Albedo

Astronomers have refined this definition by distinguishing one hand the Bond albedo , corresponding to the reflectivity of a global star for all wavelengths and all phase angles together, and secondly the geometric albedo , representing the ratio of the electromagnetic intensity reflected from a star to zero phase angle and intensity reflected electromagnetic zero phase angle by a surface equivalent to reflectance ideally Lambertian (ie, mean isotropic irrespective of the phase angle): the consequences of these definitions, the Bond albedo is always between 0 and 1, while the geometric albedo may be greater than 1.

Measurements

The dish has a very low albedo, and reflects little light: it appears black

The snow, with a very high albedo, seems very white

In practice, a body is perceived as white when it reflects at least 80% of the light source of white light. Conversely while reflecting body less than 3% of incident light appears black.

Some materials have an albedo varies as the clouds. In contrast, solids have very often fixed albedos, which characterize their chemical composition. For example, the dish has an albedo of 0.04, the sand between 0.25 and 0.30, the ice around 0.60, the snow (thick and cool) to 0.90. The average albedo is 0.30 all land areas combined.

An important climate indicator

Albedo is one indicator of preventing the surface temperature of the earth. It is a " barometer "of climate variability that affects the knowledge of the magnitude of the greenhouse effect by opposing positive feedback on surface temperature and ocean, depending on the variation of ice volume.

The cooling original astronomical causes an extension of continental ice, the ice sheet , the glacier , and thus an increase in albedo, the planet reflects the more solar radiation , absorbs less, which enhances cooling. Warming has the opposite effect. The problem today: the warming of the planet is melting the polar ice, which reduces the albedo and thus increases the temperature of the planet.

In astronomy

The albedo is used in astronomy to get an idea of the composition of a body too cold to emit its own light, by measuring the reflection of an external light source, as Sun. One can easily differentiate the gas planets , which have a high albedo, the terrestrial planets that have them, a low albedo.

For solar energy

Before installing any equipment using solar energy , it is important to know the luminance on the ground, that is to say the amount of sunlight received at the ground. For this, one of the most effective techniques is the use of satellites for earth observation. The satellite Meteosat second generation is able to provide accurate measurements every 15 minutes on the ground luminance of the European continent.

The calculation of the luminance on the ground also of interest to many other areas, such as:

• of Agriculture ;
• the architecture ;
• industry plastic , eager to adapt the security of its products depending on where they will be used;
• the medicine , to study the impact of sunlight on health, especially through the light therapy.

References

See also

Albedo is also the name given to the mesocarp of citrus.

External link

• "Why is it cold at the Poles?" An animation of the International Polar Foundation , which explains the influence of albedo on the Earth's temperature.

• Astronomy portal