Atmospheric Pressure, Winds and General Circulation

1. Atmospheric pressure and its vertical variation

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• Atmospheric pressure is the weight of a column of air over a unit area, from mean sea level to the top of the atmosphere. Gravity pulls air down, so air is densest and pressure is highest at the surface.
• Average sea-level pressure is 1,013.2 mb (the standard value is 1,013.25 mb). We do not feel it because air presses on us from all sides and the pressure inside our bodies balances it (Class 9).
• Millibar (mb): 1 mb = 1 hectopascal (hPa) = 100 pascals. 1,013.25 mb equals 760 mm of mercury. Weather maps now show hPa, which has the same numbers.
• Vertical variation of pressure: near the ground, pressure falls about 1 mb for every 10 m of height. The rate is not constant because the air gets thinner higher up.

Level	Pressure (mb)	Temperature (°C)
Sea level	1,013.25	15.2
1 km	898.76	8.7
5 km	540.48	–17.3
10 km	265.00	–49.7

• About half of all the air lies below 5.5 km. Khardung La in Ladakh has about 650 mb, so there is less oxygen and visitors need time to adjust (Class 7).
• The upward pressure force is much stronger than the sideways one, but gravity balances it (hydrostatic balance). That is why we get no strong upward winds.

2. Horizontal distribution: isobars and the world's pressure belts

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• Isobar: a line joining places of equal pressure. Readings are first reduced to sea level so that differences in height do not hide the pattern. Isobars close together mean a steep gradient and strong wind.
• A low has the lowest pressure at its centre and a high has the highest. A trough is a stretched-out low, a ridge is a stretched-out high, and a col is the saddle between two highs and two lows. Warm air rises and makes a low (cloudy, wet). Cold air sinks and makes a high (clear, dry).

Belt	Latitude	Origin	Notes
Equatorial low	0–5°	Thermal (heating makes air rise)	Doldrums
Subtropical high	~30° N/S	Dynamic (air piles up, cools and sinks)	Horse latitudes; hot deserts
Subpolar low	~60° N/S	Dynamic (westerlies meet polar easterlies and rise)	Icelandic and Aleutian lows
Polar high	Poles	Thermal (cold, dense, sinking air)	Over ice

• These are the four pressure belts. Under the subtropical highs, sinking air and an inversion stop rain. So the Sahara, Arabian, Thar, Kalahari, Atacama and Australian deserts lie at 20–30°, on the west sides of continents.
• In the Northern Hemisphere the belts break into separate cells: the Azores–Bermuda high, the Hawaiian high, the Siberian high (winter) and the Asian thermal low (summer).
• Shifting of pressure belts: the belts follow the Sun, moving about 5–10° north in the northern summer and south in winter. This gives the Mediterranean winter rain and summer drought, and gives the monsoon lands winds that reverse with the season.

3. Wind and the forces that steer it

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• Wind is the horizontal movement of air from high to low pressure. A wind is named after the direction it comes from, so a westerly blows from the west.
• Instruments (Class 7):
• The wind vane points towards the direction the wind comes from. A wind sock does the same job at airfields.
• The anemometer has cups that spin faster in stronger wind; it gives speed in km/h or knots.

• The Beaufort scale grades wind by what it does to things around us.

• Pressure gradient force: the rate at which pressure changes with distance. It acts at right angles to the isobars, from high to low, and is strong where the isobars are close together.

• Frictional force: drag from the ground that slows the wind. It is greatest at the surface, reaches up 1–3 km and is least over the sea. Slower wind is bent less, so surface winds cross the isobars at an angle towards low pressure.
• Coriolis force: an apparent force caused by the Earth's spin. It bends moving air to the right in the Northern Hemisphere and to the left in the Southern (Ferrel's law). It grows with wind speed and with the sine of latitude: it is greatest at the poles and zero at the equator.
• NCERT says it is "directly proportional to the angle of latitude" and dates Coriolis's work to 1844. Correct: sine of latitude, and a paper of 1835.
• At the equator air flows straight into a low and fills it, so no cyclones form within about 5° of the equator.

• It explains why winds blowing towards the equator become north-easterly trades.

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

• Geostrophic wind: above 2–3 km there is no friction. The pressure gradient force and the Coriolis force balance, so the wind blows parallel to straight isobars.

4. Circulation around lows and highs

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System	Centre	Northern Hemisphere	Southern Hemisphere	Weather
Cyclonic circulation	Low	Anticlockwise	Clockwise	Cloud, rain
Anticyclonic circulation	High	Clockwise	Anticlockwise	Clear, dry, settled

• Convergence and divergence:
• Over a low, surface air converges (flows in), rises and diverges (spreads out) aloft. The rising air cools and forms cloud and rain.

• Over a high, air converges aloft, sinks and diverges at the surface.

• Air is also lifted by eddies, convection, mountains (orographic uplift) and fronts. Rising air is essential for cloud and rain.

• Surface and upper-air flows are linked. Divergence aloft helps a low deepen, which is a condition for tropical cyclones.
• Heat dome (beyond NCERT): a strong, long-lasting high aloft that traps hot air like a lid on a pot. The sinking air warms as it is squeezed, the sky stays clear and the soil dries out.
• In the Pacific Northwest heat dome of June 2021, Lytton in British Columbia reached 49.6 °C, a record for Canada.
• Stalled Rossby waves (Section 7) favour such blocks. Indian heat waves often come with an anticyclone over north-west India.

5. The general circulation of the atmosphere: three cells

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• The general circulation of the atmosphere is the pattern of planetary winds. NCERT lists five factors that shape it: 1. uneven heating by latitude 2. the pressure belts 3. the shifting of the belts with the Sun 4. how land and sea are spread out 5. the Earth's rotation

• It carries heat from the tropics, which have extra heat, to the poles, which are short of heat. It also drives the large, slow ocean currents, while the oceans feed heat and water vapour back to the air.

• Hadley cell (George Hadley, 1735), 0–30°:
• Strong sunshine at the ITCZ (Inter-Tropical Convergence Zone, the belt where the trades meet) makes air rise up to about 14 km.
• Aloft, the air flows towards the poles, piles up and cools near 30°, and sinks to form the subtropical highs.

• It returns to the equator as the surface trade winds (NCERT calls them "easterlies").

• The ITCZ normally lies near the equator. It follows the thermal equator (the line of greatest heating) and in July swings far north over South Asia, where it forms the monsoon trough.

• Ferrel cell (William Ferrel, 1856), 30–60°: surface winds are the westerlies. The cell is thermally indirect: the Hadley and polar cells on either side drive it, not direct heating. In reality, travelling cyclones and anticyclones carry most of the heat in middle latitudes.
• Polar cell: cold, dense air sinks at the poles, flows towards about 60° as the polar easterlies and rises at the subpolar low.

6. The planetary winds: trades, westerlies and polar easterlies

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• Trade winds blow from the subtropical highs to the equatorial low. The Coriolis force bends them into north-easterlies in the Northern Hemisphere and south-easterlies in the Southern. They are the steadiest winds on Earth and meet at the ITCZ.

• They bring rain to tropical east coasts (eastern Brazil, eastern Madagascar, Queensland) and leave west coasts dry.

• Southeast trade winds: in the northern summer they cross the equator (between about 40° and 60°E). Bent to the right, they become India's south-west monsoon.

• Westerlies blow from the subtropical highs to the subpolar lows, between about 35° and 60°. They come from the south-west in the Northern Hemisphere and from the north-west in the Southern.

• They bring rain and cyclones to mid-latitude west coasts (British Columbia, north-west Europe, southern Chile, New Zealand) and winter rain to Mediterranean lands.

• Roaring Forties: at 40–50°S there is almost no land to slow the westerlies, so they blow strong and steady. Further south come the Furious Fifties and the Shrieking Sixties. Clipper ships used them on the Australia route.

• Polar easterlies: cold, dry winds from the polar highs to the subpolar lows. Where they meet the westerlies along the polar front, temperate cyclones form.
• Atmospheric river (beyond NCERT): a long, narrow band of water vapour carried from the tropics towards higher latitudes. When it hits mountains it can cause extreme rain or snow, as with the "Pineapple Express" that reaches California. Studies of similar plumes over India are ongoing (verify current).

7. Jet streams, Rossby waves and the polar vortex

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• Jet stream: a narrow band of very fast wind, often 150–300 km/h, in the upper troposphere at about 9–16 km, just below the tropopause. Jets form where temperature contrasts are sharp, blow nearly geostrophically and are strongest in winter. Wasaburo Ooishi discovered them over Japan in the 1920s. Eastbound flights are faster because they ride the jets.

Jet	Height / place	Key role
Polar front jet stream	9–12 km, 40–60°	Steers temperate cyclones; brings cold outbreaks when it dips
Subtropical westerly jet stream	12–14 km, ~30°	Splits around Tibet in winter; its southern branch over north India steers western disturbances; its move north helps the monsoon start
Tropical easterly jet stream	13–16 km, ~15°N over peninsular India and the Arabian Sea	Appears in summer as Tibet heats up (Koteswaram, 1958); NCERT links it to the monsoon "burst"
Somali jet	Low level, 1–1.5 km, East African coast and Arabian Sea	Crosses the equator and carries much of the monsoon's moisture (Findlater, 1969)

• Rossby waves: giant loops in the upper westerlies, caused by the Earth's spin and how its effect changes with latitude. Usually four to six of them circle the globe. When they stall ("blocking"), the weather gets stuck: the 2010 Russian heat wave and Pakistan floods, and the 2021 heat dome.
• Polar vortex: a large area of low pressure and cold air circling each pole in the stratosphere and upper troposphere. A sudden warming of the stratosphere can weaken or split it and let Arctic air spill south, as in the Texas cold wave of February 2021.
• Whether Arctic warming makes the jet wavier is still debated.

8. The circulation and the oceans: Walker circulation, El Nino and ENSO

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• Normal year:
• The trades push warm surface water west, where it piles up near Indonesia (the Western Pacific Warm Pool).
• Cold water wells up off Peru and feeds rich anchovy fisheries.

• Air rises over the warm west and sinks over the cool east. This loop is the Walker circulation, named by Bjerknes (1969) after Sir Gilbert Walker of IMD.

• El Nino: every 3–7 years, warm water from the central Pacific drifts to the Peru coast around Christmas and replaces the cold Peru current. The trades weaken, the Walker loop weakens or moves east, upwelling stops and the fisheries collapse.

• Southern Oscillation: a see-saw of pressure between the eastern Pacific and the Indian Ocean–Australia region. The Southern Oscillation Index (SOI, Tahiti pressure minus Darwin pressure) becomes strongly negative in an El Nino.
• ENSO is El Nino and the Southern Oscillation together. In strong events (Class 11) the dry coast of Peru gets heavy rain, Australia and sometimes India face drought, and China floods. La Nina is the opposite: stronger trades and a cooler eastern Pacific, which usually help the Indian monsoon.
• Nino 3.4 region: 5°N–5°S, 120°–170°W. NOAA's Oceanic Nino Index declares an event at ±0.5 °C on three-month averages, sustained over several overlapping seasons. Australia and IMD use their own criteria.
• El Nino Modoki (Ashok and others, 2007): the warmest water lies in the central Pacific, with cooler water on both sides. It can hurt the Indian monsoon even when it is weak.
• Strong El Ninos: 1982–83, 1997–98, 2015–16 and 2023–24. A "triple-dip" La Nina ran from 2020 to 2023. In 1997 India's monsoon was normal despite El Nino, helped by a positive Indian Ocean Dipole.

9. Seasonal and local winds: breezes and mountain winds

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• Seasonal winds: the belts of heating, pressure and wind shift with the seasons. The strongest result is the monsoon of South and South-East Asia.
• Local winds come from differences in how surfaces heat and cool over a day or a year.
• Sea breeze (by day):
• The land heats faster, its air rises and a low forms.
• Cooler air from the higher-pressure sea moves in.

• It is strongest in the afternoon and cools coastal towns.

• Land breeze (at night): the land cools faster, pressure becomes higher over land and air flows out to sea. It is gentler because the night-time contrast is smaller. Fishermen sail out with the land breeze and return with the sea breeze.

• Valley breeze (by day): sunlit slopes heat the air above them, which rises and pulls air up the valley. It often builds afternoon clouds over the peaks.
• Mountain breeze (at night): the slopes cool, and dense cold air sinks into the valley. This adds to valley inversions and frost.
• Katabatic wind: cold, dense air from high plateaus and ice fields that drains downhill under gravity. The strongest blow off the Antarctic and Greenland ice sheets, and the Adriatic bora is one type.

10. Named regional winds of the world

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• Foehn wind: a warm, dry wind that runs down the lee side (the side away from the wind) of a mountain range. Class 11 describes this process without naming the wind.
• The air loses its moisture as rain on the windward side.
• It then warms as it is squeezed while going down (the dry adiabatic rate).

• The foehn of the northern Alps melts snow and helps grapes ripen.

• Chinook: the foehn of the eastern Rockies (USA and Canada), blowing in winter and spring. Called the "snow eater", it clears pastures for cattle and can raise temperatures by 20 °C or more within hours. Similar winds are the zonda (Argentina) and the norwester (New Zealand).

• Sirocco: a hot, dusty south or south-east wind from the Sahara that reaches Sicily and southern Italy. It picks up moisture over the Mediterranean and can bring dusty "blood rain". Related Saharan winds are the khamsin (Egypt), ghibli (Libya) and leveche (Spain).
• Harmattan: a dry, dusty north-easterly from the Sahara over West Africa, from about November to March. It is called "the doctor" because it brings relief from the humid heat of the Guinea coast.
• Santa Ana winds: hot, dry offshore winds that blow from the inland deserts to southern California in autumn and winter. They spread wildfires, as in the Los Angeles fires of January 2025.
• Loo: a hot, dry westerly over the north-west plains of India in May–June. Australia's brickfielder is similar.
• Mistral: a cold, dry, strong northerly that funnels down the Rhone valley to the Mediterranean coast of France in winter and spring. Farmers there plant rows of cypress trees to shield crops. Other cold winds:
• bora (Adriatic)
• blizzard (North America)
• purga or buran (Siberia and Central Asia)
• pampero (Argentina)

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Exam angles

Prelims — high-yield facts and traps

• 1,013.2 mb average sea-level pressure; about 1 mb fall per 10 m near the ground; 1 mb = 1 hPa; at 1,000 mb surface pressure, the pressure at 1 km is about 900 mb.
• Equatorial low and polar high are thermal; subtropical high and subpolar low are dynamic. "Subtropical highs are caused by intense heating" — FALSE.
• Coriolis force: right in the NH, left in the SH; zero at the equator, greatest at the poles; grows with wind speed and with the sine of latitude. "Proportional to the angle of latitude" — FALSE (NCERT slip). The paper dates from 1835, not 1844.
• Geostrophic wind blows parallel to isobars above 2–3 km; friction makes surface winds cross isobars towards the low.
• Cyclone: anticlockwise in the NH and clockwise in the SH; anticyclone the reverse.
• Hadley cell 0–30°, Ferrel cell 30–60° (thermally indirect), polar cell 60–90°. Roaring Forties at 40–50°S.
• SOI = Tahiti minus Darwin; it is negative in El Nino. Nino 3.4 region: 5°N–5°S, 120°–170°W. Bjerknes named the Walker circulation after Gilbert Walker. El Nino Modoki has its warmth in the central Pacific.
• The tropical easterly jet appears in summer over peninsular India. The Somali jet is a low-level jet. The subtropical westerly jet steers western disturbances.
• Local wind matches: chinook – Rockies; foehn – Alps; mistral – Rhone valley; sirocco – Sahara to Italy; harmattan – West Africa; Santa Ana – California; bora – Adriatic; loo – north-west India.

Mains — GS-I themes

1. Discuss the factors that affect the speed and direction of wind.
2. Explain the three-cell model and why subtropical highs form near 30°N and 30°S. How well does the model fit reality?
3. ENSO and the Indian monsoon: the mechanism, El Nino Modoki and the IOD, and what they mean for farming and food security.
4. Jet streams and Indian weather: western disturbances and monsoon onset and withdrawal.
5. How are blocking highs and heat domes linked to a wavier jet stream in a warming world?
6. Local winds as a product of relief and land–sea contrast: foehn-type winds and their economic effects.

Current-affairs hooks

• El Nino and La Nina declarations and monsoon forecasts by NOAA, WMO and IMD (verify current).
• Western disturbances bringing unseasonal rain or snow to north India, and changes in how often they come.
• Heat domes and record heat in North America, Europe and India.
• Polar vortex disruptions and cold waves.
• Atmospheric-river floods in California, Chile and Europe, and studies on India.
• Wildfires driven by Santa Ana or foehn-type winds (Los Angeles, January 2025).