VSAT 2022

What is Jet Stream?

Jet streams are strong currents of air moving with a great speed at higher altitudes. These air currents are very long and narrow and contain a large mass of air. These jet streams can be found in the northern hemisphere as well as in the southern hemisphere. They move in a direction from west to east and can batter the top of Mount Everest. The jet stream also has a major effect on the weather of a region. 


Here, we will learn about jet streams and the mechanism behind jet stream formation. To understand what a jet stream is, it is important to have a clear idea about the air circulation in the atmosphere and the effect of temperature on the density of air.


Different Layers of Atmosphere

The atmosphere has several layers containing gases like nitrogen, oxygen, carbon dioxide, etc., surrounding the earth’s surface. The atmosphere of the earth extends up to around 800 km. This may vary depending on the different regions of the earth and its temperature. The atmosphere is divided into five layers. They are troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The boundary of each layer of the atmosphere is not defined sharply. It is because the extension of these layers of the atmosphere varies depending on different seasons and latitudes. 


The troposphere is the layer closest to the earth’s surface. The height of the troposphere is around 8 - 14.5 kilometers and all the weather conditions occur in this layer. The height of the troposphere also depends on the latitude. The height of the troposphere is maximum at the equator and the value of height is around 20 kilometers. The height of the troposphere is minimum at the poles and the height is around 6 kilometers. In summer, the height of the troposphere is more compared to the winter season. The formation of clouds happens in the troposphere. The temperature of the troposphere decreases with increasing altitude.


The solar radiation from the sun reaches the earth’s atmosphere and reaches the troposphere. The earth absorbs this radiation from the sun and gets heated up and radiates this heat which is absorbed by the atmosphere of the earth. The greenhouse gases in the atmosphere such as carbon dioxide, water vapour, and methane trap this heat radiated by the earth. This effect known as the greenhouse effect increases the temperature of the atmosphere of the earth.


The layer above the troposphere is the stratosphere and it extends from a height of around 10 km to a height of 50 km. The stratosphere contains little amount of water vapour. That is why very few clouds are formed in the stratosphere. The ozone layer which protects the earth from harmful ultraviolet radiation is present in the stratosphere. Unlike the troposphere, the temperature increases with increasing altitude. Since there is no turbulence of the air in this layer, commercial airplanes fly in the lower parts of the stratosphere. The boundary between the troposphere and stratosphere is called tropopause.


More About Jet Stream

Now that we have seen the different layers of the atmosphere, let us understand what is jet stream? Jet streams are narrow strong winds that move in a meandering manner at higher altitudes. The jet streams travel at an altitude of 8-11 km in tropopause which is the junction between the troposphere and stratosphere. The height at which the jet stream travels depends on the height of tropopause which varies depending on the latitude and seasons. The jet stream speed ranges from 129 to 225 kilometers per hour and it can travel at a maximum speed of more than 440 kilometers per hour. In the winter season, the temperature difference between regions of the earth such as polar regions, tropical regions is greater. Therefore, jet streams can travel in the winter season. The jet streams are hundreds of kilometers long and a few hundreds of kilometers wide. The jet stream direction is from west to east. These jet streams are so strong that even aircraft can travel easily with the help of jet streams when the aircraft is flying in the west to east direction. The aircraft travels above the jet stream or avoids the jet stream when it is travelling in the opposite direction of the jet stream.


Formation of Jet Streams

The air above the earth’s surface in the equatorial region is warmer than the air in the colder air. This is because the equatorial region receives the sunlight directly than the polar regions. The density of warmer air is less than the density of cold air. Therefore, the air in the equatorial region gets heated up and moves up and reaches the tropopause, and travels towards the polar regions. Also, the cold air in the polar regions moves towards the equatorial region. So, the warm air moving towards the polar region and the cold air moving towards the equatorial region meet each other and air mass is formed. Due to the Coriolis effect of the earth’s rotation, the air mass moves with greater speed in the west to east direction.


Types of Jet Stream

The two major types of jet streams formed most of the time are subtropical jet streams and polar jet streams. These subtropical and polar jet streams form in both the northern and southern hemispheres. The subtropical jet stream is situated at around 30° latitude and the position of the polar jet stream varies between temperate and polar regions. The polar jet stream is stronger than the subtropical jet stream because the Coriolis effect increases with an increase in altitude. The polar jet stream may circumvent the earth sometimes. The winter monsoon in India is related to the subtropical westerly jet stream.


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Now let us look at the two local jet streams. They are tropical easterly jet streams and the Somali jet stream. The tropical easterly jet stream travels at a height of around 10 - 12 km above the Gangetic plain. The air in the Tibetan plateau region gets heated up in the summer season causing the formation of low pressure at the surface and relatively higher pressure at the upper part of the troposphere. This leads to south diverging which under the Coriolis effect deflected from east to west leads to the formation of tropical jet streams. A part of the tropical easterly jet stream falls over the northeastern region of India and becomes easterly due to the Coriolis effect. The Somali jet stream is formed due to the uneven heating of the African continent and travels at a low height of 3-4 km. The Somali jet stream crosses the equator and becomes south-westerly in direction.


The jet streams are strong meandering winds moving with greater speeds at higher altitudes. The jet streams are formed due to the Coriolis effect and the uneven heating of the earth’s atmosphere. The junction of the troposphere and stratosphere is called tropopause and it is at this tropopause the jet streams travel at higher altitudes. We have also seen the direction of the jet streams which is from west to east. The two global jet streams are subtropical and polar jet streams 

FAQs on Jet Stream

1. Describe the shape of the motion of jet streams at higher altitudes?

The jet stream moves in a zig-zag manner which is known as meandering. There are two types of meandering of jet streams, horizontal meandering, and vertical meandering. Horizontal meandering is the zig-zag movement of the jet stream in the horizontal direction and vertical meandering is the zig-zag movement in the vertical direction. The horizontal meandering is due to the variation of the Coriolis effect with a change in latitude. The Coriolis effect increases with an increase in latitude and moving towards the polar regions. The vertical meandering of the jet stream is due to gravity and buoyancy.  

2. Which is stronger, polar jet stream or subtropical jet stream? Justify your answer?

The polar jet stream is stronger than the subtropical jet stream. It is based upon the effect. The greater the Coriolis effect, the stronger will be the deflection and the stronger will be the jet stream. The Coriolis effect increases with an increase in latitude and moving closer to the poles. This means the Coriolis effect is greater in the regions nearby and less in the equatorial regions. The polar jet streams are located closer to the polar regions. Therefore, the polar jet stream will have a greater Coriolis effect and it will be a stronger jet stream. 

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