Jet stream

Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"

Meaning

A jet stream is a narrow band of very fast wind that blows high in the sky, in the upper troposphere about 9–16 km up, just below the tropopause. It winds and bends as it moves around the Earth, and its fastest part (the core) often reaches 150–300 km/h.

Jet streams guide storms and cold spells in the mid-latitudes. Over India, they steer winter western disturbances and help start the south-west monsoon.

Explanation

How a jet stream forms (cause → process → result)

• Cause: warm air meets cold air
• Warm tropical air and cold polar air meet along a sharp boundary.

• This creates a steep pressure difference high in the air.

• Process: the wind is turned and balanced

• Air starts to move from high pressure to low pressure. This push is the pressure gradient force.
• The Earth's spin turns the moving air. This turning effect is the Coriolis force.
• High in the air there is almost no friction, so the two forces balance each other.

• The result is a geostrophic wind (a wind that blows parallel to the isobars, the lines of equal pressure).

• Result: a fast, narrow river of air

• Jets are near-geostrophic. Most of them flow roughly west to east around the globe.

• Season: jets are strongest in winter. That is when the temperature difference between the equator and the poles is greatest.

• Height: they blow just below the tropopause (the boundary between the troposphere below and the stratosphere above). They do not blow in the stratosphere.

The main jets

Jet	Height	Where	Season	Direction
Polar front jet stream	9–12 km	~40–60°, above the polar front	All year; strongest in winter	Westerly
Subtropical westerly jet stream (STJ)	12–14 km	~30°	Over India mainly in winter	Westerly
Tropical easterly jet stream (TEJ)	13–16 km	~15°N, over peninsular India and the Arabian Sea	Summer only	Easterly
Somali (Findlater) jet	Low level, 1–1.5 km	East African coast → Arabian Sea	Summer monsoon	South-westerly

• Polar front jet: it flows above the polar front (where cold polar air meets warmer mid-latitude air). This front has the sharpest temperature contrast, so this is the most changeable jet.
• Its wavy path guides temperate cyclones (low-pressure storms of the mid-latitudes).

• When it dips south, cold polar air follows behind it and brings cold outbreaks.

• Subtropical westerly jet (STJ): it flows above the poleward edge of the Hadley cell (the tropical loop where air rises at the equator and sinks near 30°).

• In winter, the Tibetan Plateau blocks its path. The jet splits into two branches, one north of Tibet and one south of it.
• The southern branch blows over north India and steers western disturbances into north-west India.

• In early summer, the jet jumps north of the Tibetan Plateau. This removes the westerly "lid" over north India and helps the monsoon set in.

• Tropical easterly jet (TEJ): this is the only easterly jet among the main four.

• In summer, the Tibetan Plateau heats up strongly. It acts as a raised heat source at about 4–5 km.
• Warm air rises, and a high-pressure area (anticyclone) forms in the upper air over Tibet.
• Winds flowing out of this high are turned by the Coriolis force. To the south they become strong easterlies, and these form the TEJ.
• It was first described by P. Koteswaram (1958). NCERT links it to the sudden onset, or "burst", of the monsoon.

• A satellite study of June 1977 ("Monsoon-77") found that jet cloud patterns lined up with the strongest winds at the 150 and 100 mb levels. It also found that the monsoon was active in the "right entrance" sector of the jet, where the jet starts to speed up [3].

• Somali (Findlater) jet: a low-level jet (a fast wind band near the ground), not an upper-air jet.

• It starts as south-east trades in the southern Indian Ocean. It crosses the equator near Kenya and Somalia, where the East African highlands channel it.
• The Coriolis force turns it to the right, so it becomes a south-westerly that blows to India's west coast.
• It carries much of the monsoon's moisture. It was first described by J. Findlater (1969).

Rossby waves: why jets bend

• Rossby waves (planetary waves) are giant loops in the upper westerlies and the jets. Usually four to six of them circle the globe.
• The Coriolis effect is weak near the equator and strong near the poles.
• When eastward-moving air is pushed north or south, this change with latitude pulls it back. The air overshoots and swings back again, so the flow snakes in loops.

• Mountains such as the Rockies and Tibet, and heating differences between land and sea, help start the loops.

• Ridge: a loop bending toward the pole. Warm high-pressure weather sits under it.

• Trough: a loop bending toward the equator. Cold, unsettled weather sits under it.
• These waves move heat from the tropics toward the poles and cold air toward the tropics. They help locate the jet stream and mark the track of surface low-pressure systems [4].
• Blocking: when the waves stall, the slow motion "often results in fairly long, persistent weather patterns" [4]. These can be long heat waves, cold spells or floods.

Discovery and use

• Wasaburo Ooishi measured jets over Japan in the 1920s using weather balloons.
• In the Second World War, bomber pilots flying west over the Pacific met headwinds strong enough to slow them almost to a stop.
• Eastbound flights are faster because they ride the westerly jets. Westbound flights try to stay out of them.
• Open debate: the Arctic is warming faster than the rest of the world (Arctic amplification). This may shrink the pole-to-equator temperature contrast and make the jet weaker and wavier. Observations and models do not yet agree on this.

Where it occurs

• North India (winter): the southern branch of the STJ blows along the southern side of the Himalaya and steers western disturbances from the Mediterranean region.
• 26–29 December 2024: a western disturbance met an easterly trough (a low-pressure trough in the easterly winds to the south). Together they caused deep thunderclouds and severe weather over Delhi and north-west and central India, with moisture from both the Arabian Sea and the Bay of Bengal [2].

• "Active subtropical jet stream dynamics" help such disturbances strengthen and move east [2].

• Tibetan Plateau: it splits the STJ in winter. Its summer heating drives the TEJ.

• Peninsular India and the Arabian Sea (~15°N): the TEJ in summer.
• East African coast (Kenya, Somalia) → Arabian Sea → west coast of India: the path of the Somali jet.
• North America, Europe and Central Asia: cold outbreaks when the polar front jet dips south.
• Japan: where Ooishi first measured jets. Pacific Ocean: where wartime pilots met them.
• Blocking examples:
• 2010: a blocking ridge over western Russia gave a long heat wave and wildfires. The trough downstream fed extreme monsoon rain over the Indus basin, Pakistan.
• June–July 2021 heat dome: a stalled ridge over the Pacific north-west of the USA and western Canada trapped hot air and gave record heat.

Don't confuse with

• Somali (Findlater) jet vs upper-air jets: the Somali jet is a low-level jet at 1–1.5 km. The polar front jet, STJ and TEJ all blow in the upper troposphere (9–16 km).
• Tropical easterly jet vs subtropical westerly jet: the TEJ is easterly, blows only in summer near 15°N and is driven by Tibetan heating. The STJ is westerly, sits near 30° and is over India mainly in winter.
• Polar vortex: a large area of low pressure and cold air circling each pole in the stratosphere and upper troposphere, between autumn and spring. A jet is a narrow band of fast wind just below the tropopause. A sudden stratospheric warming (a rapid rise in temperature in the polar stratosphere) weakens the vortex. Example: the February 2021 Texas cold wave, after a warming about 30 km above the North Pole [5].
• Trade winds and other surface winds: these blow near the ground and are slowed by friction. Jets blow high up where friction is almost nil, so they are near-geostrophic and much faster.

Prelims Hooks

• Lowest to highest jet: Somali jet (1–1.5 km) → polar front jet (9–12 km) → subtropical westerly jet (12–14 km) → tropical easterly jet (13–16 km).
• Only easterly jet among the main four: the Tropical easterly jet. It is a summer jet near 15°N over peninsular India, linked to Tibetan heating (Koteswaram, 1958).
• Trap: the Somali jet is a low-level jet that crosses the equator along the East African coast (Findlater, 1969). It is not an upper-tropospheric jet.
• Western disturbances are steered by the southern branch of the subtropical westerly jet in winter, not by the TEJ or the polar front jet.
• Trap: jet streams are strongest in winter, not summer. They blow just below the tropopause, not in the stratosphere.
• Eastbound flights are faster because they ride the westerly jets. Jets were first measured by Wasaburo Ooishi (Japan, 1920s).

Mains Points

• Jets and Indian weather (GS-I):
• In winter, the STJ south of the Himalaya steers western disturbances. These bring rain and snow that help the rabi crop and feed Himalayan snowpack.
• In early summer, the STJ's jump north of Tibet, the TEJ and the Somali jet together set up and supply the south-west monsoon.

• If the timing of any of these shifts, the monsoon can be delayed or weakened.

• Jet waves and blocking as disaster drivers (GS-III):

• Stalled Rossby waves turn ordinary weather into long-lasting extremes, such as the 2010 Russian heat wave and Pakistan floods, and the 2021 heat dome.

• One blocking pattern can hit regions thousands of kilometres apart. This calls for impact-based forecasting and early-warning systems.

• Climate change link (GS-I/III):

• Arctic amplification may weaken the pole-to-equator temperature contrast and make the jet wavier, with more blocking and cold outbreaks.
• Present this as a hypothesis that is still debated, not a settled fact, and stress the need for long-term observation.

Related concepts

• Polar front jet stream
• Subtropical westerly jet stream
• Tropical easterly jet stream
• Somali jet
• Rossby waves
• Polar vortex

Read more

• Jet streams, Rossby waves and the polar vortex