Insolation — Solar Energy and Its Distribution

2.1 What is Insolation?

Insolation (Incoming Solar Radiation) is the solar energy received at Earth's surface per unit time per unit area. Understanding its variation is fundamental to understanding why different parts of the world have different climates.

Solar Constant: The amount of solar radiation reaching the outer atmosphere perpendicular to incoming rays = 1,361–1,370 W/m² (watts per square metre). Only about 47–54% reaches the surface after atmospheric absorption and reflection.

Factors Controlling Insolation Distribution

Factor	Effect	Example
Angle of incidence	Low-latitude sun nearly vertical → intense radiation over small area; high-latitude sun oblique → spread over larger area	Equatorial regions receive 2× more insolation than poles
Day length	Longer days = more total energy received	Arctic midsummer: 24 hours daylight; equinox: 12 hours everywhere
Atmospheric path length	Low sun angles = longer path through atmosphere → more absorption and scattering	Polar regions: sunrise/sunset all day = long path
Albedo	High-albedo surfaces reflect more → less heating	Ice sheets reflect 80–90%; deserts 35–40%; ocean 6%
Distance from Sun	Slight variation (Earth's orbit elliptical)	Perihelion (Jan 3) — Earth 3% closer to Sun than aphelion (Jul 4)

Latitudinal Variation

• Equator (0°): Sun directly overhead at noon (twice yearly — equinoxes); highest annual insolation
• Tropics (23.5°N/S): Sun overhead at noon on solstice; high insolation
• Mid-latitudes (40°–60°): Significant seasonal variation; moderate annual insolation
• Polar regions (>66.5°): Midnight sun in summer; polar night in winter; lowest annual insolation

2.2 Heat Budget of the Earth

Of 100 units of incoming solar radiation:

• Reflected by atmosphere (clouds, aerosols): ~23 units
• Absorbed by atmosphere: ~14 units
• Reflected by Earth's surface: ~7 units (albedo)
• Absorbed by Earth's surface: ~56 units

Earth maintains energy balance — absorbed shortwave radiation is re-emitted as longwave (infrared) radiation = terrestrial radiation. The Greenhouse Effect traps some of this outgoing longwave radiation.

2.3 Albedo and Its Significance

Albedo is the reflectivity coefficient — the fraction of incoming radiation reflected by a surface, expressed as a percentage:

Surface	Albedo (%)
Fresh snow	80–90%
Sea ice	70–80%
Clouds	40–90% (varies)
Desert sand	35–40%
Grassland	20–25%
Tropical forest	12–15%
Ocean (deep)	6%
Dark soil	3–8%
Global average	~30%

Climate Implications of Albedo

• Ice-Albedo Feedback: As Arctic/Antarctic ice melts, less white surface → lower albedo → more heat absorbed → more melting → positive feedback loop amplifying warming
• Deforestation: Removes dark forest (low albedo) and may replace with crops or grassland (higher albedo) — complex, region-dependent climate effect
• Urban Heat Island: Dark paving and buildings have very low albedo → cities are warmer than surrounding rural areas by 1–3°C

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