The big idea: Albedo is the fraction of sunlight a surface reflects — a number between 0 and 1.
Light and dark surfaces
- A white T-shirt keeps you cooler in the Sun than a black one — the pale surface reflects more sunlight and absorbs less.
- A bright, shiny surface has a high albedo (reflects a lot).
- A dark surface has a low albedo (absorbs a lot).
High albedo — reflects a lot
- fresh snow / ice ≈ 0.8
- thick cloud ≈ 0.7
- stays cooler — less absorbed
Low albedo — absorbs a lot
- dark ocean ≈ 0.06
- forest / asphalt ≈ 0.1–0.2
- warms more — more absorbed
Spot it: Albedo = the reflected fraction. So the absorbed fraction is just (1 − albedo).
Earth's average (whole-planet) albedo is about 0.30 — roughly 30% of sunlight is reflected straight back to space.
Why the Earth's albedo changes: Clouds, snow and ice reflect strongly; oceans and forests reflect little. So the Earth's albedo changes from day to day with the cloud cover, and with latitude: ice-covered polar regions have a high albedo, and near the poles the Sun is low in the sky, so more light is reflected.
Albedo or emissivity?: Albedo: the fraction of incoming sunlight a body reflects. Emissivity: how well a surface emits infrared compared with a black body at the same temperature. Both are ratios between 0 and 1, with no unit.
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Albedo is a simple ratio — the power reflected divided by the power arriving:
- fraction reflected (no unit, between 0 and 1)
- power reflected/scattered back (W)
- power arriving from the Sun (W)
The two fractions: Reflected fraction = albedo.
Absorbed fraction = 1 − albedo.
So if 30% is reflected (albedo 0.30), then 70% is absorbed.
Why we use S ÷ 4 for the average: The solar constant S ≈ 1360 W m⁻² is the intensity on a surface facing the Sun. But sunlight only lands on the disc Earth shows the Sun (area πr²), while the whole sphere (area 4πr²) shares it out over a day.
So the average intensity over the whole planet is S ÷ 4 ≈ 340 W m⁻².
Earth's average albedo is 0.30 and the average incoming solar intensity is S ÷ 4 = 340 W m⁻². Find the average intensity that Earth absorbs.
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How this is tested — albedo turns up two ways:
Paper 1A
- Find the intensity that reaches the surface from the solar constant and an albedo.
- Or judge what albedo depends on (daily, latitude, cloud cover).
Paper 2
- A 'show that' the absorbed intensity is about 240 W m⁻².
- Then use emissivity to find what is re-radiated back.
The classic trap: Using albedo as the absorbed fraction. Albedo is the reflected fraction — absorbed is (1 − albedo).
The energy-balance idea: At a steady temperature, Earth is in balance: the power it absorbs from the Sun equals the power it radiates away as infrared.
energy in = energy out → that is what keeps the temperature constant.
| Step | What happens | Intensity (W m⁻²) |
|---|---|---|
| Arriving (average) | The solar constant S spread over the whole globe: S ÷ 4 | 340 |
| Reflected | Albedo 0.30 → 30% bounced straight back to space | 0.30 × 340 = 102 |
| Absorbed | The rest, (1 − albedo) = 0.70, is taken in and warms Earth | 0.70 × 340 = 240 |
| Re-radiated | At a steady temperature, Earth radiates this same amount away | 240 |
The words to know
| Word | What it means |
|---|---|
| Albedo | Total scattered (reflected) power ÷ total incident power |
| Energy balance | Power absorbed = power emitted, so the temperature is steady |
| Back-radiation | Infrared emitted by the atmosphere back down to the surface |
| Equilibrium temperature | The temperature at which absorbed and emitted power are equal |
With the Sun directly overhead, the incoming intensity at the top of the atmosphere equals the solar constant, 1360 W m⁻². The atmosphere has an albedo of 0.25 for this beam. Find the intensity of sunlight reaching the ground.
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Balance at the surface: With an atmosphere, the surface receives sunlight and infrared sent back down by the atmosphere. At a steady temperature:
intensity emitted by the surface = absorbed sunlight + back-radiation from the atmosphere.
If 0.70 I of sunlight is absorbed and the atmosphere returns 0.15 I, the surface emits 0.85 I.
A planet with no atmosphere has albedo 0.30 and receives a solar constant of 1360 W m⁻². Treating its surface as a black body, estimate its mean equilibrium temperature.
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