Jet streams are an important factor in weather forecasting and have a significant impact on aviation and maritime operations. In this article, we will discuss the jet stream in detail and its significance for the UPSC Civil Services exam.

What are Jet Streams?

• Jet streams are fast-flowing, narrow bands of air currents that exist in the atmosphere above the earth’s surface.
• These are characterized by high-speed winds that flow from west to east, generally at altitudes above 30,000 feet (9,144 meters). They are a part of upper westerlies
• The flow of Jet streams often keeps shifting in latitude, i.e. towards north and south and they also experience changes in altitude

Properties of Jet Streams

Jet Streams form in upper layers of the Troposphere

• Jet streams flow in the upper layers of the Earth’s atmosphere, specifically in the troposphere.
• Jet streams are formed in upper layers of the troposphere, at transitions between the circulation cells, .i.e. Polar, Ferrel and Hadley circulation cells. The Coriolis force further drives the jet streams.
• The troposphere is the lowest layer of the atmosphere and is characterized by decreasing temperature with increasing altitude
• Jet streams typically occur at altitudes between 10 and 15 kms above the Earth’s surface, where the temperature gradient is strongest and the winds are most intense.

Width of Jet Streams

• Horizontal width of a jet stream is typically a few hundred kilometers and the vertical thickness is usually less than 5 kilometers
• Factors affecting the width of jet streams
  • Altitude
  • Wind Strength
  • Seasons
• At higher altitudes
  • Width tends to be narrower
  • The strongest and most well-defined jet streams occur in the upper troposphere(30,000-40,000 feet)
  • Width can be as narrow as a few hundred kilometers
  • This narrow width is due to the strong horizontal temperature gradient that exists at high altitudes, which drives the formation and maintenance of the jet stream
• At lower altitudes
  • At lower altitudes width tends to be broader. This is because the temperature gradient is weaker at lower altitudes, and the winds are not strong
  • Jet streams in lower troposphere may have width of several thousand kilometers
• Wind Strength
  • Stronger winds tend to create a narrower and more well-defined jet stream, while weaker winds may create a broader and less well-defined jet stream
• Season
  • In the Northern Hemisphere, during winters the jet stream tends to be narrower and more well-defined as the temperature gradient between the poles and equator is strongest
  • In summers, the temperature gradient weakens, and the jet stream may become broader and more meandering

Altitude

• Jet streams are located at altitudes above 30,000 feet, in upper layers of the troposphere
• The exact altitude of a jet stream can vary depending on various factors such as the time of year, geographical location, and atmospheric conditions
• Jet streams at higher altitudes are less affected by surface friction and local topography, hence they are more consistent in their speed and direction
• Polar jet streams, which are the strongest and most well-known, are typically located between 30,000 and 50,000 feet above sea level
• Subtropical jet streams, are typically found at slightly lower altitudes compared to polar jet streams between 25,000 and 35,000 feet

High speeds

• On average, jet streams can have speeds ranging from 50 to 200 miles per hour (80 to 322 kilometers per hour).
• Their speeds usually range from 129 to 225 kilometers per hour
• Factors affecting speed of Jet streams are:
  • Temperature gradient
  • Position of high and low-pressure systems
  • Location in atmosphere
• Temperature Gradient: When the temperature gradient is large, the jet stream tends to be stronger and faster. The greater the temperature difference, the stronger the pressure gradient and the faster the wind speed within the jet stream.
• Position of High and Low-Pressure Systems: High-pressure systems tend to create a blocking effect on the jet stream, causing it to accelerate and increase in speed. Low-pressure systems can slow down the jet stream, causing it to weaken.
• Location in the Atmosphere: The higher altitude allows for less friction and disturbance from surface features, allowing the winds to flow more freely and at higher speeds.

Circumpolar in Nature

• Jet streams are circumpolar, which means that they flow in a circular pattern around the polar regions of the earth

Meandering Path

• Jet streams are not perfectly straight but instead, they move in a wavy pattern. These waves are known as Rossby waves, named after the meteorologist who first discovered them
• Rossby waves are caused by the difference in temperature between the polar and equatorial regions and the Coriolis force.
• These meanders themselves propagate eastward, at lower speeds than that of the actual wind 
• As jet streams move in a meandering path, they can bring different weather conditions to different regions. The peaks of the waves are associated with high-pressure systems, while the troughs of the waves are associated with low-pressure systems. As a result, the meandering jet stream can cause weather patterns such as storms and heat waves
• Due to Climate change as the Arctic is warming up faster than the rest of the planet, the temperature difference between the polar and equatorial regions may decrease, which could weaken the jet stream and cause it to meander more. This could have significant implications for weather patterns and could lead to more extreme weather conditions in certain regions

Geostrophic in nature

• Jet streams are geostrophic in nature because their formation and maintenance rely on the balance between the pressure gradient force and the Coriolis force
• Geostrophic winds refer to the horizontal wind movement that occurs when the Coriolis force and pressure gradient force are balanced.
• Pressure gradient force
  • It results from the difference in pressure between two locations.
  • It acts from high-pressure areas to low-pressure areas and tends to cause air to move in the direction of decreasing pressure.
• The Coriolis force
  • It is a deflection force caused by the rotation of the Earth. It causes moving objects in the atmosphere, including air masses, to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• The pressure gradient force in case of Jet streams
  • It is primarily generated by the temperature contrast between the equatorial and polar regions.
  • The warmer air in the equatorial regions rises, creating low-pressure areas, while the colder air in the polar regions sinks, creating high-pressure areas.
  • This temperature difference sets up a strong pressure gradient force.
• The Coriolis force in case of Jet streams
  • It acts perpendicular to the direction of motion and is proportional to the speed
  • As the air moves from high-pressure areas to low-pressure areas along the pressure gradient, the Coriolis force acts to the right in the Northern Hemisphere, resulting in a deflection of the air to the east.
  • This deflection continues until the Coriolis force and the pressure gradient force balance each other, resulting in a geostrophic wind flow.
• In the case of jet streams, the geostrophic balance between the pressure gradient force and the Coriolis force results in the formation of high-speed winds flowing from west to east in the upper atmosphere. These winds form a narrow band known as the jet stream.

Jet streams are a part of Upper-Level Westerlies

• The Westerlies refer to prevailing winds that flow from west to east in the middle latitudes of both hemispheres
• The jet stream is a narrow band within the upper-level westerlies where the wind speeds are particularly high. It is like a river of air within the larger flow of the westerlies
• Jet streams are usually located between the troposphere and the stratosphere, at altitudes ranging from about 30,000 to 50,000 feet

Strongest in Winters

• Jet streams follow the boundaries between hot and cold air. Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the northern and southern hemisphere in winters

Formation of Jet Streams

• The formation of jet streams is a complex process that involves various factors such as temperature, pressure, and the rotation of the earth.
• Formation
  • Jet streams are formed due to the difference in temperature between polar and equatorial regions.
  • The equatorial regions receive more solar radiation and hence have a higher temperature, while the polar regions receive less solar radiation and hence have a lower temperature.
  • The difference in temperature creates a pressure difference, and air moves from high-pressure areas to low-pressure areas. This movement of air creates a wind system.
  • The earth’s rotation causes a Coriolis force, which deflects moving objects to the right in the northern hemisphere and to the left in the southern hemisphere.
  • As a result, the wind system is deflected towards the east in the northern hemisphere and towards the west in the southern hemisphere.
  • The combination of the pressure difference and the Coriolis force results in the formation of the jet stream
• Why is the wind in the upper atmosphere strongest in regions around 30° N/S and 50°-60° N/S ?
  • Due to the formation of circulation cells, .i.e. Polar, Ferrel and Hadley cells, the regions around 30° N/S and 50°-60° N/S are areas which experience the maximum temperature change/contrast.
  • Due to this temperature difference between the two locations the strength of the wind increases and hence, wind in the upper atmosphere are the strongest in these regions
• Other Factors affecting the formation, shape and position of Jet Streams
  • Location of high and low pressure systems
  • Location of warm and cold air
  • Seasonal changes
  • Movement of Sun and shifting of ITCZ
  • Strength of the jet stream itself

Types of Jet Streams

Polar Jet Stream

• The polar jet stream is a high-altitude wind current that flows from west to east in the upper troposphere and lower stratosphere.
• It is one of the two major jet streams, the other being the subtropical jet stream.
• The polar jet stream is located closer to the poles, hence its name
• Formation and Characteristics:
  • Formed due to the temperature contrast between the cold polar air masses and the relatively warmer air near the mid-latitudes.
  • The temperature gradient creates a strong pressure gradient, resulting in the development of high-speed winds within a narrow band.

• The polar jet stream usually forms between 30,000 and 40,000 feet and is a few hundred miles wide, while its vertical thickness can be 5 to 6 kms
• It’s characteristics change with seasons:
  • Winter
    • During winter, the polar jet stream typically becomes stronger and shifts closer to the equator, and generally fluctuates between 30°N and 60°N in the Northern Hemisphere.
    • This is primarily because the temperature difference between the cold polar regions and the warmer mid-latitudes is more pronounced during this season. The stronger temperature gradient leads to a stronger and more pronounced jet stream
  • Summer
    • In summer, the polar jet stream weakens and tends to shift further north and is usually found around 40°N to 60°N in the Northern Hemisphere.
    • This is because the temperature difference between the polar regions and the mid-latitudes decreases during this season. As a result, the jet stream becomes less defined and less powerful
• Influence on Weather Patterns:
  • The polar jet stream’s position and intensity have a significant impact on temperature patterns, precipitation distribution, and the formation of severe weather events.
    1. Temperature Distribution: The polar jet stream helps maintain the boundary between polar air and warmer mid-latitude air. Its position can cause northward or southward shifts in the boundary, resulting in variations in temperature distribution and weather patterns.
    2. Storm Tracks: Low-pressure systems and storms often follow the path of the polar jet stream. The interaction between these weather systems and the jet stream can lead to the formation of intense cyclones, frontal systems, and precipitation patterns.
    3. Air Masses: The polar jet stream separates cold polar air masses from warmer air masses, such as tropical or subtropical air. The collision of these air masses along the jet stream can generate weather disturbances, frontal systems, and changes in atmospheric stability.
    4. Severe Weather Events: The polar jet stream can contribute to the development of severe weather events like winter storms, blizzards, and heavy precipitation.
• Polar Jet stream and cyclones
  • The interaction between the polar jet stream and cyclones can be summarized as follows:
    1. Steering Effect: The polar jet stream guides the path of cyclones by exerting a strong influence on their movement. Cyclones tend to move in the direction of the jet stream. In the Northern Hemisphere, cyclones usually move from west to east, following the path of the polar jet stream.
    2. Intensification and Weakening: When a cyclone interacts with the polar jet stream, it can gain energy and intensify. The jet stream provides additional dynamics and moisture, which can enhance the development of a cyclone. On the other hand, if a cyclone moves away from the jet stream or encounters a weaker jet stream, it may weaken or dissipate.
    3. Frontal Systems: Cyclones often develop along fronts. The interaction of warm and cold air masses along the front, combined with the influence of the jet stream, creates favorable conditions for the intensification of cyclones.
    4. Jet Streaks: Within the polar jet stream, there are regions of enhanced wind speed known as jet streaks. These jet streaks are areas of maximum wind speed within the jet stream. The presence of a strong jet streak can provide additional energy and dynamics to a cyclone, leading to its intensification

Subtropical Jet Stream

• The subtropical jet stream is primarily formed due to the temperature contrast between the tropics and subtropics.
• The equatorial regions receive more solar radiation, resulting in higher temperatures compared to the subtropical regions. This temperature gradient leads to variations in atmospheric pressure, causing air to flow from the subtropics to the equatorial regions
• Factors influencing the strength and position:
  1. Temperature Gradient: A steeper temperature gradient between the tropical and subtropical regions, tends to result in a stronger jet stream.
  2. Seasonal Variation: It tends to shift northward in the summer and southward in the winter due to the migration of the thermal equator
  3. Topography: Mountains can act as barriers, causing the jet stream to either split or strengthen
• Behaviour of subtropical jet stream during various seasons:
  1. Winter (December to February)
     • It shifts southward, closer to the equator
     • This southward shift is mainly influenced by the cooling of the northern hemisphere and the resultant decrease in temperature gradients
     • It is generally found between 20 to 30 degrees latitude in the northern hemisphere
  2. Spring (March to May)
     • It gradually starts to shift northward
     • This is due to the increasing solar radiation and the resultant warming of the northern hemisphere
     • It is usually around 30 degrees latitude
  3. Summer (June to August)
     • During summer, the subtropical jet stream is generally at its weakest.
     • This is because the temperature gradients between the equator and the poles are relatively small during this season.
     • It may meander and exhibit a more variable behavior.
     • In the northern hemisphere, it tends to retreat even further northward, often near the 40-degree latitude.
  4. Autumn (September to November)
     • In autumn, as the solar radiation decreases and the northern hemisphere starts to cool again, the subtropical jet stream gradually shifts back toward the south
     • Similar to the spring season, it is usually around 30-degree latitude mark
• Effects and Significance:
  1. Weather Systems: The subtropical jet stream influences the formation and movement of weather systems, such as cyclones, storms, and hurricanes. as it can transport moisture and energy across different regions, thereby influencing the development and intensity of these weather systems.
  2. Aviation: Pilots often take advantage of the tailwinds associated with the jet stream to reduce flight duration and save fuel.
  3. Climate Patterns: Changes in its position and strength can lead to variations in temperature, precipitation patterns, and extreme weather events. It is also closely connected to the Indian and African summer monsoons 
  4. Agriculture: Changes in precipitation patterns and water availability can impact crop growth and irrigation

Effects of ENSO events on Jet Streams

ENSO (El Niño-Southern Oscillation) events can have significant effects on jet streams:

1. El Niño weakens the intensity of polar and subtropical jet streams, making them shift southward
   • The polar jet stream tends to shift southward, causing changes in storm tracks and precipitation patterns in many regions. It’s position shows a dip in the Eastern Pacific.
   • A very strong El Niño will cause the trough to shift further south with position moving into Southern California. This can lead to increased rainfall in the southern United States, rainfall in California can be significantly above normal, leading to flash floods
   • Droughts can be experienced in Indonesia and Australia

2. La Niña strengthens the polar and subtropical jet streams, causing them to shift northward.
   • This can result in altered storm tracks and precipitation patterns, including increased rainfall in the western Pacific, enhanced snowfall in the Northwestern United States, and increased storm activity in the Atlantic basin.
3. La Niña can lead to increased tropical cyclone activity in the western Pacific and the Atlantic basin

These ENSO weather interactions are discussed in detail in Chapter 23 : ENSO Events

Significance of Jet Streams

Jet streams have several significant implications and impacts:

1. Weather Forecasting
   • Jet streams influence the movement and development of weather systems, such as high and low-pressure systems, fronts, and storms
   • Their position, strength, and direction is used to predict weather patterns, track storms, and determine the potential for severe weather events
2. Aviation Operations
   • Pilots can take advantage of jet streams by flying within or near them to benefit from the high-speed tailwinds they provide.
   • Tailwinds increase the ground speed of aircraft, resulting in shorter flight durations, reduced fuel consumption, and improved efficiency
   • The polar jet stream, which is strongest during winter months, can significantly affect flight durations between North America and Europe
3. Maritime Operations
   • Similar to aviation, maritime operators can take advantage of favorable tailwinds within the jet stream to reduce travel time and fuel consumption

Practice MCQs for UPSC Prelims Exam

Answer Key

1. a) Difference in air pressure
2. a) Troposphere
3. b) West to east
4. c) They are only found in the polar regions
5. a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
6. b) Subtropical jet stream
7. b) Increasing altitude
8. b) Increased fuel consumption
9. a) Rotation of the Earth
10. a) Climate change
11. c) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
12. b) Polar regions
13. a) A jet stream that occurs during the winter season in polar regions
14. b) Monsoons
15. a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.

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Sumit Teotia

Sumit is Creator and Author at learncrisp.com. He started this learning platform with a mission to emerge as an aspirants resource hub, especially for those preparing in self study mode. He loves writing and his interests are History, Security and Diplomacy.

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