AFRILCATE 

WHAT IS AN ALBEDO?

Albedo is the degree of reflectivity of solar radiation from the earth’s surface.

Albedo is measured on a scale of 0 to 1, where:

0 – means no reflection of incident solar radiation hence perfect absorption like a black body.

1 – total reflection of all incident solar radiation (no absorption). The material, in this case, is referred to as a perfect reflector.

A value of 0 corresponds to the lowest possible albedo while 1 represents the highest possible albedo.

High surface albedo simply means that the material can reflect almost all the light rays incident on its surface.

Let’s make a little analysis:

If a material surface has an albedo of 0.7, it means it reflects 70% of the total incident radiation and absorbs only 30%.

If a material surface has an albedo of 0.5, it means it reflects 50% of incident rays and absorbs the same 50%.

Clear right?

In other words, the higher the albedo, the higher its reflectivity of incident surface radiation and vice versa.

Albedo is a non-dimensional quantity and since it’s a ratio, it has no unit.

Let’s denote albedo with the letter A.

We can refer to albedo (A) as the ratio of the amount of solar radiation reflected back to the atmosphere to the total solar radiation incident on the surface.

Mathematically,

Where

• E_r – Reflected radiation
• E_t – total incidence radiation

Generally, Albedo applies to visible light, although it may involve some of the infrared regions of the electromagnetic spectrum.

HOW TO MEASURE ALBEDO

Albedo is more complex than assuming a single constant value for a surface.

For a given area, albedo is determined by soil composition, its effect on soil moisture, type of vegetation, levels of urbanization and few other factors.

Different surfaces on earth have different albedo’s and it varies with time.

Measuring surface albedo poses a big challenge because of the multiple variables and factors that must be taken into consideration.

Nevertheless, we can measure albedo by making simple calculations with reference to the albedo mathematical expression: A =  E_r / E_t . Incoming solar radiation can be measured in Watt/m² using pyranometers.

In the absence of one, lightmeters become a cheap alternative.

Lightmeters provide a measure of the light intensity (measured in the unit, lux) a good approximation of solar radiation. 

Example

If the incoming illuminance hitting a concrete pavement directly from the sun is 1050 W/m² within a short time period, Calculate the surface albedo?

Solution

A = E_r / E_t

A =367.5 / 1050 = 0.35

Therefore, the albedo of the concrete is 0.35

This means the concrete pavement reflects only 35% of the solar energy, incident on its surface.

ALBEDO OF VARIOUS MATERIALS

Generally, light-colored materials have a higher albedo (solar reflectance) than dark-colored materials.

This is the reason why light-colored clothing tends to keep you cooler than darker clothes.

In summer, the black asphalt road is very hot if stepped upon with bare foot because of it’s low albedo.

Sea ice has a high albedo compared to other earth surfaces.

The albedo of sea ice is usually in the range of 0.5 – 0.7 while the ocean has an albedo of approximately 0.06.

This means that sea ice reflects 50 to 70 percent of the incident radiation while the ocean reflects only 6 percent of total irradiance and absorbs the rest.

The albedo of fresh snow is 0.9, a much higher value than sea ice indicating that fresh snow reflects almost all heat energy incident on it. This is the reason why regions like Antarctica always remain cool. 

Surface albedo often depreciates with time. Once fresh snow begins to melt, its albedo drops from 0.9 to about 0.4.

The average albedo of the earth is about 0.3.

Dirt particles  also reduce albedo value.

The albedo of new concrete stands at 0.55. But over time, it falls within the range of 0.3 – 0.4.

Here is a list of various surface albedo.

FREQUENTLY ASKED QUESTIONS ON ALBEDO

1. Which of the following have a higher albedo?

2. What is the effect of a dust storm on the albedo of ice sheets?

3. The albedo of the Pacific Ocean is 0.05. Given the total solar irradiance on the water surface to be 1120W/m² .

What is the reflective irradiation of the ocean?

4. With reference to the above question, what percentage of the total irradiance is absorbed. Do the pacific ocean have a high albedo or a low albedo?