BharatNotes What is Jet Stream?
A jet stream is a fast, narrow ribbon of wind that flows from west to east in the upper troposphere, close to the tropopause. These currents are produced where strong horizontal temperature contrasts — between cold polar air and warm tropical air — combine with the Coriolis force generated by Earth's rotation, a relationship known as thermal-wind balance: the sharper the surface temperature gradient, the faster the wind aloft. Jet streams do not flow in a straight line but snake north and south in giant undulations called Rossby waves.
Per NOAA's National Weather Service, jet streams typically sit 6–12 km (4–8 miles) high and can exceed 442 km/h (275 mph / 239 knots) in their fastest cores (NWS JetStream).
Types and Key Features
Earth has four main jet streams — two polar-front jets and two subtropical jets (one of each per hemisphere).
| Feature | Polar-front jet | Subtropical jet (STJ) |
|---|---|---|
| Latitude | ~50°–60° | ~30° |
| Core altitude | ~9 km | ~12 km |
| Width | ~5° latitude | ~10° latitude |
| Behaviour | Variable, meanders strongly | Steadier, less meandering |
| Driver | Strong polar-front temperature contrast | Poleward limit of Hadley cell |
A separate polar-night jet forms in the winter stratosphere over the dark pole, driven by a steep stratospheric temperature gradient. Over India during summer, a tropical easterly jet (TEJ) also develops, flowing east-to-west, centred near 15°N (NOAA; Wikipedia, Tropical Easterly Jet).
Significance for the Indian Monsoon
Jet streams are central to the jet-stream theory of the Indian monsoon:
- In winter, the subtropical westerly jet flows along the southern slopes of the Himalayas. The Himalayas and Tibetan Plateau split it into two branches (north and south of the plateau). The southern branch steers western disturbances — mid-latitude cyclones bringing winter rain to the northwest plains and snow to the western Himalayas.
- By late May–June, the STJ withdraws north of the Himalayas. This northward shift is the first signal of monsoon onset; if the shift is delayed, the southwest monsoon is delayed too.
- Simultaneously, the tropical easterly jet establishes itself over peninsular India, and its north–south position influences where monsoon rainfall concentrates.
Current Status & UPSC Angle
Research increasingly links climate change to a weakening and erratic behaviour of these currents, including weaker western disturbances over recent decades (Down To Earth reporting). For 2026, the India Meteorological Department's forecast indicated southwest monsoon performance trending toward the lower side (IMD, 2026 forecast) — making jet-stream behaviour a live exam-relevant theme.
For UPSC, master the STJ–TEJ distinction, the bifurcation around the Tibetan Plateau, and the cause-chain linking jet migration to monsoon onset and withdrawal. Do not confuse the west-to-east subtropical westerly jet with the east-to-west tropical easterly jet — a frequently mixed-up pair.
Sources: NOAA National Weather Service (JetStream); NOAA NESDIS; Geosciences LibreTexts; Wikipedia (Jet stream; Tropical Easterly Jet); IMD 2026 forecast reporting.
