The Jet theory was propounded by Yest. The Jet stream is very fast blowing air current system in the upper atmosphere (9 to 18 km). Its speed is maximum (340 km/h) in the middle part. This system works as a cover over the Earth which affects the weather of the lower atmosphere.
During winter season the upper air westerly jet streams are positioned in Asia. These are bifurcated in two branches due to Tibet Himalayan obstruction. North branch blows north of Himalayas and the Tibetan Plateau. Southern branch blows south of the mighty mountains.
During summer season as sun falls vertically over the Tropic of Cancer the polar surface high pressure is weakened and upper air circum polar whirl shift northward as a result of which the upper air westerly jet are also withdrawn from southern slopes of the Himalayas. The removal of jet stream to north of the Tibetan plateau results in reversal of the curvature of How of free air to the north and north-west of the subcontinent. This event may well be the trigger that sets off the ‘burst’ of the monsoon.
Origin of the Easterly Jet Stream because of the Plateau of Tibet becoming Hot in summer
This theory does not explain why easterly jet stream is so much indefinite. A general interpretation is given that it is the result of the variation in insolation. However, this kind of variation needs a proper analysis.