Albedo

The albedo (Latin albedo " whiteness ", from the Latin albus "white" ) is a measure of the reflectivity of diffuse reflection, not self-luminous surfaces. It is expressed as a dimensionless number and corresponds to a percentage ( an albedo of 0.9 corresponds to 90 % reflectance ). Especially in the field of meteorology, it is important because it allows statements about how much air is heated over a variety of surfaces.

In climatology, the so-called ice albedo feedback is an essential, radiative forcing and thus the radiation balance of the earth influencing factor, which is relevant for the preservation of the global climate.

In 3D computer graphics, the albedo will also use; where she serves as a measure of the diffuse scattering power of different materials for simulations of volume scattering.

In astronomy, the albedo plays an important role as they (for example, the surface temperature of a planet ) related to the basic parameters of celestial bodies.

Albedoarten

We distinguish between different types of albedo:

  • The spherical albedo ( also planetary albedo and albedo Bondsche called ) is the ratio of light reflected from a spherical surface in all directions to the light incident on the spherical cross-section radiation. In the planetary albedo as the surface is considered the upper edge of the atmosphere. The Bond albedo is always between 0 and 1 A value of zero corresponds to complete absorption and one a complete reflection of the incident light.
  • The geometric albedo is the ratio of the observer entering from a full -irradiated surface current to the radiation, the (so-called Lambert radiator) would reach the same size at normal incidence to the observer of a diffusely reflective, absolutely white disk. The geometric albedo can also assume values ​​greater than 1, in rare cases, because real surfaces are not ideal diffusely reflecting.

The ratio of the albedo of spherical and geometric albedo is the so-called phase-integral (see phase), which takes into account the angle-dependent reflectivity of each surface element.

Measurement

Measuring the albedo via Albedometer and is expressed in percent. In astronomy, no Albedometer can be used due to the large distances. The geometric albedo can be calculated from the apparent magnitude and the radius of the celestial body and the distances between the Earth, Sun and object here, however. To determine the albedo, and the phase must be integral (and hence the phase function ) must be known. However, this is fully known only to those celestial bodies that move within the Earth's orbit (Mercury, Venus). For the upper planets, the phase function can be only partially determined, whereby the value for its albedo are not exactly known.

Satellites of the U.S. space agency NASA measure since about 2004, the albedo of the earth; her Deep Space Climate Observatory to measure the Earth's albedo from 2014 at a distance of 1.5 million kilometers from the Earth L1 Lagrange point out. At this point, the probe would have a permanent view of the sunlit side of Earth.

Influences

The surface condition of a celestial body determines its albedo. The comparison with the Albedowerten earthly substances thus makes it possible to draw conclusions on the nature of other planetary surfaces. According to the definition of the spherical albedo of the condition of parallel incident light is given because of the great distances of the reflecting celestial bodies of the sun as the light source very well. The always closed cloud cover of Venus emits much more light back than the basalt -like surface portions of the moon. Therefore, the Venus has a mean spherical albedo of 0.76, a very high, the moon with an average of 0.12, a very low albedo. The Earth has an average albedo of 0.3. The highest measured value falls to 0.99 on Saturn's moon Enceladus and the lowest mean value was found to be only 0.03 comet Borrelly.

Smooth surfaces such as water, sand or snow have a relatively high proportion of specular reflection, its albedo is therefore strongly dependent on the angle of incidence of solar radiation ( see table).

The albedo is also dependent on the wavelength of the light is analyzed, so in specifying the albedo of the corresponding wavelength range should always be given.

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