Solar Radiation, Heat Balance and Temperature

The sun is the engine that drives the earth's climate system. — Anonymous

8.1 Solar Radiation

The sun emits energy as electromagnetic radiation. Earth receives insolation (incoming solar radiation).

• Solar constant: ~1367 W/m² at the top of the atmosphere
• Only ~47% reaches the surface directly; rest is absorbed, scattered, or reflected

Albedo — the reflectivity of a surface. Snow and ice have high albedo; forests and oceans have low albedo.

8.2 Heat Budget

Earth maintains a roughly stable temperature because outgoing radiation balances incoming:

Incoming = Outgoing (over long periods)

Processes:

• Absorption by atmosphere, land, and water
• Reflection by clouds and surfaces
• Greenhouse effect — CO₂, water vapour, and methane trap outgoing longwave radiation, warming the surface

Without the natural greenhouse effect, earth's average temperature would be ~−18°C instead of ~15°C.

8.3 Factors Affecting Temperature

Diurnal range — difference between daily maximum and minimum temperature. Deserts have large ranges; humid coasts have small ranges.

8.4 Distribution of Temperature

Temperature decreases from equator to poles. Isotherms are lines connecting places of equal temperature on maps.

India shows wide seasonal variation: summer highs exceed 45°C in Rajasthan; winter lows below freezing in Ladakh. The Indian Ocean moderates coastal temperatures.

Chapter Summary

• Sun is the primary energy source; insolation drives weather and climate
• Heat budget: incoming solar radiation balanced by outgoing terrestrial radiation
• Greenhouse gases warm the earth; albedo affects absorption
• Temperature varies with latitude, altitude, continentality, and cloud cover
• India experiences extreme seasonal and regional temperature contrasts

Exercises (NCERT)

• Exercise 1 — solar radiation and the heat budget
• Exercise 2 — factors controlling temperature distribution
• Exercise 3 — greenhouse effect and temperature patterns in India