76. Climate: Insolation, Atmospheric Circulation, Humidity, Precipitation

CORE Key Points at a Glance

1. 1

   **Insolation **

   • Solar radiation received by Earth's surface; equatorial regions receive ~2× more than polar
   • Varies with angle of incidence, day length, atmospheric path length, and albedo
   • Arctic summer sees 24 hours daylight; oblique rays travel further through atmosphere
   • Annual insolation at equator: ~1,370 W/m² (solar constant)
2. 2

   **Albedo **

   • Reflectivity of a surface — fraction of incoming solar radiation reflected back
   • Fresh snow: 80–90%; ice: 70–80%; desert sand: 35–40%; ocean: 6%; tropical forest: 12–15%
   • Global average: ~30%; high albedo surfaces cool the climate
   • Low albedo surfaces (dark ocean, forest) absorb more heat
3. 3

   **Atmospheric Pressure Belts **

   • Equatorial Low (0°): Intense heating, rising air, low pressure, heavy rain
   • Subtropical High (30° N/S): Subsiding air, dry, clear skies, deserts
   • Subpolar Low (60° N/S): Meeting of cold polar and warm subtropical air
   • Polar High (90°): Extremely cold, dense, descending air
4. 4

   **Three-Cell Atmospheric Circulation **

   • Hadley Cell (0°–30°): Hot air rises at equator, moves poleward, cools and descends at 30° creating subtropical highs
   • Ferrel Cell (30°–60°): Indirect cell; surface winds blow poleward
   • Polar Cell (60°–90°): Cold air descends at poles, flows equatorward
5. 5

   **Coriolis Effect **

   • Due to Earth's rotation, winds deflect right in Northern Hemisphere, left in Southern Hemisphere (Buys-Ballot's Law)
   • Trade Winds blow from NE (NH) and SE (SH); Westerlies from SW (NH) and NW (SH)
   • Cyclones rotate anticlockwise in NH and clockwise in SH
6. 6

   **Global Wind Systems **

   • Trade Winds (0°–30°): NE Trades in NH, SE Trades in SH; most reliable winds on Earth
   • Westerlies (30°–60°): Blow toward subpolar low; stronger in SH — "Roaring Forties," "Furious Fifties"
   • Polar Easterlies (60°–90°): Cold, dry winds from polar high
7. 7

   **Jet Streams **

   • Fast-flowing, narrow upper-air currents at 9–16 km altitude, speeds 120–400 km/h
   • Subtropical Jet (30° N/S, ~12 km): Influences subtropical weather year-round
   • Polar Front Jet (60° N/S, ~9 km): Controls mid-latitude weather systems
   • Tropical Easterly Jet: Drives Indian Monsoon northward
8. 8

   **Humidity **

   • Absolute humidity: Mass of water vapour per unit volume (g/m³)
   • Relative Humidity (RH): Ratio of actual vapour to saturation vapour × 100%; measured by hygrometer/psychrometer
   • Specific humidity: Mass of water vapour per mass of moist air (g/kg)
   • Dew point: Temperature at which cooling air reaches 100% RH (saturation)
9. 9

   **Types of Precipitation **

   • Convectional rain: Heated air rises vertically — equatorial/monsoonal regions; afternoon thunderstorms
   • Orographic/Relief rain: Windward side of mountains — Western Ghats, Cherrapunji 11,430 mm/year
   • Frontal/Cyclonic rain: Cold and warm air masses meet — temperate regions, moderate, prolonged
   • Convective hail: Rapid updrafts freeze water droplets
10. 10

    **World Climate Classification **

    • A (Tropical): Af, Am, Aw — all months above 18°C
    • B (Dry): BWh (hot desert), BSh (semi-arid) — evaporation exceeds precipitation
    • C (Temperate): Csa (Mediterranean), Cfb (Oceanic) — coldest month -3°C to 18°C
    • D (Continental): Cold winters; E (Polar): ET (Tundra), EF (Ice cap)
11. 11

    **El Niño–Southern Oscillation / ENSO **

    • El Niño: Anomalous warming of central/eastern Pacific (every 2–7 years); weakens trade winds
    • Effects: Drought in Australia, India, SE Asia; floods in Peru/Ecuador; warmer global temperatures
    • La Niña: Opposite — stronger trades, cooler Pacific; strengthens Indian Monsoon, floods in Australia
    • SOI: Pressure difference between Tahiti and Darwin measures Southern Oscillation
12. 12

    **Greenhouse Effect and Global Warming **

    • Natural greenhouse effect maintained by CO₂, water vapour, CH₄, N₂O; without it Earth avg = −18°C
    • Enhanced greenhouse effect from fossil fuel burning — CO₂ at 425 ppm (2024) vs pre-industrial 280 ppm
    • Global avg temperature risen 1.1°C above pre-industrial level (2024)