Introduction
Air expands when heated and gets compressed when cooled. This creates variations in atmospheric pressure, which make air move from high pressure to low pressure. Air in horizontal motion is wind. Winds redistribute heat and moisture across the planet, keeping its overall temperature constant. The vertical rising of moist air forms clouds and brings precipitation. This chapter explains pressure, the forces that control winds, the general circulation of the atmosphere, air masses, fronts, and violent storms like cyclones, thunderstorms and tornadoes.
Key Concepts
1. Atmospheric Pressure
The weight of a column of air in a unit area from mean sea level to the top of the atmosphere is atmospheric pressure.It is expressed in millibars (mb). Average sea-level pressure is 1,013.2 mb.It is measured with a mercury barometer or an aneroid barometer.Air at the surface is denser because of gravity, so pressure is higher there.2. Vertical Variation of Pressure
In the lower atmosphere, pressure falls about 1 mb for each 10 m of rise, though not always at the same rate.Standard atmosphere (Table 9.1): sea level 1,013.25 mb, 15.2°C; 1 km 898.76 mb, 8.7°C; 5 km 540.48 mb, –17.3°C; 10 km 265.00 mb, –49.7°C.The vertical pressure gradient force is much larger than the horizontal one, but it is balanced by an almost equal and opposite gravitational force, so we do not feel strong upward winds.3. Horizontal Distribution of Pressure
Isobars are lines connecting places of equal pressure. Pressure is reduced to sea level before plotting, to remove the effect of altitude.A low is enclosed by isobars with the lowest pressure in the centre; a high has the highest pressure in the centre.4. World Distribution of Sea-Level Pressure
Equatorial low near the equator.Subtropical highs along 30° N and 30° S.Sub-polar lows along 60° N and 60° S.Polar highs near the poles.These belts are not permanent; they shift with the apparent movement of the sun — southward in the northern winter and northward in summer.5. Forces Affecting Wind
Pressure gradient force — the rate of change of pressure with distance. Strong where isobars are close, weak where they are far apart.Frictional force — slows the wind; greatest at the surface and felt up to 1–3 km; minimal over the sea.Coriolis force — due to the earth's rotation; described by a French physicist in 1844. It deflects wind to the right in the northern hemisphere and to the left in the southern hemisphere. The deflection is greater at higher wind speeds. The book states it is directly proportional to the angle of latitude — maximum at the poles, absent at the equator.Because the Coriolis force is zero at the equator, wind blows straight across isobars into a low and fills it — which is why tropical cyclones do not form near the equator.6. Pressure and Wind
Around a low, the circulation is cyclonic: anticlockwise in the northern hemisphere, clockwise in the southern.Around a high, it is anticyclonic: clockwise in the north, anticlockwise in the south.Over a low, air converges and rises; over a high, air subsides and diverges.Air also rises through eddies, convection, orographic uplift and uplift along fronts — rising air is essential for clouds and rain.Geostrophic wind: 2–3 km up, free of friction, when the pressure gradient force is balanced by the Coriolis force, the wind blows parallel to the isobars.7. General Circulation of the Atmosphere
The pattern of planetary winds depends on latitudinal heating, pressure belts, their migration with the sun, land–sea distribution and the earth's rotation.
At the ITCZ (Inter Tropical Convergence Zone), strong insolation causes convection and low pressure. Air rises to about 14 km, moves poleward and piles up near 30° N and S, where it sinks to form the subtropical high. Near the surface it returns to the equator as the easterlies. This tropical cell is the Hadley cell.In middle latitudes: the Ferrel cell, with surface westerlies.At the poles: cold dense air subsides and flows out as the polar easterlies — the polar cell.Heat transfer from low to high latitudes maintains this circulation. The winds also drive slow ocean currents, and the oceans feed energy and water vapour back to the air.8. El Niño, Southern Oscillation and ENSO
Warm water from the central Pacific drifts to the South American coast and replaces the cool Peruvian current — this is El Niño.The associated pressure change between the central Pacific and Australia is the Southern Oscillation. Together they form ENSO.In strong ENSO years: heavy rain on the arid west coast of South America, drought in Australia and sometimes India, and floods in China. ENSO is used for long-range forecasting.9. Seasonal and Local Winds
Monsoons are the most pronounced seasonal shift, especially over south-east Asia.Land and sea breezes — by day, land heats faster → low pressure over land → sea breeze from sea to land. At night land cools faster → land breeze from land to sea.Valley breeze — by day, heated slopes draw air upslope from the valley.Mountain wind — at night, cooled dense air descends into the valley. Cold air draining from high plateaus and ice fields is the katabatic wind.A warm dry wind on the leeward side of mountains, warmed adiabatically as it descends, can melt snow quickly.10. Air Masses
An air mass is a large body of air with little horizontal variation in temperature and moisture. The homogeneous surfaces where they form are source regions.Five source regions: warm tropical oceans; subtropical hot deserts; relatively cold high-latitude oceans; very cold snow-covered continents; permanently ice-covered Arctic and Antarctica.Types: Maritime tropical (mT), Continental tropical (cT), Maritime polar (mP), Continental polar (cP), Continental arctic (cA). Tropical air masses are warm; polar ones are cold.11. Fronts
The boundary zone where two different air masses meet is a front; its formation is frontogenesis.Cold front — cold air moves towards warm air. Warm front — warm air moves towards cold air. Stationary front — the front doesn't move. Occluded front — an air mass is fully lifted above the surface.Fronts occur in middle latitudes, with steep temperature and pressure gradients, abrupt temperature changes, rising air, clouds and precipitation.12. Extra-Tropical Cyclones
Form along the polar front in mid and high latitudes. Warm air moves north and cold air south, setting up anticlockwise circulation in the northern hemisphere, with a warm front and a cold front.Warm air glides over cold air ahead of the warm front, causing clouds and precipitation; cumulus clouds form along the cold front. The faster cold front overtakes the warm front, the warm air is lifted completely, the front is occluded and the cyclone dissipates.Differences from tropical cyclones: they have a clear frontal system, cover a larger area, can form over land and sea, have lower wind speeds, and move west to east (tropical cyclones move east to west).13. Tropical Cyclones
Violent storms over tropical oceans that bring violent winds, very heavy rain and storm surges.Names: Cyclones (Indian Ocean), Hurricanes (Atlantic), Typhoons (Western Pacific and South China Sea), Willy-willies (Western Australia).Favourable conditions: (i) large sea surface above 27°C; (ii) presence of the Coriolis force; (iii) small variations in vertical wind speed; (iv) a pre-existing weak low or low-level cyclonic circulation; (v) upper divergence.Energy comes from condensation in towering cumulonimbus clouds. On reaching land the moisture supply is cut off and the storm dies. The place where it crosses the coast is the landfall. Cyclones that cross 20° N generally recurve and are more destructive.Structure: a calm eye with subsiding air, surrounded by the eye wall where winds reach up to 250 km/h with torrential rain. The circulating system is 150–250 km across; storms over the Bay of Bengal, Arabian Sea and Indian Ocean are 600–1,200 km across and move about 300–500 km per day.14. Thunderstorms and Tornadoes
Thunderstorms are caused by intense convection on moist hot days — a well-grown cumulonimbus cloud with thunder and lightning. Hail forms if clouds reach sub-zero heights; with little moisture they can cause duststorms.A tornado is a spiralling wind descending like an elephant's trunk from a severe thunderstorm, with very low central pressure. Tornadoes occur mostly in middle latitudes; over the sea they are water spouts.Diagrams to Know
Isobars around a low and a high in the northern hemisphere, with wind arrows.Geostrophic wind blowing parallel to straight isobars.Three-cell model — Hadley, Ferrel and polar cells with easterlies, westerlies and polar easterlies.Land and sea breezes (day and night).Vertical sections of warm, cold and occluded fronts.Vertical section of a tropical cyclone showing the eye and eye wall.Key-Facts Sheet
Sea-level pressure ≈ 1,013.2 mb; drop ≈ 1 mb per 10 m.Pressure belts: equatorial low, subtropical highs (30°), sub-polar lows (60°), polar highs.Coriolis: right in NH, left in SH; zero at equator.Cyclone: NH anticlockwise, SH clockwise; anticyclone the reverse.Tropical cyclones need sea temperature > 27°C; winds up to 250 km/h.Exam Focus
MCQ from the exercise: if surface pressure is 1,000 mb, pressure at 1 km ≈ 900 mb (about 1 mb per 10 m); the ITCZ is near the equator; wind around a low in the NH is anticlockwise; the Siberian Plain is an air-mass source region.Why are the tropical winds north-easterlies when the gradient is north to south? Outline: the Coriolis force deflects wind to the right in the NH, turning a northerly flow into a north-easterly.Why do tropical cyclones form over seas? Outline: they need warm water above 27°C and continuous moisture; latent heat from condensation powers them, and the eye wall has the strongest winds and heaviest rain.Why are subtropical highs at 30°? Outline: air rising at the ITCZ moves poleward, accumulates and cools near 30°, and sinks.Key Terms
Atmospheric pressure (वायुमंडलीय दाब)Isobar (समदाब रेखा)Pressure gradient (दाब प्रवणता)Coriolis force (कोरिऑलिस बल)Geostrophic wind (भू-विक्षेपी पवन)Inter Tropical Convergence Zone (अंतः उष्णकटिबंधीय अभिसरण क्षेत्र)Hadley cell (हेडली कोष्ठ)Air mass (वायु राशि)Front (वाताग्र)Frontogenesis (वाताग्र उत्पत्ति)Tropical cyclone (उष्णकटिबंधीय चक्रवात)Landfall (स्थलावतरण)Tornado (टॉरनेडो)Katabatic wind (अवरोही पवन)Common Mistakes
Reversing cyclone directions — low = anticlockwise in the NH, clockwise in the SH.Saying the Coriolis force is strongest at the equator — it is absent there.Confusing geostrophic wind (upper air, parallel to isobars) with surface wind.Mixing up warm and cold fronts — name the front after the air mass that is advancing.Forgetting that tropical cyclones move east to west, extra-tropical ones west to east.