Albedo
As mentioned earlier, an ideal blackbody will absorb all incident light, but in the real world, things absorb only part of the incident light. The fraction of light that is reflected by an object is called the albedo, which means whiteness in Latin. Black objects have an albedo close to 0, while white objects have an albedo of close to 1.0. The table below lists some representative albedos for Earth surface materials. Most of these albedos are sensitive to the angle at which the sunlight hits the surface; this is especially true for water. When the Sun is at angles of 40° and higher relative to the horizon, the albedo of the water is fairly constant, but as the angle decreases from 40°, the albedo increases dramatically, so that it is about 0.5 at a Sun angle of 10° and 1.0 at a sun angle of 0°. You are aware of this in the form of glare coming off the water in the early morning or in the evening before sunset.
| Substance | Albedo (% reflectance) |
|---|---|
| Whole Planet | 0.31 |
| Cumulonimbus Clouds | 0.9 |
| Stratocumulus Clouds | 0.6 |
| Cirrus Clouds | 0.5 |
| Water | 0.06 - 0.1 |
| Ice & Snow | 0.7 - 0.9 |
| Sand | 0.35 |
| Grass lands | 0.18 - 0.25 |
| Deciduous forest | 0.15 - 0.18 |
| Coniferous forest | 0.09 - 0.15 |
| Rain forest | 0.07 - 0.15 |
Most people have an intuitive sense for the effects of albedo on reflectance and solar energy absorption. This is why people wear white clothes in hot sunny climates and dark clothes in cold sunny climates. What should you wear if it is cloudy and cold?
In the above table, we see that the Earth’s average albedo is 0.31, but there is considerable variation in this value over the surface of the Earth and over time as well — this spatial and temporal variation in albedo of the Earth is shown in the figure below.
