Albedo

Albedo is the fraction of incoming electromagnetic radiation that a surface or body reflects, usually referring to sunlight. It is a dimensionless quantity commonly expressed between 0 and 1, or as a percentage: an albedo of 0.30 means that 30% of the incident radiation is reflected. Albedo helps determine how much solar energy is absorbed by land, oceans, and planets, making it important in climate science, remote sensing, and astronomy. The word derives from the Latin term for whiteness, although albedo is not simply a measure of visible color. (usgs.gov)

Definition and physical meaning

For a surface receiving radiation, albedo can be written as

where the two fluxes are measured over the same wavelength range and compatible angular domains. Surface albedo normally includes radiation reflected into the entire hemisphere above the surface, rather than just toward one observer. For a passive surface, the reflected fraction cannot exceed the incident energy; ordinary energy-fraction albedo therefore lies between 0 and 1. Astronomical geometric albedo, discussed below, uses a different normalization. (www-cdn.eumetsat.int)

For an opaque surface, the absorbed fraction is $1-\alpha$, giving an absorbed solar flux of

This complement must not automatically be treated as absorption when radiation can pass through the material: transmitted energy must also be accounted for. Albedo concerns reflection and scattering of incident radiation, not the thermal radiation subsequently emitted by a warmed surface. Solar reflectance and thermal emittance are consequently distinct properties. (user.eumetsat.int)

Spectral and broadband albedo

Spectral albedo describes reflectance at a particular wavelength. Broadband albedo combines a range of wavelengths, weighted by the incident spectrum:

Here, $E_{\mathrm{incident}}(\lambda)$ is spectral irradiance. Broadband albedo is therefore not generally an unweighted average of spectral values. A material can have different visible, near-infrared, and total shortwave albedos; even the same surface can have different broadband values under different illumination spectra. Visible brightness alone is insufficient to establish solar albedo. (www-cdn.eumetsat.int)

Directional dependence

Albedo depends on illumination geometry as well as material properties. Real surfaces do not necessarily scatter radiation equally in all directions. Their angular behavior is described by the bidirectional reflectance distribution function (BRDF), which relates reflected radiance to illumination and viewing directions. A satellite measurement of brightness in one direction is thus not, by itself, a measurement of hemispherical albedo. (user.eumetsat.int)

Land-surface products commonly distinguish three quantities:

• Black-sky albedo is the hemispherical reflectance under direct illumination, without a diffuse component. It depends on the solar zenith angle.
• White-sky albedo is the reflectance under wholly diffuse, isotropic illumination from the upper hemisphere.
• Blue-sky albedo, or actual albedo, applies to the mixture of direct and diffuse radiation present under ambient conditions. (data.nasa.gov)

If diffuse illumination is approximated as isotropic, actual albedo can be estimated by weighting black-sky and white-sky albedos by the direct and diffuse fractions of incoming radiation. Atmospheric conditions must therefore be considered when comparing surface measurements with standardized satellite products. (ntrs.nasa.gov)

Surface and planetary albedo

Surface albedo measures reflection by the ground, vegetation, snow, or water surface. Planetary albedo measures the fraction of incoming sunlight returned to space by the entire planet, including its atmosphere. These are not interchangeable: planetary reflection includes clouds and atmospheric particles as well as the surface beneath them. (science.nasa.gov)

Earth reflects approximately 30% of incoming sunlight to space, giving a global planetary albedo of about 0.30. This is a spatially and temporally averaged quantity, not a fixed property of every location. Cloud cover, snow and ice cover, and airborne particles contribute to its variability. (science.nasa.gov)

Illustrative surface values show the contrast between open water and ice:

These values are representative rather than universal. Melt ponds reduce the reflectivity of snow-covered sea ice, and the albedo falls further as ponds grow and deepen. (nsidc.org)

Surface appearance also depends on viewing direction. Smooth water can redirect reflected light away from a satellite, making the ocean appear dark, whereas the many droplets in a cloud scatter light toward a wider range of directions. This distinction illustrates why image brightness must not be equated directly with total reflected energy. (eros.usgs.gov)

Role in climate and energy balance

Albedo controls the shortwave component of Earth’s energy budget. At otherwise unchanged conditions, a higher planetary albedo reduces absorbed solar energy; a lower albedo increases it. The resulting temperature response also depends on thermal emission, atmospheric processes, and energy transport. (science.nasa.gov)

For an idealized planet with Bond albedo $A_{\mathrm B}$, uniform thermal emission, unit thermal emissivity, and no internal heat source, radiative equilibrium gives

or

Here, $S$ is stellar irradiance at the planet and $\sigma$ is the Stefan–Boltzmann constant. The factor of four comes from the ratio of a sphere’s total surface area to its projected disk area. This equilibrium temperature is not necessarily the actual surface temperature: the greenhouse effect and nonuniform heating alter the relationship. (ntrs.nasa.gov)

Ice–albedo feedback

The ice–albedo feedback is an important positive feedback in the climate system. Warming can reduce snow and sea-ice cover, exposing darker surfaces that absorb more sunlight. Increased absorption promotes further warming and melting. Conversely, expanding reflective snow or ice can reinforce cooling. “Positive” here means that the process amplifies an initial change, not that its effects are beneficial. (nsidc.org)

Human activities can also alter albedo through deforestation, agriculture, and urbanization. An albedo change represents only one part of the resulting environmental response; it does not by itself describe changes in carbon storage, moisture exchange, or the full local energy balance. (usgs.gov)

Astronomical definitions

Bond albedo

Bond albedo is the fraction of the total incident radiant power that an astronomical body scatters back into space, integrated over outgoing directions and, in its bolometric form, over the incident spectrum. It is the appropriate albedo for calculating a planet’s absorbed stellar power and radiative energy balance. (ntrs.nasa.gov)

Geometric albedo

Geometric albedo compares a body’s brightness at zero phase angle—when its illuminated face is directed toward the observer—with that of an ideal, perfectly reflecting diffuse flat disk having the same projected area. It describes apparent brightness relative to a reference, rather than the total fraction of energy reflected into all directions. Geometric and Bond albedo therefore cannot be substituted for one another. (psg.gsfc.nasa.gov)

At a specified wavelength, the two are related by

where $p$ is geometric albedo and $q$ is the phase integral:

The normalized phase function $\Phi$ describes brightness as the star–body–observer phase angle $\theta$ changes, with $\Phi(\lambda,0)=1$. Integrating the resulting spherical albedo over the incident stellar spectrum yields bolometric Bond albedo. Thus, observations at one wavelength or a narrow range of phase angles do not directly determine the complete energy-reflection fraction. (ntrs.nasa.gov)

Measurement and applications

At ground level, albedo can be measured using paired radiation sensors that record incoming and reflected shortwave fluxes. Satellite retrievals must account for atmospheric effects, viewing geometry, and the surface’s directional reflectance before converting observed radiances into albedo. They may also require conversion from narrow spectral bands to broadband quantities. (www-cdn.eumetsat.int)

Albedo datasets are used in climate models, weather forecasting, and studies of land-surface energy exchange. Repeated observations can help identify changes in vegetation, soil moisture, erosion, and land cover, although the cause of a reflectivity change must be established using additional information. (lsa-saf.eumetsat.int)

In building design, high-albedo roofing reduces absorbed sunlight and can help limit roof temperatures and the urban heat island effect. Performance also depends on thermal emittance: a highly reflective material and a material that efficiently emits absorbed heat have different, complementary radiative properties. (epa.gov)

Interpretation and limitations

An albedo value is incomplete unless its wavelength range, illumination conditions, angular definition, and spatial and temporal averaging are specified. Surface and top-of-atmosphere measurements answer different questions, while geometric and Bond albedos use different definitions. Comparisons must preserve these distinctions. (user.eumetsat.int)

Validation is complicated when a ground sensor samples a heterogeneous area that is not representative of an entire satellite pixel. Instrument calibration, atmospheric corrections, and measurement uncertainty are also important when detecting small changes. Short records can confuse natural variability with a persistent trend; reliable interpretation requires consistent measurements and an appropriate observation period. (lpvs.gsfc.nasa.gov)

References

1. Landsat Science Team's Crystal Schaaf Discusses Albedo, Its Importance, and How It Can Affect Climateusgs.gov
2. Earth’s Albedo in Declinescience.nasa.gov
3. Climate and Earth’s Energy Budgetscience.nasa.gov
4. Measuring Earth’s Albedoscience.nasa.gov
5. Planetary Spectrum Generator Handbookpsg.gsfc.nasa.gov
6. NASA Technical Memorandum 78558ntrs.nasa.gov
7. PDS: Data Set Informationpds.nasa.gov
8. Meteosat Surface Albedo Retrieval: Algorithm Theoretical Basis Documentuser.eumetsat.int
9. EUMETSAT Symposium 2008: Manninen et al.www-cdn.eumetsat.int
10. METimage Science Planuser.eumetsat.int
11. Approach — UMass Bostonumb.edu
12. MODIS/Terra+Aqua BRDF/Albedo White Sky Albedo Band7 Daily L3 Global 30ArcSec CMG V061data.nasa.gov