CLIMATE CHANGE

ALBEDO

By Charles Rhodes, Xylene Power Ltd.

GLOSSARY OF TERMS

SOLAR POWER ABSORBED BY ELEMENT OF AREA dAc:
Let dAc be an element of cross sectional area normal to the solar flux Ho.
The solar power absorbed by cross section dAc of an ideal black body located in the earth's orbit is:
Ho dAc
A real body, such as th Earth, reflects fraction Fr of the solar power incident upon it, where Fr is the planetary albedo or Bond albedo. Hence the solar power absorbed by element of cross sectional area dAc is:
Ho (1 - Fr) dAc

A major cause of albedo is Rayleigh scattering by molecules in the atmosphere. Rayleigh scattering is dominant at the high frequency end of the visible spectrum and gives the sky its apparent blue color. Rayleigh scattering continues on into the ultra-violet frequency range.

When the sky is overcast by cloud the larger particle sizes of water droplets in the cloud cause Mie scattering to dominate. Mie scattering is not strongly wavelength dependent, so clouds usually appear white or grey.

Clouds also reflect back into space a large portion of the solar light incident upon them. The albedo of a cloud is generally in the range .36 to .56

Albedo is a also function of ground cover (snow, ice, water, trees, grass, sand, etc.). Typical local albedos for open sky conditions are tabulated below:

GROUND COVER	LOCAL ALBEDO
Open Ocean	.035
Charcoal	.04
Smooth Standing Water	.07 - .08
Pine Forest	.09
Swampland	.09 - .14
Deciduous Trees	.13
Grassy Fields	.20
Sand	.25
Ice	.3 - .4
Old Snow	.4 - .7
Fresh Snow	.7 - .9

Notice that the albedo of the open ocean at .035 is extremely low. When floating ocean ice melts there is a large change in local albedo. Even if the resulting change in local albedo is moderated by partial cloud cover, the change is still significant.

A decrease in local albedo due to melting of snow or ice has the effect of increasing local warming, and vice versa.

The local albedo for flat open water is also affected by the angle of incidence of the solar radiation.

The best available value of planetary albedo Fr is from Earthshine observations of the earth's reflectance which give the planetary albedo as:
Fr = .297 +/- .005
by photometric measurements of earthshine reflected off the moon conducted during the period late 1998 to early 2001. The report notes a slight decrease in the planetary albedo as compared to a similar measurement made in 1994-1995. A slight decrease in planetary albedo is expected due to ongoing melting of floating ice near the north and south poles.

ALBEDO REFERENCES:

Planetary Albedo

Best Planetary Albedo Value

Albedo Range

Disk Averaged Earth's Reflectance Spectra

The Atmospheric Absorption Spectrum

Atmospheric Extinction

Blue Sky

GLOSSARY OF TERMS

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This web page last updated July 26, 2008