Atmosphere, Winds and Ocean Currents

Composition of air (dry air, by volume)

Water vapour varies from almost 0 to about 4%.

Layers of the atmosphere

1. Troposphere – all weather happens here. It is about 8 km thick at the poles and about 18 km at the equator. Temperature falls with height (about 6.5°C per km).
2. Stratosphere – contains the ozone layer, which absorbs harmful UV rays. It is calm, so jet aircraft fly in the lower stratosphere.
3. Mesosphere – the coldest layer; meteors burn up here.
4. Thermosphere – includes the ionosphere, which reflects radio waves; auroras form here.
5. Exosphere – the outermost layer, merging into space.

Pressure belts and planetary winds

• Belts: the Equatorial Low (doldrums), the Subtropical Highs (horse latitudes, ~30°), the Subpolar Lows (~60°) and the Polar Highs.
• Winds blow from high to low pressure and are deflected by the Coriolis force – to the right in the Northern Hemisphere and to the left in the Southern.
• Trade winds (30° → equator), Westerlies (30° → 60°; the "Roaring Forties" in the south) and Polar Easterlies.

Ocean currents

Warm currents usually flow from the equator towards the poles, and cold currents the other way. Cold currents along west coasts help create deserts (Atacama, Namib).

Background: the air around us

The atmosphere is mostly nitrogen (about 78%) and oxygen (about 21%), with argon, carbon dioxide (about 0.04% and rising), water vapour and dust. CO₂ and water vapour are small in volume but crucial: they trap heat (the greenhouse effect). Dust particles act as condensation nuclei for clouds.

Insolation and heat balance

The Earth receives solar energy (insolation) as short waves and returns it as long-wave terrestrial radiation. The albedo (reflectivity) of ice and snow is high, so melting ice makes the Earth absorb more heat – a feedback that speeds up warming. Heat moves through conduction, convection and advection (horizontal movement – the loo is an example).

Pressure belts and planetary winds

Air moves from high to low pressure. The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator and greatest at the poles.

Winds: trade winds (from 30° to the equator – north-east and south-east trades); westerlies (30°–60°, strongest in the south – "Roaring Forties, Furious Fifties, Screaming Sixties"); polar easterlies.

Local winds to remember:

• Foehn (Alps) and Chinook (Rockies, "snow eater") – warm, dry winds descending the leeward side;
• Sirocco (from the Sahara to Italy);
• Mistral (cold, France);
• Harmattan ("the doctor", West Africa – dry wind bringing relief from humidity);
• Bora (Adriatic);
• Loo (north India).

Air masses, fronts and cyclones

Naming: cyclones in the North Indian Ocean are named in rotation from a list submitted by member countries of the WMO/ESCAP Panel (including India, Bangladesh, Pakistan and others). The IMD's RSMC in New Delhi is the regional centre for the area.

Ocean currents – rivers within the sea

Causes: planetary winds, differences in temperature and salinity (density), the Coriolis force and the shape of coastlines. Warm currents flow from the equator towards the poles; cold currents flow from the poles towards the equator (usually along the western coasts of continents in the tropics).

Effects:

• Climate: the North Atlantic Drift keeps NW Europe's ports ice-free; cold currents create coastal deserts (Atacama – Humboldt; Namib – Benguela).
• Fishing: where warm and cold currents meet, plankton grows, making rich fishing grounds (the Grand Banks off Newfoundland, near Japan).
• Fog: where currents meet.
• Navigation.

Case study: El Niño – when the Pacific changes the world's weather

Normally, trade winds push warm water west towards Indonesia, while cold, nutrient-rich water upwells along the Peru coast (excellent anchovy fishing). During El Niño, the trades weaken and warm water spreads east. Peru gets heavy rain and fish catches collapse, while Australia and Indonesia face drought. India's monsoon often weakens. La Niña is the opposite. Together they make up ENSO. The lesson for policy: seasonal forecasting helps farmers, insurers and the RBI plan for food inflation.

Common misconceptions

• "The ozone hole causes global warming." – They are different problems. Ozone depletion lets in more UV; global warming comes mainly from greenhouse gases. Some ozone-depleting chemicals (CFCs) are also greenhouse gases.
• "Cyclones form at the equator because it is hottest." – The Coriolis force is zero there, so cyclones rarely form within about 5° of the equator.
• "All ocean currents are warm on the east coasts of continents." – This holds broadly for low latitudes, but patterns change in higher latitudes.

How UPSC asks it

Prelims asks about layers and their features, the Coriolis effect, local winds with their regions, and named currents. Mains themes include why cyclones are increasing in the Arabian Sea, the effects of ocean currents on climate and fisheries, and jet streams and the Indian monsoon.

Mains answer framework

Q-type: "Discuss the factors responsible for ocean currents and their influence on regional climates."

• Intro: define currents; warm vs cold.
• Factors: primary (insolation, wind, gravity, Coriolis) and secondary (temperature, salinity, coastlines).
• Influence: temperature and precipitation, deserts, fog, fisheries, navigation – with examples.
• Conclusion: climate change is altering circulation (the slowing of the Atlantic overturning circulation, AMOC, is being studied).

Quick fact check

Warm currents (Gulf Stream, Kuroshio) raise coastal temperatures and rainfall; cold currents (Humboldt/Peru, Benguela, California) are associated with fog and coastal deserts such as the Atacama and the Namib. Areas where warm and cold currents meet, such as off Newfoundland, are rich fishing grounds.