- SECS, Chapter 5--Meteorology: the Many Facets of the Sky (section on Air Masses)
Case Goal: Develop a working concept of the atmosphere as a case study of a selective surface, interacting in concert with the Earth surface to contribute to the global energy budget. Then, move on to more specific reflective surfaces that we might use as a secondary shortwave resource (diffuse reflectors and specular reflectors) in designing SECS.
The Earth is a vast solar energy conversion system! The atmosphere encasing Earth's land and water mass is a collection of gases and particles that vary in pressure, temperature, and chemistry continuously. If we collectively imagine for just a moment that the atmosphere is a simple cover on top of our main absorber (the Earth), we can begin forming a concept of the atmosphere interacting with electromagnetic radiation across broad bands of wavelengths.
Now, let us consider the optical properties of a single material that reflects light for some bands and transmits or absorbs light for alternate bands, just like the atmosphere represented above. We call the surface of such a material a selective surface, because the light interaction occurs at the surface. A selective surface is non-reflective to some bands of light, while being reflective to other bands of light. The atmosphere is a case study for a selective covering surface between shortwave and longwave bands.
For any given wavelength or band of similar wavelengths, the following three simple phenomena will occur when light interacts with a material surface (where light is either being absorbed or emitted):
This means that each of the simple optical phenomena can be represented by fractions from 0 to 1, with the sum of these equating to 1.
For opaque materials, there is a relation between reflectivity and emissivity (the glow of an object) and between reflectivity and absorptivity.
- A surface that is highly reflective for a certain band of wavelengths is also a surface with a low emissivity.
- Meaning: reflective materials don't 'glow' effectively.
- In contrast, a surface that has low reflectivity for certain wavelengths of light will have a high emissivity.
- Meaning: non-reflective materials tend to 'glow' effectively.
We now know the relation for surfaces in optics called Kirchoff's Law of Radiation. When a surface is in thermal equilibrium with the surroundings, the emissivity is equal to its absorptivity at each wavelength ($\epsilon = \alpha$). This allows us to make the same relations among reflectivities and absorptivities.
- A surface that is highly reflective for a certain band of wavelengths is also a surface with a low absorptivity.
- Meaning: reflective materials don't absorb light effectively either.
- In contrast, a surface that has low reflectivity for certain wavelengths of light will have a high absorptivity.
- Meaning: non-reflective materials really do absorb light effectively.
We can show the ways in which the Earth-Atmosphere is selective in the relative properties of each to absorb, reflect, and transmit different bands of light.
Video: Solar Case Study (6:47)
We have already described shortwave (280-2500 nm at ground level) and longwave (>2500 nm) bands of irradiation incident upon a surface. The study of optics is that of light-matter interactions, regardless of wavelength. We know that materials like glass are semi-transparent in most of the shortwave band, and materials like pure aluminum are reflective in the shortwave band. We are not so familiar with the way that materials behave in the longwave band, and we often have trouble with materials that are opaque vs. semi-transparent.
Let's review the transmittance graphic that I introduced earlier. This plot has a logarithmic x-axis, so, we can count from 0.2 micrometers (200 nm) up to 1 micrometer wavelengths in increments of 0.1 micrometer. Then, we count from 1 micrometer to 10 micrometers in increments of 1 micrometer, and so on...
We see that from about 0.3 micrometers to about 2.5 micrometers, there is a significant amount of white showing on the plot of the Total Absorption and Scattering, meaning that the absorption and reflection of visible light by the atmosphere is relatively small. In other words, the atmosphere transmits 70-75% of the sun's shortwave light from the top down to the Earth's surface. Along the way, clouds can back-scatter (reflect) some of the visible light into space. In times and places where transmitted sunlight reaches the Earth's surface, then the land, oceans, deserts, grasses, and trees, etc., reflect some of the surface-incident shortwave light back into space (again, with limited absorption along the way).
We also see that from 8-13 micrometers, there is an atmospheric window where the longwave band light is not absorbed or scattered. This is the way that the Earth and the Atmosphere can "leak" energy back into space. If you like, go back to take a look at a similar figure from Granqvist. From which side of the sky window is water vapor absorbing the longwave light, and from which side is CO2?
Using this example, when incident light (irradiance) is not absorbed or transmitted through the surface and bulk of a material, it is reflected by the surface (the fraction of reflection is called the albedo).