Albedo

Albedo means reflecting power and comes from the Latin word, albus, for white or whiteness. It is used to describe areas of the Earth (terrestrial albedo) or the overall brightness of reflective astronomical objects such as planets and moons (astronomical albedo). An object’s or area’s albedo is its ability to reflect a proportion of the electromagnetic radiation (including visible light) that falls upon it. High albedo equals high brightness or reflectivity; low albedo equals low brightness. A perfect mirror has an albedo of 100%; smooth, unoxidized surfaces of white metals like aluminum or silver comes close to that figure. Some metals, like brass or copper, however, are colored, and do not reflect all visible light equally well.

In astronomy and meteorology, albedo describes the proportion of sunlight reflected back into outer space. Without reflection all the planets and their satellites would be invisible to us since, unlike the Sun, they are not luminous (with some exceptions, such as Jupiter, which radiates more heat from its interior than it absorbs from the Sun). An object with zero albedo would be perfectly black and therefore perfectly undetectable, except when it passed in front of a radiation source and blocked it temporarily.

The albedo of Earth is around 30–35%. It is higher over snow-covered surfaces or where there is a cloud cover, and lower over clear oceans or land. After 30–35% of the sunlight is reflected back to space, the remaining 65–70% is first absorbed by Earth and its atmosphere and is re-emitted at the much longer wavelengths corresponding to the average temperature of our planet, 60°F (15.5°C). This re-emitted radiation is in the infrared part of the spectrum, and while we feel it as heat, it is not visible. Earth is known as the blue planet because the albedo reflects the particular wavelength that lies in the blue area of the spectrum. Mars, on the other hand, appears reddish to us, probably because its surface formations contain a large proportion of iron oxide, which reflects red light.

The albedo plays a crucial role in determining Earth’s climate, as the average temperature at its surface is closely tied to the 65–70% of absorbed sunlight or solar energy. The light which is directly reflected back does not contribute to the warming of our planet. If, therefore, the albedo were to increase, the temperature at the Earth’s surface would drop. If the albedo were to decrease, temperature would rise.

A moon of Saturn, Enceladus, has the highest albedo of any body in the Solar System: approximately 99%. Its surface is nearly pure water ice. Some asteroids have albedos as low as .05%

See also Weather.