Albedo

Albedo originates from a Latin word albus, which means white. Albedo is the amount of sunlight (of all wavelengths) that is reflected back from an object or a substance. The more the amount of light reflected back, the brighter the color of the object. A lesser amount of light is reflected back from darker objects. The albedo of an object varies from 0 to 1. Black objects have zero albedo, while white objects have an albedo of one. Sometimes, it is also expressed in terms of percentage, 1–100. An ideal white body thus has an albedo of 100 percent, while an ideal black body has an albedo of zero percent. Some standard amounts of sunlight reflected from certain objects.

Usually, albedo is used in the field of astronomy to describe reflective properties of planets, satellites, and asteroids. There are two types of astronomical albedo: normal and bond albedo. Normal albedo is a measure of a surface’s brightness when illuminated and observed vertically, while bond albedo is defined as the fraction of total solar light reflected back to space and is a measure of a planet’s energy balance. The value of bond is defined over the entire range of wavelengths.

Surface reflectance values vary across the globe, mainly because of variation in the presence or absence of snow, ice, or clouds, which increases albedo values in those areas. The presence of ice and snow, for example, at the poles and the absence of snow and ice at the equator reflects the difference in albedo values at the poles and the equator.

The reflectance value (and hence the albedo value) changes with the change in dust concentration, thickness of the clouds (or amount of cloud cover), and zenith of sunlight falling in that zone, which is also reflected in seasonal variation in albedo value for the same region. This can be observed best at higher altitudes, where in winter the surface is covered significantly by snow (or ice), thus increasing the surface reflectance values; while in spring, when most of this snow (or ice) melts, the surface (bare soil) absorbs a lot more sunlight, thus decreasing the albedo values for the same place.

Albedo is an important concept in climatology. When albedo is expressed in percentages, snow has an albedo of 90 percent and charcoal has an albedo of 4 percent. When seen from a distance, the ocean surface has a low albedo as do most forests, while desert has one of the higher albedo values.

The role of the concept of albedo in climate change can be seen in the following example: Ice reflects back more sun radiation than water; with the snow cover getting smaller and the water in lakes (and seas and oceans) increasing, the amount of sunlight absorbed (and, hence, heat retained) is increasing, leading to further increases in the temperature of lake, sea, and ocean water. On the other hand, if more snow is formed, a cooling cycle starts. The amount of sunlight (radiation) absorbed or reflected back causes fluctuations in temperature, wind, ocean currents, and precipitation. In a way, the hydrological cycle changes with the fluctuations in temperature (which is related to how much evapotranspiration takes place). Also, the climate system equilibrium is dependent on the balance between the amount of solar radiation absorbed and the amount of terrestrial radiation emitted back to space.