset of winter, it suddenly disappears while its arrival heralds the onset of the South West Monsoon.

Polar Night Jet Stream/ Stratospheric Jet Stream

  • This Jetstream is found during winter above poles at an altitude ranging from 40-50km.
  • They are called polar nights as jet streams are formed in winter nights of 6 months at poles. It is found due to the thermal contrast between sub-polar layers and polar layers, and become westerly due to Coriolis force.
  • This jet has been found in the stratosphere little below the maximum concentration of ozone thus also known as Stratospheric Jet Stream.

Influence on Weather

  • Polar Night Jet Stream help in increasing the intensity of Polar Anticyclone.

Findlater/ Cross equatorial Jet Stream/Somali Jet Stream

  • Formed due to the unequal heating of continent of Africa and located at a very low height of 3-4 km, since the warm air above the Mozambique Channel produces a horizontal thermal contrast with the cold air from the African Highlands.
  • It also crosses the equator and becomes south-westerly in direction and due to this reason, it is known as Cross Equatorial Jet Stream.

Influence on Weather

  • Helps in the intensification of S-W Monsoon over Indian Subcontinent.

Uses of Jet Stream

  • Jet Stream has its uses in aviation. It can be safely said that it is a crucial component. The jet stream around the northern polar regions is the most important one. It is also used for weather forecasting.
  • The first commercial use of jet streams was on 18 November 1952 when a Pan American flight flew from Honolulu to Tokyo at an altitude of 7600 metres. The usual time to complete the journey was at 18 hours. WIth the use of jet streams it was cut down to 11.5 hrs. By cutting down on the flight time, jet streams help in cutting down costs for the airline industry.
  • A phenomenon known as clear-air-turbulence (CAT) is often associated with jet streams, it is caused by vertical and horizontal wind shear. The CAT is at its strongest on the cold air side of the jet and just under the axis of the jet. This phenomenon can cause aircraft to lose control and plummet and does present a safety hazard for passengers.

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