Jet streams

Jet streams are long, narrow, high-speed, meandering, circumpolar winds that typically flow north-eastward, eastward, and south-eastward in the middle and upper troposphere or lower stratosphere. 

Jet streams are characterized by wind motions that generate strong vertical shearing action, which is thought to be largely responsible for clear air turbulence.

FORMATION

• Jet streams form when warm air masses meet cold air masses in the atmosphere. The Sun does not heat the whole Earth evenly. That is why areas near the equator are hot and areas near the poles are cold. So, when Earth’s warmer air masses meet cooler air masses, the warmer air rises up higher in the atmosphere while cooler air sinks down to replace the warm air. This movement creates an air current, or wind. A jet stream is a type of air current that forms high in the atmosphere.
• As the winds uplift from low pressure regions of the Earth a strong air current is developed.
• Coriolis force is higher on the fast-moving objects and hence fast-moving currents experience higher Coriolis force.
• Because of the movement of Earth from West to East, the air current up in the atmosphere are also deflected towards the East. 
• As the difference in temperature increases between the two locations the strength of the wind increases. Therefore, the regions around 30° N/S and 50°-60° N/S are also regions where the wind, in the upper atmosphere, is the strongest.
• The 50°-60° N/S region is where the polar jet located with the subtropical jet located around 30°N.
• With increase in pressure gradient, the speed of air current keeps on increasing which in turn increases the Coriolis deflection and hence the speed of Jetstream towards East.

CHARACTERISTICS OF JET STREAMS

• They generally move from west to east in a narrow belt of a few thousands km of length, hundred km of width and few km of thickness moving at the height of 7.5 -14 km in the upper tropo­sphere.
• Generally, their circulation is observed between poles and 20° latitudes in both the hemispheres. 
• The minimum velocity of jet stream is 30m/second (108 km/hour).
• The vertical wind shear of jet streams is 5- 10m/second (18-36 km/hour), meaning thereby the wind velocity above or below jet stream decreases by 18-36 km/hour. 
• Their circulation path (trajectory) is wavy and meandering. Meandering jet streams are called as Rossby waves. 

• Their velocity increase during winter season and the wind velocity becomes twice the velocity during summer season. Maximum wind velocity is 480 km (per hour).
• The extent of jet streams narrows down during summer season because of their northward shifting while these extend up to 20° latitudes during winter season.

TYPES OF JET STREAMS

• Polar Front Jet Streams: formed above the convergence zone (40-60 lats.) of the surface polar cold air mass and tropical warm air mass. These are rather irregular. 
• Subtropical Westerly Jet Streams: formed in the upper troposphere to the north of subtropical high-pressure belt above 30°-35° latitudes. They are most regular and continuous. 
• Tropical Easterly Jet Streams: develop in the upper troposphere above surface easterly trade winds over India and Africa during summer season due to intense heating of Tibetan plateau and play important role in the mechanism of Indian monsoon.
• Polar Night Jet Streams: also known as stratospheric sub-polar jet streams, develop in winter season due to steep temperature gradient in the strato­sphere around the poles at the height of 30 km. 
• Local Jet Streams: formed locally due to local thermal and dynamic conditions and have limited local importance. Ex. Somali Jetstream. 

SIGNIFICANCE OF JET STREAMS

• Climate: Jet streams can transport weather systems across the world, affecting temperature and precipitation. Ex. they carry temperate cyclones from eastern coast of USA to Western coast of Europe. Their presence & withdrawal over Gangetic planes directly affects the monsoonal pattern in India. 
• Predicting weather: Weather satellites, such as the Geostationary Operational Environmental Satellites-R Series (GOES-R), use infrared radiation to detect water vapor in the atmosphere. With this technology, meteorologists can detect the location of the jet streams. Monitoring jet streams can help meteorologists determine where weather systems will move next. 
• Ozone layer depletions: Jet streams may transport ozone depleting substances higher up in the atmosphere up to stratosphere. This vertical air circulation causes rapid rate of mixing of air between troposphere and stratosphere, which helps in the transport of an­thropogenic pollutants from troposphere to stratosphere.
• Disasters: Recently climate change studies have proved the link of Jet streams and disasters like floods, fires and cyclones. 
• Travel & Transportation: if an airplane flies in a powerful jet stream and they are traveling in the same direction, the airplane can get a boost reducing the fuel need. Opposite may lead to turbulence and resistance.