Westerly Jet stream and its Impact
During winter, at about 8 km. above sea level, a westerly jet stream blows at a very high speed over the subtropical zone.
This jet stream is bifurcated by the Himalayan ranges.
The northern branch of this jet stream blows along the northern edge of this barrier.
The southern branch blows eastwards south of the Himalayan ranges along 25° N latitude.
This jet stream is responsible for bringing western disturbances from the Mediterranean region into Indian sub-continent.
Winter rain and hail storms in northwestern plains and occasional heavy snowfall in hilly regions are caused by these disturbances.
These are generally followed by cold waves in whole of northern plains.
Easterly Jet and its Influence
During summer, due to the apparent shift of the sun in northern hemisphere, the reversal in upper air circulation takes place.
The westerly stream is replaced by easterly jet stream which owes its origin to the heating of the Tibetan plateau.
This leads to the development of an easterly cold jet stream centered around 15°N latitude and blowing over peninsular India.
This helps in the sudden onset of monsoons.