How Earth's geography shapes climate
- Latitude: the Sun is higher near the equator, so it is hotter there; poles get slanted, weak sunlight.
- Air circulation: Hadley cells: air rises at the equator (rainforests), sinks near 30° N/S (deserts like the Sahara and the Thar edge), then returns as trade winds. Further cells (Ferrel, polar) give westerlies and polar easterlies.
- Altitude (elevation): air cools about 6.5 °C for every 1,000 m. Shimla is cooler than Delhi.
- Nearness to water: water heats and cools slowly (high specific heat), so coasts have mild, maritime climates; inland areas have hot summers and cold winters (continental).
- Ocean currents: warm currents (Gulf Stream) warm nearby coasts; cold currents (Humboldt) cool them and make coastal deserts.
- Mountains (relief): block winds and create rain shadows.
Rain shadow effect
When moist wind meets a mountain, it must rise (orographic lift). Rising air expands and cools; its water vapour condenses into clouds and rain on the windward side. The air that crosses the top has lost much of its moisture, and it warms as it sinks down the leeward side. So the leeward side is dry: a rain shadow. Examples: the Western Ghats (wet coast, drier Deccan), the Himalaya (wet south, dry Tibetan plateau), the Andes and Patagonia.
El Niño and La Niña (ENSO)
The El Niño–Southern Oscillation (ENSO) is a natural swing in the tropical Pacific every 2–7 years.
Normal conditions
Trade winds blow from east to west along the equator. They push warm surface water west, so the west Pacific is warm with rising air and heavy rain. In the east, cold, nutrient-rich water rises from below (upwelling), feeding fish.
El Niño
- Trade winds weaken or reverse; warm water spreads east.
- Upwelling weakens: fish catches fall off South America.
- Rain shifts east: floods in Peru and Ecuador; drought in Indonesia and Australia; often a weaker Indian monsoon.
- Global average temperature rises a little for a year.
La Niña
- Trade winds are stronger than normal; the east Pacific is colder.
- More rain in Southeast Asia and Australia; often a good Indian monsoon; drier conditions in parts of the Americas.
The Southern Oscillation is the matching seesaw in air pressure between the west and east Pacific.
Try it: make a rain shadow and a mini El Niño
Fill a long tray with water and add a few drops of food colour at one end. Blow steadily along the surface with a straw (trade wind): the coloured water piles at the far end. Stop blowing: it sloshes back. That is how warm water spreads east when trade winds weaken. For a rain shadow, breathe on a cold glass held behind a book: the side facing you fogs up, the side behind the book stays clear.
Key formulas and definitions
- Air cools ≈ 6.5 °C per 1,000 m of height
- Hadley cell: rise at 0°, sink near 30°
- Windward = wet, leeward = rain shadow
- El Niño: weak trade winds, warm east Pacific, less upwelling
- La Niña: strong trade winds, cold east Pacific
- ENSO cycle: about every 2–7 years
Worked examples
1. A town at sea level is 30 °C. Estimate the temperature at a hill station 2,000 m above it.
Cooling ≈ 6.5 °C × 2 = 13 °C, so about 30 − 13 = 17 °C.
2. Why are many of the world's large deserts found near 30° N and 30° S?
Air from the equator sinks there in the Hadley cell. Sinking air warms and dries, so clouds rarely form.
3. Mumbai and Pune are about 150 km apart. Why does Pune get much less rain?
Monsoon winds from the Arabian Sea rise over the Western Ghats and drop most rain on the windward (Mumbai) side. Pune is on the leeward side, in the rain shadow.
4. During an El Niño year, what happens to fishing off Peru, and why?
It falls. Weak trade winds reduce upwelling of cold, nutrient-rich water, so plankton and fish decline.
5. Why do coastal cities have smaller temperature ranges than inland cities at the same latitude?
Water has a high specific heat, so it warms and cools slowly, keeping nearby air milder in both summer and winter.
Common mistakes
- Thinking El Niño is a single storm. It is a months-long change in Pacific winds and ocean temperatures.
- Saying the rain shadow is on the windward side. The windward side is wet; the leeward side is dry.
- Believing El Niño always causes drought in India. It often weakens the monsoon, but not every time.
- Forgetting that altitude matters: a place on the equator can be cold if it is high (e.g. snow on Kilimanjaro).