Narrow belts of very fast, high-altitude winds in the upper troposphere that flow generally from west to east and steer weather systems.
- Found in the upper troposphere (about 9 to 12 km up); blow west to east at speeds often exceeding 100 km per hour.
- Major types: the polar jet stream and the subtropical jet stream (one in each hemisphere); their position shifts with the seasons.
- The subtropical westerly jet stream lies over north India in winter; its withdrawal northwards (over the Tibetan plateau) helps trigger the onset of the south-west monsoon.
- The tropical easterly jet stream forms over India in summer and is linked to the monsoon's progress.
- Western disturbances, which bring winter rain to north-west India, are steered eastwards by the subtropical westerly jet stream.
The link between the subtropical westerly jet's withdrawal and monsoon onset, and between the jet and western disturbances, is a recurring climatology item.
Subtropical westerly jet (winter over north India, linked to monsoon onset and western disturbances) versus the tropical easterly jet (summer); jet streams flow west to east.
Fast upper-air west-to-east winds; the subtropical westerly jet's withdrawal helps trigger the Indian monsoon onset.