Not all the sunlight that lands on the ground stays there. Some bounces back to the sky. How much depends on the surface, and it changes the climate a great deal. This lesson uses percents of a number to follow the sunlight.
Key idea
Albedo is the fraction of sunlight a surface reflects: $$\text{albedo} = \frac{\text{reflected}}{\text{incoming}}$$ The rest is absorbed: absorbed = (1 − albedo) × incoming. Fresh snow has an albedo of about 0.8 to 0.9; the open ocean about 0.06; Earth as a whole about 0.30.
Why it works
Light that hits a surface is either reflected or absorbed (sunlight does not pass through the ground). So the two shares must add to 100%. A white surface reflects most colors of light; a dark one absorbs them, and absorbed light becomes thermal energy. That is why a black car seat burns in summer and a white one stays cooler: same sunlight, different albedo.
| Surface | Albedo (about) | Share absorbed |
|---|---|---|
| fresh snow | 0.80–0.90 | 10–20% |
| sea ice | 0.50–0.70 | 30–50% |
| desert sand | 0.40 | 60% |
| grassland | 0.25 | 75% |
| forest | 0.10–0.15 | 85–90% |
| open ocean (Sun high) | 0.06 | 94% |
| Earth as a whole | 0.30 | 70% |
Worked example: Snow and ocean
Sunlight of 600 W/m² falls on fresh snow (albedo 0.85) and on open ocean (albedo 0.06). How much does each absorb?
- Snow absorbs $1 - 0.85 = 0.15$ of the light: $0.15 \times 600 = 90$ W/m².
- Ocean absorbs $1 - 0.06 = 0.94$: $0.94 \times 600 = 564$ W/m².
- The ocean absorbs more than six times as much as the snow under the same Sun.
Answer: Snow 90 W/m², ocean 564 W/m²
Worked example: Finding an albedo
A satellite measures 340 W/m² of sunlight arriving at a region and 102 W/m² reflected back. What is the albedo?
- Albedo = reflected ÷ incoming = $102 \div 340 = 0.30$.
- So 30% is reflected and 70% absorbed: the same as Earth's average.
Answer: 0.30
Common mistake
Reading a high albedo as "soaks up a lot of sunlight." Albedo measures what is reflected, so a high albedo means the surface absorbs less. Check: snow, with the highest albedo, stays cold in bright sunshine; dark asphalt, with one of the lowest, gets hot.
Albedo can change itself. When snow or sea ice melts, the dark ground or ocean underneath absorbs far more sunlight, which warms the area and melts more ice. This ice–albedo feedback is one reason the Arctic is warming faster than the rest of the planet.