Meaning and Formation
Jet streams are narrow bands of very strong winds found in the upper atmosphere, mainly near the tropopause.
They generally flow from west to east at altitudes of around 8–15 kilometres.
They form because of strong temperature differences between adjoining air masses. These temperature contrasts create pressure gradients in the upper atmosphere, while Earth’s rotation causes deflection through the Coriolis effect.
Jet streams are not continuous straight lines. They often develop large meanders known as Rossby waves.
Major Types
The two principal jet streams in each hemisphere are:
Polar Front Jet Stream
It forms near the boundary between cold polar air and warmer mid-latitude air.
It is generally stronger in winter because the temperature contrast between polar and tropical regions is greater.
Subtropical Jet Stream
It occurs closer to about 30° latitude and is associated with upper-level circulation between tropical and subtropical regions.
Over South Asia, two seasonal jet streams are especially important:
- Subtropical Westerly Jet
- Tropical Easterly Jet
Role in Indian Monsoon
During winter, the Subtropical Westerly Jet flows south of the Himalayas.
It helps guide Western Disturbances, which bring:
- winter rainfall to north-western India;
- snowfall to the western Himalayas.
During summer, intense heating over the Indian subcontinent and Tibetan Plateau contributes to the northward shift of the westerly jet.
At the same time, the Tropical Easterly Jet develops over peninsular India and adjoining regions.
These seasonal changes in upper-air circulation are closely associated with the establishment of the southwest monsoon.
Jet-stream position and strength can therefore influence:
- monsoon onset;
- rainfall distribution;
- Western Disturbances;
- heatwaves;
- cold waves.
Weather and Aviation Importance
Jet streams influence the movement of major weather systems.
They can:
- steer cyclones and storms;
- guide weather fronts;
- influence rainfall patterns;
- separate contrasting air masses;
- contribute to persistent weather conditions.
They are also important for aviation.
Aircraft travelling with a jet stream can benefit from strong tailwinds, reducing flight time and fuel use. Flying against a jet stream can increase both.
Strong wind shear along the edges of jet streams can generate Clear Air Turbulence, even when no clouds or thunderstorms are visible.
Rossby Waves and Climate
Jet streams frequently develop large north-south bends called Rossby waves.
When these waves become strongly amplified or remain nearly stationary, they may contribute to prolonged weather extremes such as:
- heatwaves;
- cold spells;
- heavy rainfall;
- drought;
- persistent storm tracks.
Their behaviour depends on:
- seasonal temperature gradients;
- land-ocean contrasts;
- ocean-atmosphere circulation;
- polar and tropical temperature patterns.
Climate change can alter the temperature gradients that influence jet streams, but its effects on jet-stream position, speed and waviness vary by region and season and remain an active area of research.
Conclusion
Jet streams are powerful upper-atmospheric winds that play a major role in global weather, aviation and the Indian monsoon. Changes in their position and strength can significantly influence rainfall, storms and temperature extremes.
