Albedo

The amount of diffuse light a surface reflects—its reflectivity—is known as the albedo of that surface. The word is derived from the Latin albus,

Albedo. At full moon, sunlight is reflected directly at the Earth. This makes the Moon shine more brightly than it does when the sunlight is reflected at a greater angle, as it is at a quarter moon, for example.

Albedo. At full moon, sunlight is reflected directly at the Earth. This makes the Moon shine more brightly than it does when the sunlight is reflected at a greater angle, as it is at a quarter moon, for example.

which means "white." A value for the albedo of a surface is obtained by measuring the amount of light falling onto the surface—the incident light—and the amount of light being reflected by it, and comparing the two. The term describes only the diffuse light reflected from an uneven surface. A perfectly smooth mirror, reflecting all of the light falling upon it, does not have an albedo. The diagram shows the relationship between direct incident light and indirected reflected light.

AVERAGE CLOUD COVER (PERCENT) Latitude (degrees) Northern Hemisphere Southern Hemisphere

0-10

63

46

10-20

51

57

20-30

47

79

30-40

55

82

40-50

64

74

50-60

70

63

60-70

68

54

70-80

70

55

80-90

65

57

Total

59

63

Earth total

61

Incident light and reflected light

Albedo is always reported as the percentage of the incident light that it reflects. The percentage is usually expressed as a decimal fraction, where 100 percent is 1, for example, and 50 percent is 0.5. A body with an albedo of 1 would reflect all of the light falling on it, and a body with an albedo of 0 would reflect no light at all. The table lists the typical albedos for a number of surfaces and also the average albedo for the Earth as a whole and the albedos for the Moon, Mars, and Venus. The range for the albedo of Mars is due to the haze that usually covers the planet. The albedo in red light is about 0.30 and that in blue and ultraviolet light is 0.04. The albedo of Venus changes according to the angle at which light is reflected. The albedo of an open water surface varies greatly depending on the angle of the incident radiation. When the Sun is directly overhead, water is almost black, with an albedo of 0.02. When the Sun is on the horizon, most of its light is reflected and the albedo is 0.99. That is why people sailing in small boats are at a high risk of sunburn early in the morning and late in the afternoon.

ALBEDO

Surface

Value

Fresh snow

0.75-0.95

Old snow

0.40-0.70

Cumuliform cloud

0.70-0.90

Stratiform cloud

0.59-0.84

Cirrostratus

0.44-0.50

Sea ice

0.30-0.40

Dry sand

0.35-0.45

Wet sand

0.20-0.30

Desert

0.25-0.30

Meadow

0.10-0.20

Field crops

0.15-0.25

Deciduous forest

0.10-0.20

Coniferous forest

0.05-0.15

Concrete

0.17-0.27

Black road

0.05-0.10

Open water

0.02-0.99

Earth

0.31

Moon

0.068

Mars

0.04-0.30

Venus

0.55-0.90

Albedo and temperature

Measuring albedo

The table shows that, despite shining so brightly on clear nights, the Moon is a dark body, with an albedo of only 0.068—it reflects only 6.8 percent of the sunlight falling upon it. Venus, in contrast, reflects up to 90 percent. Venus is so bright because it is permanently enshrouded by light-colored cloud. Its surface is completely hidden. Many years ago this led some people to suppose that oceans and swamps, inhabited by strange beasts, covered most of the surface of Venus.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment