B.2.3—Albedo

Albedo

Albedo is the fraction of incident radiation scattered or reflected by a macroscopic system:

$$a=\frac{P_{\mathrm{scattered}}}{P_{\mathrm{incident}}}$$

Interpret the value

An albedo near 0 means most incident energy is absorbed; an albedo near 1 means most is reflected. It has no units and is often reported as a decimal or percentage.

Use ratios safely

Calculate each albedo from reflected divided by incident power before comparing surfaces. A snow surface can have a higher albedo than concrete because it reflects a larger fraction of the same incoming intensity.

Worked example from the mapped local textbook

Radiation of intensity $610\,\mathrm{W\,m^{-2}}$ reaches water with albedo $a=0.18$. The absorbed fraction is $1-a=0.82$, so

$$I_{\mathrm{absorbed}}=(1-a)I_{\mathrm{incident}}=(0.82)(610)=5.0\times10^2\,\mathrm{W\,m^{-2}}$$

About $110\,\mathrm{W\,m^{-2}}$ is reflected; the absorbed and reflected intensities add back to the incident intensity, within rounding.

Common trap

Albedo is a ratio of powers, not the reflected power by itself. Do not compare two reflected intensities unless their incident intensities are the same or you have normalized them.