The Jet Stream and India’s Weather Explained

A jet stream is a narrow ribbon of very fast wind flowing high in the atmosphere, and over India the jet stream decides much of the year’s weather: its northward shift in early summer helps unlock the arrival of the monsoon, a different easterly jet strengthens the monsoon rains, and in winter the westerly jet steers rain-bearing storms into north India. Three distinct jets matter for the subcontinent, and their seasonal swaps are among the least visible but most powerful clockwork mechanisms in the Indian climate.

Because jet streams sit near the top of the troposphere, they cannot be seen from the ground, yet their effects are felt in monsoon onset dates, western disturbances, cold waves, and even airline flight times. Last updated: 30 October 2026. This explainer describes each jet, how and why it moves, and what that means for India.

Key Takeaways

• A jet stream is a high-altitude river of fast-moving air, usually found near the top of the troposphere and often exceeding 100 km/h.
• India experiences the subtropical westerly jet in winter, which flows south of the Himalaya and steers western disturbances towards north India.
• The subtropical westerly jet moves north of the Tibetan Plateau in early summer, and this shift is closely linked with the onset of the Indian monsoon.
• The tropical easterly jet blows from east to west over peninsular India during the monsoon, and a strong easterly jet is generally associated with good monsoon rainfall.
• Western disturbances, which bring winter rain and snow to north India, travel along the subtropical westerly jet from the Mediterranean region.
• Jet streams are driven by temperature contrasts between air masses and by the rotation of the Earth, and they shift with the seasons.

What is a jet stream?

A jet stream is a strong, narrow current of wind concentrated near the tropopause, the boundary between the lower atmosphere (troposphere) and the stratosphere. It is typically thousands of kilometres long, a few hundred kilometres wide and only a few kilometres deep, and its core winds can exceed 200 km/h in the strongest cases.

Jet streams form where air masses of different temperatures sit side by side. Warm air is lighter and its pressure surfaces are higher, so a steep temperature contrast sets up a steep pressure gradient aloft. The Earth’s rotation then bends the resulting airflow so that it runs along the boundary rather than across it. This is why the main jets mostly flow west to east in the middle latitudes.

The jets are not straight. They bend into large north-south meanders called Rossby waves, and the troughs and ridges of these waves steer weather systems at the surface. A deep southward dip in the jet can drag cold air over north India, while a northward bulge can allow heat to build.

What are the main jet streams that affect India?

Three jet systems matter for India: the subtropical westerly jet in winter, the tropical easterly jet in summer, and a low-level jet over the Arabian Sea during the monsoon known as the Somali or Findlater jet. The first two are upper-level jets, while the third is a low-altitude stream important for moisture transport.

• Subtropical westerly jet (STWJ): A west-to-east jet near 12 km altitude that is strongest in winter over the Himalaya and northern India.
• Tropical easterly jet (TEJ): An east-to-west jet at about 14 to 16 km altitude that develops over the Indian region in June to September.
• Somali (Findlater) jet: A powerful southwesterly low-level current, around 1 to 1.5 km above the sea, that carries moisture from the southern hemisphere across the Arabian Sea to India.

Only the first two are generally called jet streams in the strict upper-air sense. Together, they bookend the Indian year: the westerly jet dominates the cool season, and the easterly jet rules the monsoon.

How does the subtropical westerly jet behave over India in winter?

In winter, the subtropical westerly jet sits at roughly 25 to 30 degrees north, directly over or just south of the Himalaya, and flows from west to east. Because the Tibetan Plateau is a massive obstacle at that height, the jet divides into two branches: a northern branch that flows around the plateau’s northern edge, and a southern branch that runs along the Himalayan arc across north India.

The southern branch is the one that matters most for Indian weather. It channels disturbances eastward over Pakistan, Jammu and Kashmir, Himachal Pradesh, Punjab, Haryana and Uttar Pradesh. Wind speeds in the jet core can reach well above 150 km/h in January and February, which is why aircraft flying west to east across the region often gain a large tailwind.

The jet position also affects temperatures. When the jet dips south, cold air from higher latitudes is drawn towards India, contributing to a cold wave across the plains. When it sits farther north, milder weather generally prevails.

How does the jet stream help bring the monsoon to India?

The jet stream helps bring the monsoon to India because the subtropical westerly jet must retreat from its winter position south of the Himalaya and shift north of the Tibetan Plateau before the summer monsoon circulation can establish itself. This northward jump generally occurs in late May to June and is closely tied in time with the onset of the monsoon over Kerala.

As the sun moves north and the land heats, the Tibetan Plateau becomes a high-level heat source. The upper-level temperature contrast reverses, with warm air now sitting to the north of the Indian peninsula, and this reversal removes the westerly jet from its winter track. The flow aloft over India turns to easterly, and the upper-level Tibetan anticyclone, a vast area of high pressure, builds above the plateau.

The withdrawal of the westerlies makes room for the monsoon trough and the deep moist flow. If the westerly jet lingers over north India, it can delay or weaken the monsoon’s advance by keeping dry, descending air in place. The reverse happens in autumn: as the sun moves south, the westerly jet returns to its winter position, usually from October, and this helps drive the monsoon’s retreat.

What is the tropical easterly jet?

The tropical easterly jet is a narrow band of strong easterly winds in the upper troposphere that develops over south Asia during the summer monsoon, running from the South China Sea across the Indian peninsula and the Arabian Sea into Africa. It is at its strongest in July and August, with core speeds around 100 to 180 km/h at about 14 to 16 km altitude.

The jet is driven by the sharp temperature difference between the intensely heated Tibetan Plateau to the north and the cooler Indian Ocean to the south. This contrast sets up an upper-level flow from east to west south of the Tibetan anticyclone. Its core is typically located near 10 to 15 degrees north, passing over peninsular India.

The easterly jet is part of the monsoon’s engine room. It helps ventilate the rising air from monsoon thunderstorms, carrying it away at height so that fresh moist air can keep flowing in at the surface. For that reason, a vigorous tropical easterly jet is generally associated with a healthy monsoon, while a weak one is often associated with deficient rain. The jet was identified in the 1950s through high-altitude wind observations, and Indian meteorologists played a leading role in describing it.

Which jet streams matter in India each season?

The dominant jet over India changes with the seasons, as summarised in the table below. This seasonal handover is one of the clearest examples of how the upper atmosphere organises the Indian year.

How does the jet stream steer western disturbances?

The subtropical westerly jet steers western disturbances by acting as a conveyor belt that carries moisture-bearing low-pressure systems eastward from the Mediterranean and Caspian regions across Iran, Afghanistan and Pakistan into north India. Western disturbances are the main source of winter rain and Himalayan snowfall in the north, and they are crucial for rabi crops such as wheat.

These systems ride along the jet’s flow and are often strengthened where the jet is strong and the upper-air troughs are deep. As they meet the Himalayan ranges, the moist air is forced upward, producing snow at height and rain over the plains. A series of disturbances in a single winter can supply a large share of the snowpack that later feeds Himalayan rivers.

When a western disturbance interacts with the monsoon’s moisture, as can happen in the pre-monsoon or retreat period, the result can be extreme rain over the hills. The jet’s position determines how far south and east such storms travel, and how often they arrive.

What happens when the jet stream shifts early or late?

When the jet stream shifts early or late, the timing of the monsoon and the character of the pre-monsoon season change with it. An early retreat of the westerly jet from north India can ease the path for monsoon advance, while a late retreat tends to extend dry, hot conditions and delay rain.

The effect can be seen in heat patterns as well. In spring, a jet that is positioned unusually far north or that has weakened can allow hot, dry continental air to build over northwest India, contributing to heatwaves. In winter, a jet that makes deep southward meanders is more likely to drive severe cold waves and prolonged fog, as cold, still air settles over the Indo-Gangetic plain.

Scientists are studying whether climate change and Arctic warming are making jet streams more wavy or more persistent. The evidence is still debated for the Indian region, and caution is warranted. What is clearer is that variations in the jet from year to year are a recognised contributor to variations in Indian seasonal weather.

Why does the jet stream matter for India’s weather forecasting and daily life?

The jet stream matters for India’s forecasting and daily life because it is a leading indicator of weather days ahead, and it directly affects farming, aviation and water supply. Forecasters at the India Meteorological Department (IMD) examine upper-air maps showing jet positions to anticipate western disturbances, cold waves, and the timing of monsoon changes.

• Agriculture: Winter rain from jet-steered western disturbances supports wheat, mustard and other rabi crops.
• Water security: Snowfall delivered by these storms feeds glaciers and the Himalayan rivers that sustain the northern plains.
• Aviation: Eastbound flights can use the westerly jet as a tailwind, while westbound flights often route to avoid it.
• Disaster planning: Jet position helps predict cold waves, heatwaves and heavy rain episodes in the hills.
• Monsoon outlooks: The timing of the westerly jet’s retreat and the strength of the easterly jet are watched as signals of how the monsoon will behave.

Bottom Line

The jet stream is the atmosphere’s great switchboard over India: the westerly jet delivers winter storms and then steps aside to let the monsoon arrive, and the easterly jet takes over to help the rains thrive. Understanding both jets makes the timing of India’s seasons much easier to explain.

Frequently Asked Questions

What is a jet stream in simple terms?

A jet stream is a fast, narrow river of air flowing high in the atmosphere, usually near the top of the troposphere. It can blow at well over 100 km/h and moves weather systems along with it. Over India, jets change with the seasons and strongly influence the monsoon and winter storms.

How does the jet stream affect the Indian monsoon?

The jet stream affects the Indian monsoon in two ways. The subtropical westerly jet must move north of the Tibetan Plateau in early summer for the monsoon to set in, and the tropical easterly jet that follows helps maintain the rain-bearing circulation. A delayed westerly retreat can delay the monsoon, while a strong easterly jet is generally linked with good rainfall.

What is the tropical easterly jet?

The tropical easterly jet is an upper-level band of strong easterly winds that forms over south Asia and the Indian peninsula during the summer monsoon. It blows from east to west at roughly 14 to 16 km altitude and is strongest in July and August. It results from the temperature contrast between the heated Tibetan Plateau and the Indian Ocean.

Which jet stream brings western disturbances to India?

The subtropical westerly jet brings western disturbances to India. It carries these moisture-bearing low-pressure systems from the Mediterranean region across Iran, Afghanistan and Pakistan into north India during the cool season. They produce winter rain on the plains and snowfall in the Himalaya.

Why does the westerly jet move north in summer?

The westerly jet moves north in summer because the Tibetan Plateau and surrounding land heat up strongly, reversing the north-south temperature contrast in the upper atmosphere. As a result, the jet that sat south of the Himalaya in winter shifts north of the plateau. This shift is closely timed with the monsoon onset over Kerala.

Does the jet stream affect flights over India?

Yes, the jet stream affects flights over India and the surrounding region. Aircraft flying west to east can gain a strong tailwind from the winter westerly jet, which shortens journey times, while those travelling the opposite way often adjust routes to avoid headwinds. Pilots and airlines use jet stream forecasts to plan fuel and flight paths.

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