4.4 The Impact of the Atmosphere on Insolation
This figure also illustrates what happens to the solar radiation as it passes through the atmosphere.
Geosystems in Action 4.1. Shortwave Radiation Inputs and Albedo. Solar energy cascades through the lower atmosphere where it is absorbed, reflected, and scattered. Clouds, atmosphere, and the surface reflect 31% of this insolation back to space. Atmospheric gases and dust and Earth’s surface absorb energy and radiate longwave radiation (as shown in GIA 4.2).
Christopherson, R.W., Birkeland, G. H., Byrne, M., Giles, P. T. (2016). Geosystems: An Introduction to Physical Geography, Fourth Canadian Edition. Toronto, Ontario: Pearson Canada. View full-size image.
Some of the insolation is scattered out of the direct beam from the Sun by the gases and particulates of the atmosphere. This process divides the incoming solar radiation into the direct beam insolation and the scattered or diffuse insolation.
Image of Earth taken by NASA's Lunar Reconnaissance Orbiter (LRO) from orbit around the moon.
Image credit: NASA/Goddard/Arizona State University. View full-size image.
NASA. (2017, August 7). NASA Releases New High-Resolution Earthrise Image. Retrieved from https://www.nasa.gov/image-feature/goddard/lro-earthrise-2015
NASA's Lunar Reconnaissance Orbiter (LRO) captured a unique view of Earth from the spacecraft's vantage point in orbit around the moon on October 12, 2015.
Notice that in this photograph of the Earth above the horizon of the moon there is no evidence of an atmosphere on the moon, and the shadows are black.
Layers of Earth's atmosphere are shown in a view looking across Earth's surface from the International Space Station.
NASA. (2017, August 7). Layers of Earth's atmosphere are shown in a view looking across Earth's surface from the International Space Station. Retrieved from https://www.nasa.gov/audience/forstudents/k-4/dictionary/Atmosphere.html View full-size image.
But the photo of the Earth from space illustrates the layering in the atmosphere with the blue sky. This is because without an atmosphere scattering part of the solar insolation, you cannot see an atmosphere, and without scattered light there is no illumination of the shadows. This leads to an explanation of why the Earth’s sky is blue and why photographs taken of people in shade may have a blue cast to them.
There is a process known as Rayleigh scattering which summarizes the physics that describes short wavelengths, such as blue, being more effectively scattered than the longer red wavelengths. The actual relationship is:
Rayleigh scattering is inversely proportional to the fourth power
of wavelength (scattering ~λ−4).
This explains the blue colour to the sky, and the blue cast in shadows. The short blue wavelengths are scattered much more than the long red wavelengths. The blue wavelengths are scattered away from the direct beam and colour the surrounding sky. The red wavelengths are not scattered and found predominantely within the direct beam radiation from the sun.
When you look at the Sun near sunset, it has a very intense red for this reason as well. The Sun’s light is passing a long distance through the atmosphere and much of the shorter blue wavelengths are scattered out of its direct beam. You see the remaining red wavelengths. With more gases in the atmosphere, such as occurs after intense volcanic eruptions, the remaining red is intensified and you get very rich sunsets. You may be familiar with the rich skies of the Victorian paintings of J.M.W. Turner. These may be the result of the intense scattering after the major eruption of Mount Tambora in Indonesia in 1815.