Jet stream
These are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere. Their speed varies from about 110km/h in summer to about 184 km/h in winter.A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub tropical jet stream.

These jet streams are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet streams. Over India, these jet streams blow south of the Himalayas, all through the year except in summer. The western cyclonic disturbances experienced in the north and north-western parts of the country are brought in by this westerly flow. In summer, the subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the sun. An easterly jet stream, called the tropical easterly jet stream blows over peninsular India, approximately over 14°N during the summer months.

Answer referred from NCERT book Class 9 .
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Air current in the upper layers of the atmosphere is known as jet stream. It could determine the arrival and departure of monsoon wind in India.