The jet stream is a river of wind that blows horizontally through the upper layers of the troposphere, generally from West to East at an altitude of 20,000 - 50,000 feet (6,100 - 9,144 metres) or about 7 miles (11 kilometres) up.
There are different jet streams and in respect of the climate and monsoons of India, it is the Subtropical Jet Stream (STJ) and the countering easterly jet that are most important.
As the summertime approaches, there is increased solar heating of the Indian subcontinent. This has a tendency to form a cyclonic monsoon cell situated between the Indian Ocean and southern Asia. This cell is blocked by the STJ which tends to blow to the South of the Himalayas; as long as the STJ is in this position, the development of summer monsoons is inhibited. During the summer months, the STJ deflects northwards and crosses over the Himalayan Range. The altitude of the mountains initially disrupts the jet, but once it has cleared the summits, it is able to reform over central Asia. With the STJ out of the way, the sub-continental monsoon cell develops very quickly indeed, often in a matter of a few days.
In winters, an easterly jet stream, called the tropical easterly jet stream blows over peninsular India, approximately over 14°N during the summer months. Thus, it affects most part of Indian climate throughout the year.