In the article What is heat flux?, radiation is mentioned as a form of heat flux. To know the heat flux onto or through an object it is required to know the quantity of radiation entering and exiting the object. When radiation meets an object, not all the energy is absorbed by the object. Some of the radiation may be reflected. An object’s ability of reflecting radiation is albedo. The albedo of an object can refer to 2 different things. It can mean the ‘bond albedo’ or the ‘geometric albedo’.
The bond albedo measurement
The bond albedo is the fraction of radiation hitting an object that gets reflected. If 100 joules of energy hit an object and the object reflects 71 joules, then the objects bond albedo is 71/100 or 0.71. Naturally the bond albedo can never be more than 1 as an object cannot reflect more energy than it receives. The bond albedo is often referred to as the spheric albedo or the planetary albedo as this definition is used particularly often in astronomy.
The geometric albedo
The geometric albedo has a more difficult definition. This coefficient accounts for the problem that an object can reflect radiation unequally in different directions. The geometric albedo is a ratio of reflectance of an object to an observer, and the reflectance of an idealized reflector to an observer. This ‘observer’ is placed on the normal axis above the object.
Imagine a perfect mirror, or idealized reflector. This mirror has a bond albedo of 1, reflecting all incoming radiation. This mirror reflects all radiation in all directions equally.
If, for instance, 100 joules of energy hit the mirror, then only 10 of them are reflected to the observer, because the other 90 joules are reflected in other directions. No joules are absorbed and a tenth of the reflected energy (10 joules) finds the observer.
Then, 100 joules of energy hit the object of which we want to know the geometric albedo. The object has a bond albedo of 0.75. That means that 25 joules are absorbed and 75 are reflected by the object. Because the object preferentially reflects light in a specific direction, 15 joules are reflected to the observer and 60 joules are reflected to different directions. 25 joules are absorbed by the object and a fifth of the reflected energy (15 joules) finds the observer.
The geometric albedo for this particular object is: the observed reflected joules from the object, divided by the observed reflected joules from the mirror. That results in a geometric albedo of 15/10, or 1.5. See the two images below for clarification.
The geometric albedo of an object can be more than 1 but is often of a similar value as the bond albedo of the object. Diffuse reflectors reflect incoming radiation in all directions resulting in a relatively low geometric albedo. Specular reflectors reflect the incoming radiation from a given direction at the same angle to the normal axis of the surface. They have a relatively high geometric albedo. The geometric albedo is more important when looking at smaller problems of reflection.
How to measure albedo?
What does albedo measure? The albedo measurement of a surface is an expression of its capability to reflect radiation. The measurement of albedo is done with an albedometer like the Hukseflux SRA30. An albedometer is a devise composed of two pyranometers. One pyranometer faces up to the sun and one faces down to the ground. The pyranometers are able to convert the incoming radiation into an electric signal by using a thermopile. If you want to learn more about how thermopiles work, please read the article: How to measure heat flux.
The upward facing pyranometer measures the global solar radiation, while the downward facing pyranometer measures the reflected solar radiation. The ratio of the reflected radiation to the global radiation is the regions albedo.
The height of installment of the albedometer is very important because of ‘self-shading’ errors. The measurement accuracy of an albedometer can be affected because the device casts a shadow onto the ground it is measuring, resulting in a (slightly) lower albedo value of the measured ground.
Cause of different albedo values
The reflectivity of a material is dependent on the smoothness of the texture, type of radiation and the electromagnetic properties of the materials forming the boundary surface. For real applications, other factors like humidity, frost, vegetation and urbanization can influence a region’s albedo greatly.
The smoothness of a surface can influence the reflectivity of a material. When a beam of radiation hits a very smooth surface, it is reflected at the angle of incidence. When the same beam hits a very rough surface, the angle of incidence is different for every square tenth of a millimeter. This results in the light being reflected in all directions.
The type of radiation also influences the reflectivity. A material reflects different amounts of light for different wavelengths. Grass reflects most wavelengths of the visual spectrum (wavelengths 0.4 ∼ 0.7 micrometers) with about 5 percent. The wavelength 0.55 nanometers is reflected with 11 percent. This wavelength is reflected more than twice as much and corresponds to the color green. This is the reason we perceive grass as green. Larger wavelengths can be reflected up to ∼ 53 percent. The reason grass still looks green to us is because these larger wavelengths cannot be seen by the human eye.
Reflectance of different wavelengths
The reason why grass reflects certain wavelengths at higher percentages is because of its electromagnetic properties. Every material has its own unique properties. As the radiation meets the boundary layer of air and grass, the medium through which the radiation travels changes. Part of the radiation undergoes refraction and is absorbed by the grass. The other part is reflected. What part is reflected is dependent on the wavelength and the two media. For metals, for instance, a much larger part is reflected due to the property of free electrons being present in metals. The physics of these processes will not be explained further as they stray too far from the initial subject.
Why are reflection and albedo important?
The albedo of a landscape has a very big influence on the net radiation of the land. If you want to know what the total heat flux is onto a surface, you have to know how much energy from the sun is reflected by the ground. Snow and ice have a high albedo, meaning they reflect a lot of radiation, preventing a lot of energy to be absorbed as heat. When a region heats up and the snow or ice melts, the albedo of the region lowers, meaning it will absorb more radiation. This way, the region will heat up even more, causing even more snow and ice to melt. This is called a positive feedback-loop. The change of albedo of a region can enforce even more change. For this reason, it is of vital importance for climatologists and governments to monitor the albedo values of regions.
The albedo measurement can be of great importance to, for instance, in albedo solar radiation of bifacial PV modules. Find out more about albedometers.