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Albedo means reflecting power and comes from the Latin word, albus, for white or whiteness. The scientific meaning of albedo is the ability of a surface to reflect a certain proportion of visible light. A perfect mirror has an albedo of 100%; the polished surface of white metals like aluminum or silver comes close to that figure. Some metals like brass or copper, however, are colored, and they do not reflect all visible light equally well. This shows that albedo is dependent on the wavelength of the light being reflected.

In astronomy and meteorology, albedo describes the proportion of sunlight reflected back into outer space, for example, by a planet or a satellite. Without this reflection all the planets and their satellites would be invisible to us since, unlike the Sun, they are not self-luminous. The light from the Sun is white or yellowish because it is emitted from a star whose temperature is very high, close to 9,900°F (5,482°C).

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. 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. Astronauts in outer space would not be able to see Earth if it had no albedo. 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.

See also Weather.

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