Pressure Belts, Winds and Jet Streams

Prerequisites: Climatology — overview · Earth’s Revolution and the Seasons

The Hook: Why the Flight Home Is Faster

Connect: Everyday Puzzle → Concept

Look at any airline timetable between New York and London: the eastbound flight to London is routinely scheduled shorter than the westbound return, even though the distance is the same. In winter, north India gets rain and Himalayan snow from western disturbances — weather systems that arrive from the Mediterranean side, thousands of kilometres to the west.

Both have the same cause: a fast, narrow river of westerly wind high in the troposphere, the jet stream. To understand it — and the monsoon it helps to switch on — we have to start with pressure and the forces that move air.

The Core Concept

Pressure and the forces that move air

Atmospheric pressure is the weight of the column of air above a place. Average sea-level pressure is about 1013.2 millibars (mb). Pressure falls with height — in the lower atmosphere by about 1 mb for every 10 m of ascent (NCERT). Lines joining places of equal pressure are isobars.

Wind is air moving from high to low pressure. Three forces control it:

Force	What it does
Pressure gradient force	Pushes air from high to low pressure; closely spaced isobars = steep gradient = strong wind
Coriolis force	Deflects moving air because the Earth rotates: to the right in the Northern Hemisphere, to the left in the Southern (Ferrel’s law). Zero at the Equator, maximum at the poles; stronger for faster winds
Friction	Slows surface winds; greatest over land, least over the sea; negligible above about 1–3 km

In the upper atmosphere, where friction is absent, the pressure gradient force and the Coriolis force balance, and winds blow parallel to the isobars — geostrophic winds. Jet streams are geostrophic.

Because of the Coriolis force, winds spiral around pressure centres:

System	Pressure at centre	Northern Hemisphere	Southern Hemisphere
Cyclone	Low	Anticlockwise, converging	Clockwise, converging
Anticyclone	High	Clockwise, diverging	Anticlockwise, diverging

Buys Ballot’s law: in the Northern Hemisphere, if you stand with your back to the wind, low pressure is on your left.

The pressure belts

Belt	Latitude (approx.)	Origin	Character
Equatorial low	0°–5°/10° N and S	Thermal — intense heating, rising air	Calm, rising air — the doldrums; the ITCZ (Inter-Tropical Convergence Zone)
Subtropical high	~30° N and S	Dynamic — air descending from upper troposphere	Calm, dry, descending air — the horse latitudes; hot deserts lie here
Subpolar low	~60° N and S	Dynamic — Earth’s rotation, convergence of westerlies and polar easterlies	Stormy; zone of temperate cyclones (polar front)
Polar high	90° N and S	Thermal — intense cold, sinking air	Cold, dense air

The belts shift with the overhead Sun — northward in the northern summer, southward in winter. This seasonal shift explains the monsoons and the Mediterranean climate’s winter rain.

The three-cell model of circulation

graph TD
    A[Equator: heating, air rises - equatorial low] --> B[Hadley cell 0-30 degrees: thermally direct]
    B --> C[30 degrees: air sinks - subtropical high]
    C --> D[Ferrel cell 30-60 degrees: thermally indirect]
    D --> E[60 degrees: air rises - subpolar low]
    E --> F[Polar cell 60-90 degrees: thermally direct]
    F --> G[Poles: cold air sinks - polar high]

The Hadley and polar cells are driven directly by heating and cooling; the Ferrel cell in between is driven by the other two, like a gear between two wheels.

Planetary (permanent) winds

Wind	Blows from → to	Direction (NH / SH)	Notes
Trade winds	Subtropical high → equatorial low	North-east trades / south-east trades	Steady; bring rain to the east coasts of continents in the tropics; leave the west sides dry
Westerlies	Subtropical high → subpolar low	South-westerlies / north-westerlies	Stronger and steadier in the Southern Hemisphere (little land): Roaring Forties, Furious Fifties, Shrieking Sixties
Polar easterlies	Polar high → subpolar low	North-easterly / south-easterly	Cold, dry; meet the westerlies at the polar front

Winds are named after the direction they come from.

Periodic and seasonal winds

Monsoons — seasonal reversal of winds caused by differential heating of land and sea and the seasonal shift of pressure belts (details in Indian Climate and the Monsoon).
Land and sea breezes — by day, land heats faster, so a sea breeze blows onshore; at night the land cools faster and a land breeze blows offshore.
Valley and mountain breezes — by day, warm air moves up the slopes (anabatic valley breeze); at night, cool dense air drains downslope (katabatic mountain breeze), often causing frost in valley bottoms.

Local winds

Wind	Region	Nature
Loo	North and north-western India (May–June)	Hot, dry, dusty afternoon wind
Foehn	Northern slopes of the Alps	Warm, dry wind descending the lee side; melts snow
Chinook	Eastern slopes of the Rockies	Warm, dry descending wind; called the “snow eater”
Sirocco	From the Sahara across the Mediterranean to Italy	Hot, dusty; can turn humid over the sea
Harmattan	From the Sahara over West Africa	Dry, dusty; called “the doctor” because it relieves humid heat
Mistral	Down the Rhône valley (France) to the Mediterranean	Cold, dry, strong
Bora	Eastern Adriatic coast	Cold, dry, gusty
Khamsin	Egypt	Hot, dry, dusty
Santa Ana	Southern California	Hot, dry; fans wildfires

Foehn and Chinook are both warm because descending air is compressed and heats up at the dry adiabatic rate.

Jet streams

Jet streams are narrow, fast-flowing bands of wind in the upper troposphere, near the tropopause, blowing mostly from west to east and following a wavy path (the meanders are called Rossby waves).

Jet stream	Location	Season / role
Subtropical westerly jet (STJ)	Around 30° latitude, above the subtropical highs	In winter it lies south of the Himalaya over north India and steers western disturbances; in summer it shifts north of the Tibetan Plateau
Polar front jet	Around 50°–60° latitude, above the polar front	Steers temperate cyclones; most variable
Tropical easterly jet (TEJ)	Over peninsular India and the Indian Ocean in summer	An easterly jet; appears in June after the STJ moves north; associated with the active south-west monsoon

The monsoon link: the summer heating of the Tibetan Plateau helps push the subtropical westerly jet north of the Himalaya; its withdrawal from over India and the appearance of the tropical easterly jet are closely tied to the sudden onset of the monsoon. In winter, the westerly jet’s return south brings western disturbances that give winter rain to Punjab, Haryana and the western Himalaya — valuable for rabi crops such as wheat.

Memory Anchors

Pressure belt sequence, Equator to pole: “Low, High, Low, High” at 0°, 30°, 60°, 90°.

Thermal vs dynamic: the two ends (equatorial low, polar high) are thermal; the two middle belts (subtropical high, subpolar low) are dynamic.

Coriolis: “Right in the North” — RN; “zero at zero” — no Coriolis at 0° latitude.

Southern westerlies: Roaring 40s, Furious 50s, Shrieking 60s — “Really Fast Sailing”.

Chinook = Canada/Rockies; Foehn = Alps — “Chinook Crosses the Continental divide; Foehn Falls off the Alps”.

For Prelims

Facts worth a mark

The Coriolis force is zero at the Equator and maximum at the poles; it deflects winds to the right in the Northern Hemisphere and to the left in the Southern.
The equatorial low and polar highs are thermal in origin; the subtropical highs and subpolar lows are dynamic.
The subtropical high-pressure belt is the zone of the horse latitudes; the equatorial low is the zone of the doldrums.
Trade winds blow from the north-east in the Northern Hemisphere and from the south-east in the Southern.
Geostrophic winds blow parallel to isobars because pressure gradient and Coriolis forces balance.
The subtropical westerly jet brings western disturbances to north India in winter; the tropical easterly jet appears over India in the monsoon season.
Chinook (Rockies) and Foehn (Alps) are warm, dry descending winds.

For Mains

Dimensions: forces (pressure gradient, Coriolis, friction); general circulation (cells, belts, planetary winds); seasonal shift of belts; jet streams and Rossby waves; impact on regional climates (deserts at 30°, Mediterranean winter rain, monsoon onset); climate-change angle (changes in jet-stream behaviour are an active research area).
Keywords: differential heating, ITCZ, Hadley circulation, subsidence, geostrophic balance, Rossby waves, western disturbances, tropical easterly jet, blocking highs.
Value-addition: the location of the world’s great hot deserts (Sahara, Arabian, Kalahari, Australian) near 30° is explained by subsiding air of the subtropical highs plus cold ocean currents on western coasts; the jet-stream shift over Tibet is a standard element of modern explanations of the Indian monsoon.

Common Traps

Trap #1: 'Westerlies blow towards the west'

Winds are named for where they come from. Westerlies blow from the west towards the east. Trade winds in the Northern Hemisphere blow from the north-east.

Trap #2: 'The Coriolis force is strongest at the Equator'

It is the opposite: the Coriolis force is zero at the Equator and greatest at the poles. That is why tropical cyclones do not form within about 5° of the Equator.

Trap #3: All pressure belts are caused by temperature

Only the equatorial low and polar highs are thermal. The subtropical highs form from descending air in the Hadley circulation, and the subpolar lows from the Earth’s rotation and the convergence of air masses — both are dynamic.

Practice

Prelims

UPSC Practice · Prelims GS I Qpbw-1

Consider the following statements about the Coriolis force:

It deflects winds to the right in the Northern Hemisphere.
It is maximum at the Equator.
It increases with the speed of the wind.

Which of the statements given above is/are correct?

A 1 only
B 1 and 3 only
C 2 and 3 only
D 1, 2 and 3

Solution

Statement 1 — correct (Ferrel’s law).
Statement 2 — wrong. It is zero at the Equator and maximum at the poles.
Statement 3 — correct. The deflecting force is proportional to the wind speed (and to the sine of the latitude).

Answer: B

UPSC-pattern practice question

UPSC Practice · Prelims GS I Qpbw-2

The “horse latitudes”, a zone of calm, descending, dry air associated with many of the world’s hot deserts, coincide with which of the following?

A Equatorial low-pressure belt
B Subpolar low-pressure belt
C Polar high-pressure belt
D Subtropical high-pressure belt

Solution

The subtropical high-pressure belt (around 30° N and S) has calm, subsiding air — the horse latitudes. Subsidence warms and dries the air, which is why many hot deserts lie near these latitudes. The equatorial low is the zone of the doldrums, a different calm belt with rising air.

Answer: D

UPSC-pattern practice question

UPSC Practice · Prelims GS I Qpbw-3

Consider the following pairs:

Local wind	Region
1. Chinook	Eastern slopes of the Rocky Mountains
2. Mistral	Rhône valley
3. Harmattan	West Africa
4. Foehn	Andes

How many of the pairs given above are correctly matched?

A Only one
B Only two
C Only three
D All four

Solution

Pairs 1, 2 and 3 are correct. Pair 4 is wrong: the Foehn blows down the northern slopes of the Alps. (Similar warm descending winds occur in many ranges, but the name “Foehn” belongs to the Alps.) So three pairs are correct.

Answer: C

UPSC-pattern practice question

Mains

UPSC Practice · Mains GS I 10 marks · 150 words Qpbw-m1

What are jet streams? Explain their influence on the weather and climate of the Indian subcontinent.

Solution

Introduction: Jet streams are narrow bands of fast, mostly westerly winds in the upper troposphere, near the tropopause. They are geostrophic and follow a wavy (Rossby wave) path.

Body:

Types: subtropical westerly jet (~30°), polar front jet (~50°–60°), and the seasonal tropical easterly jet over the Indian Ocean in summer.
Winter: the subtropical westerly jet lies south of the Himalaya and steers western disturbances into north-west India — winter rain and snow, useful for rabi crops; also cold waves when it dips south.
Summer and monsoon onset: heating of the Tibetan Plateau helps the westerly jet shift north of the Himalaya; the tropical easterly jet then develops over peninsular India — associated with the sudden onset and strength of the south-west monsoon.
Withdrawal: the westerly jet’s return south in autumn coincides with the retreat of the monsoon.
Variability: unusual jet positions are linked to delayed onset, breaks in the monsoon and extreme events.

Conclusion: Jet streams act as the upper-air switch for India’s seasons — they help explain both the winter western disturbances and the timing of the monsoon.

UPSC-pattern practice question

Quick Revision

Point	One-liner
Sea-level pressure	~1013.2 mb; falls ~1 mb per 10 m near the surface
Coriolis	Right in NH, left in SH; zero at Equator, maximum at poles
Pressure belts	Low (0°), high (30°), low (60°), high (90°)
Thermal / dynamic	Equatorial low and polar high thermal; others dynamic
Trade winds	NE in NH, SE in SH; towards equatorial low
Westerlies	Towards subpolar low; Roaring Forties in SH
Cells	Hadley (direct), Ferrel (indirect), Polar (direct)
Jet streams	Upper troposphere; STJ brings western disturbances; TEJ in monsoon
Warm descending winds	Foehn (Alps), Chinook (Rockies)

Key Takeaways

Uneven heating creates pressure differences; the Coriolis force turns the resulting winds into belts and spirals.
The four pressure belts and three wind belts shift with the Sun — that shift drives monsoons and Mediterranean winter rain.
Jet streams link the upper atmosphere to India’s surface weather: western disturbances in winter, monsoon onset in summer.

The Hook: Why the Flight Home Is Faster

The Core Concept

Pressure and the forces that move air

The pressure belts

The three-cell model of circulation

Planetary (permanent) winds

Periodic and seasonal winds

Local winds

Jet streams

Memory Anchors

For Prelims

For Mains

Common Traps

Practice

Prelims

Mains

Quick Revision

Key Takeaways

Related Topics

Keep going in this chapter