https://sciencetopic03.blogspot.com/2021/12/what-is-global-air-circulation-and-how.html
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Jet Streams produced by the Global Air circulation |
A jet stream is mainly
categorized by its speed and its way of flowing. Jet Streams flow at a speed
of 275mph and at the height of 5 to 9 miles in the atmosphere. It flows in a
meandering shape altered by pressure difference and gravity. It is named a
stream, because of its meandering shape (shape and flow like river). Also, as
Jets fly over in this height to get a greater push, it is named as Jet Streams.
A sample flow of the jet stream can be observed from the below video.
A Jet Stream is
basically a geostrophic wind, which forms due to the principle of Pressure
Gradient Force (PGF) and the Coriolis Force (CF). The word “Geo” means
“earth”, and the word “strophe” means “turning.” Thus it refers to the type of
wind that revolves around the earth. Now let us look into the formation of jet
stream by taking a quick look at the Pressure gradient force and the Coriolis
force.
We know that, a low
pressure is created when the surface air gets heated and rises up due to its
low density. Similarly, high pressure is created, when the cold air
sinks to the surface, due to its high density. When this happens, the air
from the high pressure region moves to the low pressure region, causing a flow
of wind. This principle is called the Pressure Gradient Force.
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Principle of Pressure Gradient Force |
When we take this
principle in a global pattern, we term it as Global Air Circulation. But there
is also one more fact that needs to be considered in the Global Pattern. It is
called as Coriolis Effect. This force plays a major role in the direction of
the movement of the wind from one pressure to another pressure. This
Coriolis force makes the movement of the air to the right, when moving from
the equator to the poles, and to the left when moving from poles to the equator.
To know more about the process of pressure gradient and the Coriolis effect,
please visit the below article,
https://sciencetopic03.blogspot.com/2021/12/what-causes-winds-science-behind.html
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Principle of Coriolis Force |
There is also one more
thing that needs to be considered to understand the principle of geostrophic
wind. When we say a place is equipped with a certain pressure, it can be
classified as both high and low pressure. But to denote the pressure correctly,
it must be specified as surface-level pressure or upper-level pressure.
If the surface pressure has high-level pressure, then the top portion will be
empty, which will be considered as low pressure. Similarly, the upper-level zone
of a low pressure zone will be high pressure, as all the air had been lifted
off. Since the jet stream flows at a higher altitude, we are always going to consider
the upper-level pressure for its operation. The demonstration can be observed
clearly from the below figure.
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Difference between Surface level pressure and upper-level pressure |
Now let’s take a look
at how the upper-level pressure gradient force and the Coriolis effect play a
vital role in creating the geostrophic wind. Consider an air particle as shown
in the below figure. When there is an upper-level high pressure on the equator
and upper-level low pressure on the Northern side of the equator, the air
particle will move from high pressure region to low pressure region due to
pressure gradient force (PGF). It also tends to move right due to the
Coriolis force. The Coriolis force acts upon the particle continuously as
it moves towards the north. When the Pressure gradient force is equal to the
Coriolis Effect, the air particle flows parallel to the isobars. Isobars
are the areas of constant pressure. So in this case, the high pressure region
is one isobar and the low pressure region is another isobar. The same thing
applies to the southern hemisphere. So the main thing that is necessary for the
formation of the jet stream is two different kinds of temperature and a
little bit of spin. The process has been clearly depicted in the below
diagram.
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Formation of Jet Stream |
The subtropical jet stream is produced by the warm air from the tropics and the cold air from the subtropics. Similarly, the polar jet stream is produced by the warm air from the subtropics and the cold air from the poles.
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Formation of Subtropical Jet Stream |
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Formation of Subtropical Jet Stream |
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Meandering of Jet Streams |
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Formation of Rossby waves and Storms |
How do Jet Streams impact our weather?
Even though the jet
stream flows in the upper layer of the atmosphere, it will have a significant
impact on the surface. As explained, there will be always an upper-level low
pressure zone to the north of the jet stream and upper-level high pressure zone
to the south of the jet stream. When there is an upper-level low pressure zone,
air from the surrounding area is sucked up. Such that the air from the
below also rises, which creates a low pressure zone in the surface, leading to cloudy conditions. A non-meandering jet stream keeps the upper-level low
pressure system over the same areas and won’t have much impact on the surface
pressure as the pressure gradient gets equaled to the Coriolis force.
But a meandering jet stream will create new low pressure systems within it, and
changes the surface pressure drastically, as the balance between the
pressure gradient force and the Coriolis force is not maintained well. Let’s
take a look at a simple demonstration.
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Influence of weather by the Jet streams |
As shown in the above
picture, the air in the meandering jet stream moves faster at the bending. When
it moves, the air particles are subjected to different amounts of Pressure
Gradient Force and Coriolis Force. As shown in the image when the air enters
into the bending, it begins to accelerate, as the pressure gradient is
larger than the Coriolis force. So the air particle will move to the north and
will be a loss of air in the south. To fill the depletion, air from the
surface is sucked up leading to lower pressure on the surface. This low
pressure condition leads to heavy rain and cloudy condition. As the air
particle proceeds, the Coriolis force gets matched up with the pressure
gradient force and the balance between them is maintained. But next, as the air
particle reaches the exit, the pressure gradient decreases. And the
Coriolis force will not respond quickly, leading to an imbalance again. But
now, as the Coriolis force is larger than the pressure gradient force, it leads
to a southward component of the movement of air. So there will be a loss
of air in the north and accumulation of air in the south leading to a low
pressure condition in the north. This is how the jet streams affect the
weather.
Types of Jet Streams:
The main streams that
we have seen so far, are called the Subtropical Jet Streams and Polar Jet
Streams. But there are also other jet streams that originate in different parts
of the world. They are,
- Tropical Easterly Jet Stream
- Somali/Findlater Jet Stream
- Polar Night Jet Stream
- African Easterly Jet Stream.
These jet streams are non-permanent
winds that originate only at a specific location and by a specific
duration. Let’s look at each jet stream and see about their properties of
occurrence.
Tropical Easterly Jet Stream: (Speed: 70-120mph; Height: 15km)
The tropical Easterly
Jet stream (also called as Equatorial Easterly Jet) originates in the month of
June to September across the places of Philippines to West Africa by
crossing the southern tip of India. It is mainly caused by the intense heat
of the Tibetan plateau, which creates an upper-level high pressure zone,
thereby diverting a branch of subtropical zones as a tropical easterly jet
stream. It is one of the helping natures of the south-west monsoon.
We will look about its uses, particularly in the Indian monsoon in our next post.
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Formation of Tropical Easterly Jet Stream |
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Formation of Tropical Easterly Jet Stream |
Somali or Findlater Jet: (Speed: 100mph; Height: 1.5km)
It is a kind of low-level
jet stream that originates on the coast of Somalia in the month of July and
August. It originates from the heavy temperature contrast between the Indian
Ocean and the Somalian country. But, as it originates in the month of July
and August, the shifting of ITCZ turns the direction of the jet from the
west side to the east side, which further enhances the strength of the southwest
monsoon to India. Due to this property, it is also called as Cross Equatorial
Jet Stream.
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Formation of Somali or Findlater Jet Stream |
Polar Night Jet Stream: (Speed: 100-200mph; Height: 25km)
This Jet occurs in the
Polar Regions during the winter months. This Jet occurs, due to the
upper-level low pressure region in the polar area created by the heavy
temperature contrast between the polar region and the tropical region.
As both the warm and cold air meets at the subtropical zone it creates a
front and makes the warm air to rise over the polar region. Then due to
Coriolis Effect, it is made to spin like a vortex in an anticlockwise direction in
the northern hemisphere and in a clockwise direction in the southern hemisphere. The subtropical Jet stream holds the polar vortex from escaping into the tropics. If the
vortex and the jet stream were weaker caused by the less temperature contrast,
several places will get affected by the severe cold, as this jet is mainly made
of severe cold conditions.
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Formation of Polar Night Jet Stream |
African Easterly Jet: (Speed-30mph; Height: 3km)
As the name denotes,
this jet arises in the African continent between the Sahara desert and the
gulf of guinea. This jet plays an important role in the West African
Monsoon. It also plays an important role in bringing huge storms in the
Atlantic Ocean which are greatly experienced by the eastern coast of America
and Mexico.
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| Formation of African Easterly Jet |
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