Jet streams are concentrated narrow bands of fast flowing and strong winds in the upper troposphere of the earth that significantly affect global weather phenomena. There are three types of jet streams:

• Polar jet stream – between Ferrel and Polar cells
• Sub-Tropical jet stream – between Hadley and Ferrel cells
• Temporary jet streams – e.g.- Somali Jet Stream, Tropical Easterly Jet Stream

Properties

• Geostrophic winds – due to Coriolis force, they blow in a direction perpendicular to the pressure gradient force.
• Circumpolar – circle around the earth with poles as their centres.
• Westerlies – i.e. blow in west to east direction
• Rossby waves because of meandering path 
• High velocity – 150-250 kmph
• Upper atmospheric wind circulation – blow just below the tropopause

Role of Jet Streams in Weather Phenomenon

• Help in maintenance of latitudinal heat balance by mass exchange of air.
• Influence the weather of mid-latitudes by influencing the path of temperate cyclones and the distribution of precipitation.
• Impact the movement of air masses, which may cause prolonged drought or flood conditions. Eg. polar vortex cold wave over North America in 2014 winters.

Role of Jet Streams in Rainfall in India

Sub-tropical jet stream and some temporary jet streams together influence Indian Monsoon patterns, winter rainfall and tropical cyclones.

1. Winter rainfall: The Sub-Tropical westerly jet stream transports the western disturbances (temperate cyclones) originating over the Mediterranean Sea and brings rain to northwestern regions of India – Punjab, Haryana, Himachal Pradesh.

2. Southwest monsoon: The sub-tropical jet stream and easterly jet stream play an important role in the monsoon system of India.
a. The withdrawal of sub-tropical jet stream from the south of Himalayas paves the way for the onset of monsoon in Indian sub-continent.                        

b. The easterly jet stream steers the tropical depressions into India, which play a significant role in the distribution of rainfall during the SW monsoon period.

3. Tropical cyclones – The Easterly jet stream steers tropical depressions and cyclones from the Pacific ocean towards Indian Ocean region causing rainfall predominantly over the eastern coastal region. Eg. cyclone Mora in 2017.