Atmosphere, Insolation, Pressure Belts and Winds

The air around us

The atmosphere is a thin envelope of gases held by gravity. It protects life from harmful radiation and meteors, keeps the Earth warm through the greenhouse effect, and moves heat and moisture around the planet as weather and climate. Climatology is a high-weight area for UPSC because it explains the Indian monsoon, cyclones, droughts and climate change. This lesson covers the composition and structure of the atmosphere, the heat budget, temperature patterns, pressure belts, planetary and local winds, and jet streams.

Composition

By volume, dry air is about 78 per cent nitrogen, 21 per cent oxygen, 0.93 per cent argon and about 0.04 per cent carbon dioxide (over 420 parts per million in recent years, up from about 280 ppm before industrialisation). Small amounts of ozone, methane, nitrous oxide and noble gases are also present. Water vapour varies from almost zero in deserts to about 4 per cent in humid tropics and is the most important greenhouse gas by quantity. Dust and aerosols act as condensation nuclei for clouds and scatter sunlight, giving red skies at sunrise and sunset.

Structure of the atmosphere

The troposphere is thicker at the equator because strong heating and convection lift air higher.

Insolation and the heat budget

Insolation is incoming solar radiation, mostly in short wavelengths. Of the energy reaching the top of the atmosphere, roughly 35 units out of 100 are reflected back to space by clouds, dust and the surface – this is the Earth's albedo. About 14 units are absorbed by the atmosphere and 51 units by the surface. The surface then radiates energy as long-wave (terrestrial) radiation, which greenhouse gases absorb and re-radiate, warming the lower atmosphere. Over a year, incoming and outgoing energy balance – the heat budget – although the tropics have a surplus and the poles a deficit, which winds and ocean currents correct by moving heat poleward.

Factors affecting temperature: latitude (angle of the Sun's rays), altitude, distance from the sea (continentality), ocean currents, prevailing winds, relief and cloud cover. Temperature inversion – a layer where temperature rises with height – occurs on long winter nights with clear skies and calm air, especially in valleys; it traps pollution and fog, as seen in north India's winter smog.

Pressure belts

Air pressure falls with height and varies with temperature. The Earth has a pattern of seven pressure belts:

1. Equatorial low (Doldrums), about 5°N–5°S – rising hot air, calm winds, heavy convectional rain. The meeting zone of trade winds is the Inter-Tropical Convergence Zone (ITCZ).
2. Subtropical highs (Horse latitudes), about 30°N and 30°S – descending air, dry; most of the world's hot deserts lie here.
3. Subpolar lows, about 60°N and 60°S – meeting of warm westerlies and cold polar air; frontal cyclones.
4. Polar highs at the poles – cold, dense air.

The equatorial and polar belts are thermally induced; the subtropical and subpolar belts are dynamically induced by the rotation of the Earth and air circulation. The belts shift about 5° to 10° north and south with the apparent movement of the Sun, which is vital for the Indian monsoon and Mediterranean climates.

Planetary winds

Wind blows from high to low pressure, but is deflected by the Coriolis force – to the right in the Northern Hemisphere and left in the Southern (Ferrel's law). The Coriolis force is zero at the equator and greatest at the poles, which is why cyclones do not form within about 5° of the equator.

• Trade winds – from the subtropical highs to the equatorial low; north-east trades in the north and south-east trades in the south. Steady and reliable; they bring rain to the eastern coasts of continents in the tropics.
• Westerlies – from the subtropical highs to the subpolar lows; strongest in the Southern Hemisphere because there is little land, giving the Roaring Forties, Furious Fifties and Shrieking Sixties.
• Polar easterlies – from the polar highs to the subpolar lows.

The three-cell model describes the circulation: the Hadley cell (equator to 30°), the Ferrel cell (30°–60°) and the Polar cell. The Walker circulation is an east–west circulation over the Pacific linked with El Niño.

Periodic and local winds

• Land and sea breezes – daily reversal caused by differential heating of land and water: sea breeze by day, land breeze by night.
• Mountain and valley breezes – valley breeze up the slopes by day (anabatic), mountain breeze down by night (katabatic).
• Monsoons – seasonal reversal of winds over large areas, most striking in South and South-East Asia (covered in detail under Indian geography).

Local winds of the world:

Jet streams

Jet streams are narrow belts of very fast winds (often over 150–300 km/h) blowing from west to east near the tropopause, at about 9–14 km height. The subtropical westerly jet lies around 30° latitude and the polar front jet around 40°–60°. They meander in large waves called Rossby waves. Jet streams steer weather systems, affect flight times and play a key role in the onset and withdrawal of the Indian monsoon – the westerly jet shifts north of the Himalayas in summer, and a tropical easterly jet develops over peninsular India during the monsoon. Western disturbances, which bring winter rain and snow to north-west India, travel along the westerly jet from the Mediterranean.

Mains practice question

Explain the formation of the planetary pressure belts and wind systems. How does their seasonal shift influence the climate of India? (250 words)

Answer framework

• Intro: unequal heating and Earth's rotation.
• Belts: thermal (equatorial low, polar high) and dynamic (subtropical high, subpolar low); three-cell model; Coriolis force.
• Winds: trades, westerlies, polar easterlies.
• Seasonal shift: ITCZ moves north in summer → south-west monsoon; jets shift; western disturbances in winter.
• Conclusion: global circulation links India's weather to the world.