• Jet streams are high-speed winds that occur in narrow bands of upper air westerlies.
• The width of this airband can be 160-480km wide and 900-2150m thick, with core speed exceeding 300km/hr.
• Jets are coincident with major breaks in the tropopause.
• Jet Streams develop where air masses of differing temperatures meet.
• The greater the difference in temperature, the faster the wind velocity inside the jet stream.
• Jet Streams extend from 20 degrees latitude to the poles in both hemispheres.
• 

Geostrophic Wind

• Geostrophic flow is a theoretical wind resulting from exact balance between Coriolis force and pressure gradient force.
• Geostrophic wind is the velocity and direction of the wind as a net result of wind generating forces.
• Winds in upper atmosphere (2-3 km above surface) are free from surface friction and controlled by pressure gradient and Coriolis force.
• Air parcel initially moves from high to low pressure due to Pressure Gradient Force (PGF).
• Coriolis force deflects moving air parcel to right in northern hemisphere (left in southern hemisphere).
• Deflection increases as wind gains speed until Coriolis force equals pressure gradient force.
• At this point, wind blows parallel to isobars (perpendicular to Pressure Gradient Force).
• Wind is called geostrophic wind when it blows parallel to isobars.
• 

Why winds don’t flow from tropical high pressure (in upper troposphere) to polar low (in upper troposphere) directly as shown in the figure below?

• Because these winds are geostrophic, i.e., they flow at great speeds due to low friction and are subjected to greater Coriolis force.
• So they deflect greatly giving rise to three distinct cells called Hadley cell, Ferrel Cell, and Polar cell.
• Instead of one big cell (as shown in fig) we have three small cells that combinedly produce the same eff

Genesis of Jet Streams

• The genesis of the Jet-streams is provided by three kinds of gradients:
  • The thermal gradient between pole and equator
  • The pressure gradient between pole and equator
  • The pressure gradient between surface and subsurface air over the poles.

Characteristics of Jet Stream

• High velocity winds- 400-500km/hr. High velocity is due to great thermal contrast creating powerful pressure gradient force.
• Meandering- jet streams encircle the globe, thus follow a curved path. Flow is 3 dimensional and develop crests and trough
• They cover hundreds of km in width and thousands of km in length.
• size and dimension- width-10-12km
• depth-2-3 km
• length-3000km
• altitude – below the tropopause
• They have seasonal variations and shift with the apparent movement of the sun
• Jet streams travel from west to east in both hemispheres

Types of jet Streams

• Polar front jet streams
• Subtropical Westerly Jet streams
• Tropical easterly Jet streams
• Polar night Jet Streams
• Local Jet Streams

Permanent jet streams – subtropical jets at lower latitudes and polar front jets at mid latitudes.

Temporary jet streams – Tropical Easterly Jet or African Easterly Jet ,and Somali Jet (southwesterly).

• Polar Front Jet Streams: formed above convergence zone of polar cold air mass and tropical warm air mass, irregular easterly movement.
• Subtropical Westerly Jet Streams: formed above 30-35 latitude, move north of subtropical surface high-pressure belt in upper troposphere, also called stratospheric subpolar jet streams.
• Tropical Easterly Jet Streams: develop in upper troposphere above surface easterly trade winds over India and Africa during summer due to intense heating of Tibetan plateau, important for Indian Monsoon.
• Polar Night Jet Streams: develop in winter season due to steep temperature gradient in stratosphere around poles.
• Local Jet Streams: formed locally due to thermal and dynamic conditions, limited local importance.

Index cycle of jet streams

• Stage 1:
  • Cold air from poles and warm air from subtropics converge along a horizontal line
  • They do not mix and create a stationary zone
• Stage 2:
  • Cold polar air is pushed by easterlies and warm air is pushed by westerlies
  • Stationary situation transforms into an oscillating wave, known as Rossby waves
• Stage 3:
  • Cold and warm air further invades each other’s territory, and waves meander
  • Jet streams of high sinuosity develop and attain maturity
• Stage 4:
  • Cold air mass moves into warm air and latitudinal heat exchange occurs
  • Stationary front situation is reattained.
• 

Significance of jet streams

• The close relationship between the intensity of Mid-latitude cyclones and jet streams. The cyclones become very strong and stormy when the upper air tropospheric jet streams are positioned above temperate cyclones
• The monsoon of South Asia is largely affected and controlled by jet streams

Influencing factors on the Jet Stream Flow

• Factors influencing the flow of the jet stream are landmasses and the Coriolis effect.
• Landmasses interrupt the flow of the jet stream through friction and temperature differences.
• The spinning nature of the earth accentuates these changes.
• The jet stream meanders across the earth, like a river meanders before it reaches the sea.
• Meandering sections of the jet stream continue to change as they interact with landmasses.
• This creates an ever-changing state of flux and subsequent temperature differences.
• In winter, the temperature of the stratosphere can also affect the strength and position of the jet stream.
• The cooler the polar stratosphere, the stronger the polar/tropical differential becomes; encouraging the jet stream to gain in strength.
• The warmth of the landmasses and oceans (such as the El Nino Southern Oscillation) can also have a bearing on the strength and amplitude of the jet stream.

Jet streams & the weather

• Jet streams separate colder air and warmer air, which affects weather patterns
• Jet streams move weather systems around and can cause them to stall if they move too far away
• Changes in the jet streams are linked to global warming, especially the polar jet streams
• Warmer jet streams cause more extreme ups and downs, leading to unexpected weather patterns in certain areas
• When jet streams dip south, they bring colder air masses with them.

Air travel

• Jet streams play a significant role in air travel.
• Eastbound flights take less time than westbound flights because of the assistance from fast-moving air.
• Jet streams can contain wind shear, which is a sudden and violent change in wind direction and speed.
• Wind shear is a significant threat to air travel, causing planes to suddenly lose altitude and putting them in danger of crashing.
• In 1988, the FAA mandated that all commercial aircraft must have wind-shear warning systems.
• However, it took until 1996 for all airlines to have these systems on-board.

Jet Streams affecting the Monsoons and the Indian Sub Continent

• The Subtropical Jet Stream (STJ) and countering easterly jet are important for the climate and monsoons of India.
• Increased solar heating of the Indian subcontinent forms a cyclonic monsoon cell between the Indian Ocean and southern Asia.
• The STJ blocks the development of summer monsoons by blowing to the south of the Himalayas.
• During summer, the STJ shifts northwards and crosses over the Himalayan Range, allowing the subcontinental monsoon cell to develop quickly.
• The lower level tropical jet stream brings warm and moisture-laden air masses from the Indian ocean into the monsoon cell.
• The mountainous terrain of north India cools and compresses the air, causing excess moisture to be dissipated in the form of monsoon rains.
• The end of the monsoon season is brought about by the cooling atmosphere over the Tibetan Plateau, allowing the STJ to transition back across the Himalayas.
• This leads to the formation of a cyclonic winter monsoon cell characterized by sinking air masses over India and relatively moisture-free winds that blow seaward, bringing settled and dry weather over India during the winter months.