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Solar Radiation and the Earth’s Albedo

The Energy That Fuels The Planet Earth

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Solar Radiation and the Earth’s Albedo

The energy from the sun powers life on Earth.

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Nearly all of the energy arriving on planet Earth and driving the various weather events, oceanic currents, and distribution of ecosystems originates with the sun. This intense solar radiation as it is known in physical geography originates in the sun’s core and is eventually sent to Earth after convection (the vertical movement of energy) forces it away from the sun’s core. It takes approximately eight minutes for solar radiation to reach the Earth after leaving the sun’s surface.

Once this solar radiation arrives on Earth, its energy is distributed unevenly across the globe by latitude. As this radiation enters the Earth’s atmosphere it hits near the equator and develops an energy surplus. Because less direct solar radiation arrives at the poles, they in turn develop an energy deficit. To keep energy balanced on the Earth’s surface, the excess energy from the equatorial regions flows toward the poles in a cycle so energy will be balanced across the globe. This cycle is called the Earth-Atmosphere energy balance.

Solar Radiation Pathways

Once the Earth’s atmosphere receives shortwave solar radiation, the energy is referred to as insolation. This insolation is the energy input responsible to moving the various Earth-atmosphere systems like the energy balance described above but also weather events, oceanic currents, and other Earth cycles.

Insolation can be direct or diffuse. Direct radiation is solar radiation received by the Earth’s surface and/or atmosphere that has not been altered by atmospheric scattering. Diffused radiation is solar radiation that has been modified by scattering.

Scattering itself is one of five pathways solar radiation can take when entering the atmosphere. It occurs when insolation is deflected and/or redirected upon entering the atmosphere by dust, gas, ice, and water vapor present there. If the energy waves have a shorter wavelength, they are scattered more than those with longer wavelengths. Scattering and how it reacts with wavelength size are responsible for many things we see in the atmosphere such as the sky’s blue color and white clouds.

Transmission is another solar radiation pathway. It occurs when both shortwave and longwave energy pass through the atmosphere and water instead of scattering when interacting with gases and other particles in the atmosphere.

Refraction can also occur when solar radiation enters the atmosphere. This pathway happens when energy moves from one type of space to another, such as from air into water. As the energy moves from these spaces, it changes its speed and direction when reacting with the particles present there. The shift in direction often causes the energy to bend and release the various light colors within it, similar to what happens as light passes through a crystal or prism.

Absorption is the fourth type of solar radiation pathway and is the conversion of energy from one form into another. For example, when solar radiation is absorbed by water, its energy shifts to the water and raises its temperature. This is common of all absorbing surfaces from a tree’s leaf to asphalt.

The final solar radiation pathway is reflection. This is when a portion of energy bounces directly back to space without being absorbed, refracted, transmitted, or scattered. An important term to remember when studying solar radiation and reflection is albedo.

Albedo

Albedo (albedo diagram) is defined as the reflective quality of a surface. It is expressed as a percentage of reflected insolation to incoming insolation and zero percent is total absorption while 100% is total reflection.

In terms of visible colors, darker colors have a lower albedo, that is, they absorb more insolation, and lighter colors have high albedo, or higher rates of reflection. For example, snow reflects 85-90% of insolation, whereas asphalt reflects only 5-10%.

The angle of the sun also impacts albedo value and lower sun angles create greater reflection because the energy coming from a low sun angle is not as strong as that arriving from a high sun angle. Additionally, smooth surfaces have a higher albedo while rough surfaces reduce it.

Like solar radiation in general, albedo values also vary across the globe with latitude but Earth’s average albedo is around 31%. For surfaces between the tropics (23.5°N to 23.5°S) the average albedo is 19-38%. At the poles it can be as high as 80% in some areas. This is a result of the lower sun angle present at the poles but also the higher presence of fresh snow, ice, and smooth open water- all areas prone to high levels of reflectivity.

Albedo, Solar Radiation, and Humans

Today, albedo is a major concern for humans worldwide. As industrial activities increase air pollution, the atmosphere itself is becoming more reflective because there are more aerosols to reflect insolation. In addition, the low albedo of the world’s largest cities sometimes creates urban heat islands which impacts city planning and energy consumption.

Solar radiation is also finding its place in new plans for renewable energy- most notably solar panels for electricity and black tubes for heating water. These items’ dark colors have low albedos and therefore absorb nearly all of the solar radiation striking them, making them efficient tools for harnessing the sun’s power worldwide.

Regardless of the sun’s efficiency in electricity generation though, the study of solar radiation and albedo is essential to the understanding of Earth’s weather cycles, ocean currents, and locations of different of ecosystems.

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