Solar Radiation
Incoming solar radiation is known as shortwave (SW) energy. When incoming solar radiation passes through Earth’s atmosphere, one of three things may happen: scattering, reflecting, and absorbing.
Scattering
Gases are selective about the wavelengths they absorb. Some gas molecules are more reflective. As a result, some incoming solar radiation will bounce off these gas molecules in the atmosphere, and be redirected into space. This energy is lost to Earth.
Reflecting
Of the remaining solar energy that isn’t scattered, another portion of it will be reflected back into space. This accounts for roughly 31% of all incoming solar energy. Reflected solar energy results in “Earthshine” seen in pictures taken from space. This is also how we can see the moon: we are actually seeing insolation being reflected off the moon’s surface.
An important variable in reflection is albedo, or the reflective quality of a surface. Albedo can be impacted by a surface’s composition, color, roughness, or angle relative to the sun. Albedo is typically reported as a percentage: 0% albedo means total absorption, and 100% means total reflection.
Typically, darker surfaces tend to have lower albedo, while lighter surfaces tend to have higher albedo. Take a look at the following list:
- Open ocean: 3-6%
- Asphalt: 4-12%
- Conifer forest: 8-15%
- Deciduous forest: 15-18%
- Bare soil: 15-25%
- Green grass: 20-25%
- Thin clouds: 30-50%
- Old/dirty snow: 50-60%
- Thick clouds: 70-80%
- Aluminum: 85%
- Fresh snow: 80-85%
Absorbing
Any remaining radiation that was not scattered or reflected is absorbed at or near Earth’s surface.
About 24% of SW energy is absorbed by atmospheric gases, dust, clouds, and stratospheric ozone. However, the atmosphere is overall a poor absorber of oncoming solar energy, because gases are selective about the wavelengths they absorb. The main atmospheric absorbers of solar energy are ozone, oxygen, and water vapor.
About 45% of incoming solar energy is absorbed by land and water surfaces on Earth. This process raises the temperature of the absorbing surface – just think of a time when you stepped barefoot on some asphalt that has been sitting in direct sunlight in the summer!
But what happens to that solar energy once it has been absorbed by Earth?
Terrestrial Radiation
Absorbed solar energy (shortwave/SW radiation) actually changes into heat energy (or Longwave/LW radiation) inside Earth. Earth then radiates this energy, which like solar energy can also be reflected, scattered, and absorbed.