Jet Streams are the fast blowing winds of a steady velocity moving in a narrow zone of the upper layer of the atmosphere. Their speed varies from about 110 km/h in summer to about 184 km/h in winter. Jet streams are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet  streams. A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub tropical jet stream. The western cyclonic disturbances experienced in the north and north-western parts of the country are brought in by this westerly flow.
The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer is an important facts to keep in mind to also understand the mechanism of Mansoon