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Geology LibreTexts

3.3: Energy Pathways in the Atmosphere

Recall our discussion of the electromagnetic spectrum from last week. 

  • Waves with a short wavelength have high energy
  • Waves with a long wavelength have low energy

The majority of the spectrum of electromagnetic waves that reach the earth do not reach the surface of the earth.  The various gasses and particulate matter in the atmosphere are very important in maintaining the balance of energy on our planet.  Some key gasses, such as ozone, act as ‘gatekeepers’ allowing visible electromagnetic waves to pass through but stopping harmful ultraviolet electromagnetic waves. When insolation falls on a surface of any object it can be:

  • absorbed
  • reflected, or
  • transmitted.

The way in which electromagnetic radiation interacts with the earth helps us understand a common question: Why is the sky blue?

The English scientist Lord Rayligh predicted in 1881 that particles will scatter inversely proportional to their wavelength. Therefore, short wavelength light scatters more than long. Recall the graphic of the electromagnetic spectrum from last week that blue is the shortest visible wavelength so it scatters the most, and is most visible. For example, when you look straight up, the sky is darkest blue because you are looking through the shortest atmospheric path. When you look toward the horizon, the sky may look more blue-white. This is because you are looking through a longer path length (more atmosphere) and the short wavelengths have scattered so much that you can no longer detect them. This can also be used to explain why sunsets appear red and orange (longer wavelengths) -- the sunlight is traveling through a longer atmospheric path so the short wavelengths are 'scattered away' and all you can see are the longer wavelengths.

Albedo is the percentage of short wave radiation reflected by a surface. Light colored things have a higher albedo than dark colored things. The insolation that is not reflected is absorbed and turned into heat. That is why on a sunny day, a person wearing a black T-shirt will have a hotter back than a person wearing a white T-shirt. Albedo plays an important role in the earth's radiation budget. See figure 4.5 in your text for examples of surfaces with different albedos.

To review: 

  • All energy the earth receives comes from the sun in the form of electromagnetic radiation
  • The amount of energy in electromagnetic radiation depends on the wavelength.  Short wave length = high energy, long wavelength = low energy
  • Electromagnetic radiation can either be absorbed, transmitted or reflected
  • When an object absorbs electromagnetic radiation it gains heat energy

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