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Jet stream:

What is Jet Stream:

Jet Stream is winds of higher altitude that blow below the tropopause and it is largely governed by pressure gradient on the upper atmosphere and Coriolis forces.
It has a higher speed usually above 120 km per hour as it is free from surface friction.
It flows like river streams and zig-zag motion as it follows the isobar path and isobars are not straight lines.

Origin and formation of Jet Stream:

Due to higher insolation, air rises in the equatorial region, and rises air reaches up to the tropopause. Due to adiabatic cooling, cloud formation happens; after condensation and moisture from air come down via precipitation.
At the higher altitude at the equatorial region, there is high atmospheric pressure as compared to the higher latitude at the same altitude.
Piled air from the equatorial region starts flowing toward the poleward as the air always flows from high pressure to low pressure.
This high altitude wind is called a Jet Stream.

Types of Jet Stream:

There are primarily two jet streams in each Hemisphere:
Subtropical Jet Stream originated in the Equator but flow between 20 to 30 degrees latitude from west to east
Polar Jet stream; originated between 50 to 60 degrees latitude in both hemispheres and flow around 70 to 80 degrees latitudes.

Effect of Jet Stream on Weather:

Jet stream usually separated cold air masses and warm air masses, jet stream generally follows the Zig-zag path and formed ridge on war air masses side and trough at the cold airmasses side.
On the ridge side, there is a cyclonic condition at the surface and causes instability in the atmosphere and can cause rainfall.
On the trough side, there is anty-cyclonic condition and air sink into the ground and make stable atmospheric conditions.
The jet stream is also linked with global warming, if the north pole is warmer than usual, the Polar Jet Stream follows more Zig zag path and at the resulted encroachment of cold air masses southward. Extreme winter in the USA is the result of global warming and the encroachment of cold air masses southward.
The withdrawal of the subtropical Jet stream also causes the sudden burst of Monsoon in India.
The presence of Subtropical Jet Stream causes the western disturbance in winter in India.

For a detailed explanation, watch the below video:

Civil Services Self Study ( NCERT Based)