Narrow ribbons of fast wind (150–400 km/h) at 9–12 km altitude that steer weather systems and affect monsoon timing.
phenomenaWhat is a Jet Stream?
A jet stream is a narrow, fast-flowing ribbon of wind in the upper atmosphere, typically at altitudes of 9–12 kilometres (in the upper troposphere). Jet streams flow generally west-to-east, with wind speeds ranging from 100 km/h to over 400 km/h in the strongest jet cores.
Jet streams form along strong horizontal temperature gradients — where cold polar air meets warm tropical air. The atmosphere tries to balance this gradient, and the result is a fast-moving river of air aloft. There are typically four major jet streams:
- Polar jet (one in each hemisphere, near 50°–60° latitude)
- Subtropical jet (one in each hemisphere, near 25°–30° latitude)
- Tropical Easterly Jet (only over Africa/Asia in summer; flows east-to-west)
For South Asia, the subtropical jet and the tropical easterly jet are the most important.
The Subtropical Jet over the Himalayas
The Subtropical Westerly Jet (STJ) sits over the Himalayan ridge during winter — flowing west-to-east at altitudes of 10–12 km, with core speeds of 200–300 km/h.
Its position has profound effects on South Asian winter weather:
- It steers western disturbances — mid-latitude low-pressure systems that originate in the Mediterranean and cross Iran, Afghanistan and Pakistan to reach northern India.
- Each western disturbance brings snow to Kashmir, Himachal and Uttarakhand, and winter rain to Punjab, Haryana, Delhi and western UP.
- The strength and southward extent of the jet controls how cold the winter is in north India.
In summer (June), the subtropical jet shifts north of the Himalayas, retreating to Central Asia. This shift is one of the triggers for monsoon onset — once the jet moves out of the way, moist southwest winds can sweep up into the Indian landmass.
The Tropical Easterly Jet (TEJ)
The Tropical Easterly Jet (TEJ) is unique to the Asian-African summer. As the southwest monsoon establishes, a strong easterly jet forms at 12–14 km altitude over peninsular India and northern Africa, flowing east-to-west with speeds of 40–100 km/h.
The TEJ is critical for the monsoon:
- It transports air away from the rising convection zone over India, allowing more air to rise from below — sustaining the heavy rainfall.
- A strong TEJ correlates with above-normal monsoon rainfall.
- A weak TEJ (e.g., in El Niño years) often coincides with below-normal monsoon.
The TEJ disappears by October when the monsoon retreats and the subtropical jet returns south over the Himalayas.
Jet streams and aviation
Long-distance commercial aircraft fly at 10–12 km altitude — the same level as jet streams. Airlines deliberately use jet streams to save fuel:
- Westbound flights (Europe → India, US → Europe) try to avoid the jet (it would slow them).
- Eastbound flights (India → Europe, Asia → US) ride the jet (gaining 100+ km/h tailwinds).
A flight from Mumbai to London might take 9 hours westbound but only 8 hours eastbound. Pilots monitor jet-stream forecasts daily to optimise routes.
Severe clear-air turbulence often occurs at jet-stream edges, where wind shear is strongest. This is the kind of turbulence that strikes without warning even on a sunny day.
Jet streams and cyclones
Jet streams influence tropical cyclone tracks. Specifically:
- The subtropical ridge below the jet steers Atlantic and Pacific hurricanes/typhoons westward.
- Strong upper-level westerlies can recurve cyclones poleward.
- A weakening or split jet can allow tropical systems to drift further north.
For Bay of Bengal cyclones, the typical track depends on the steering pattern: most move northwest (toward Odisha/Bengal/Bangladesh), but some recurve toward Myanmar.
Jet streams and climate change
A warming Arctic is weakening the polar jet stream. The temperature gradient between the pole and the tropics is decreasing, and the jet is becoming more wavy and slower in some regions.
The implications:
- More persistent weather patterns — heatwaves, cold spells and droughts may last longer.
- More extreme winter storms when waves of the jet stream dip deep south.
- Indirect effects on monsoon timing if the subtropical jet position shifts.
Scientists are still debating the magnitude of these effects, but the trend toward a “wavier” jet stream is observable in satellite records since 1979.
How jet streams are observed
- Radiosonde balloons released twice daily worldwide measure wind at all altitudes.
- Satellite cloud-tracking infers upper-level winds from cloud motion.
- Aircraft reports (AMDAR) provide tens of thousands of wind observations per day.
- Weather models (ECMWF, GFS) ingest all of this and produce jet-stream forecasts up to two weeks ahead.
Frequently asked questions
How fast is the jet stream? Typically 150–250 km/h, but the strongest cores can exceed 400 km/h. The world record was recorded over Japan in 2004 at 408 km/h.
Can you see the jet stream from the ground? Sometimes — narrow streaks of high cirrus clouds aligned in the wind direction can mark a jet-stream axis. Pilots flying at the jet’s level can see clear air with sudden velocity changes.
Does the jet stream cause winter rain in north India? Indirectly. The subtropical jet steers western disturbances — the actual rain producers — from the Mediterranean across to Pakistan and India. Without the jet’s steering, these systems would not reach South Asia.
Why does the jet stream blow eastward? The Coriolis force, driven by Earth’s rotation, combined with the north-south temperature gradient, produces a west-to-east geostrophic wind. The faster the gradient, the faster the jet.
Where can I see jet-stream forecasts? Specialised aviation weather sites publish jet-stream charts; ECMWF and GFS provide free 300-hPa wind forecasts. For surface impacts (cyclone tracks, monsoon trough), Mausam Online provides city-level forecasts. See Srinagar, Delhi, Mumbai, Patna.