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All 11 Flashcards — Albedo and Earth's energy balance
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Question
Define albedo.
Answer
The **fraction of incident sunlight that a surface reflects** (scatters back). A number between 0 and 1, with no unit.
Question
Formula for albedo?
Answer
$\text{albedo} = \dfrac{\text{total scattered power}}{\text{total incident power}}$ — the reflected fraction. **Given** in the data booklet.
Question
If the albedo is 0.30, what fraction is absorbed?
Answer
**0.70** — the absorbed fraction is 1 − albedo.
Question
Roughly what is Earth's average albedo?
Answer
About **0.30** — roughly 30% of sunlight is reflected back to space.
Question
Which surfaces have a high albedo? A low albedo?
Answer
**High:** fresh snow/ice (~0.8), thick cloud (~0.7). **Low:** dark ocean (~0.06), forest/asphalt (~0.1–0.2).
Question
Why is the average incoming intensity S ÷ 4?
Answer
Sunlight lands on the **disc** Earth shows the Sun (πr²) but is shared over the whole **sphere** (4πr²): πr² ÷ 4πr² = 1/4.
Question
What is the average absorbed intensity for Earth?
Answer
About **240 W m⁻²**: (1 − 0.30) × (S ÷ 4) = 0.70 × 340 ≈ 240 W m⁻².
Question
What does 'energy balance' mean for a planet?
Answer
At a steady temperature the power **absorbed** from the Sun equals the power **radiated** away. Energy in = energy out.
Question
What does Earth's albedo depend on?
Answer
The **surface** (ice/cloud high, ocean/forest low), **cloud cover**, and the Sun's angle — so **latitude** and **time of day**.
Question
Common albedo mistake to avoid?
Answer
Treating albedo as the **absorbed** fraction. Albedo is the **reflected** fraction; absorbed = 1 − albedo.
Question
How does emissivity enter the balance?
Answer
A real surface radiates **emissivity ×** the black-body value. Use it when the surface is not a perfect black body (emissivity < 1).
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