Albedo is the fraction of incident solar power that a surface scatters or reflects back into space. Earth's albedo varies because cloud cover, surface conditions, and the angle of incoming sunlight change with location and time.
This concept is part of B.2 Greenhouse effect and is studied by both SL and HL students. Albedo, usually represented by , is defined as
It has no unit and ranges from 0 to 1. A value near 0 represents strong absorption, while a value near 1 represents strong reflection. Earth's average albedo is approximately 0.30, meaning roughly 30% of incoming solar radiation is returned to space.
| Factor | Effect on Earth's albedo |
|---|---|
| Cloud cover | Thick or extensive clouds reflect more incoming solar radiation, increasing albedo. Changing cloud formations therefore produce daily variation. |
| Latitude | Polar regions generally have greater albedo because snow and ice reflect strongly. The lower angle of incoming sunlight also increases reflection. |
| Surface type | Snow, ice, and light-coloured deserts have high albedo; oceans and forests usually have lower albedo. |
| Seasonal change | Changes in snow, ice, vegetation, and cloud distribution alter the proportion of radiation reflected. |
Albedo affects Earth's energy balance. If the solar constant is , the mean incoming intensity over the whole spherical Earth is . After reflection, the mean absorbed intensity is
For example, if , Earth absorbs , or 70%, of the mean incoming solar radiation.
A common misconception is that albedo and emissivity are the same. Albedo concerns reflected incoming solar radiation, whereas emissivity compares a surface's emitted thermal radiation with that of an ideal black body at the same temperature.
Exam technique: For an IB explain question, define albedo as a ratio, then link variation specifically to changing cloud formations and latitude. In calculations, use for the absorbed fraction, not .