Introduction
Interactions between atmospheric energy and matter and with the surface of the earth play an important role in remote sensing. There is a constant interaction between the surface of the earth, electromagnetic radiation and the atmosphere. This interaction determines the spectral areas through which remote sensing can be done (Bucholtz, 1995, 2769). Electromagnetic radiation is propagated via the earth's atmosphere at the speed of light once it has been generated. Since the earth's atmosphere is not a vacuum, it has a different impact on the wavelength, spectral distribution, speed and direction of radiation.
Scattering
One of the results obtained for the interaction between the surface of the earth, atmosphere and electromagnetic waves is scattering. Apart from scattering, reflection represents the other outcome associated with the interaction between electromagnetic waves, the earth’s surface and the atmosphere (Reiners and Kenneth, 2003, 110). While the direction associated with reflection is predictable, the direction related to scattering is unpredictable. The predictability of the direction associated with scattering and reflection can be used to differentiate the two occurrences. Scattering occurs in three different types; Mie, Rayleigh and Non-selective scattering.
Composition of the earth’s surface
Different layers of the atmosphere demonstrate various and different types of constituents. The troposphere is a region 8 km above the earth’s surface. It is majorly made of water and other tropospheric aerosols. Rayleigh scattering occurs in the region in the troposphere between the 3rd kilometer and the 8th kilometer above the surface of the earth. Oxygen, carbon dioxide and trace gasses form most of the atmosphere. However, the percentages of the gasses vary as the distance further way from the surface of the earth. At the stratosphere (the region 20 kilometers above from the surface of the earth) the major constituents include ozone layer and other stratospheric aerosols.
Rayleigh scattering
For Rayleigh scattering to occur, the wavelength of the incident electromagnetic radiation must be incredibly larger than the diameter of the molecules. All scattering is attained through re-emission and absorption of radiation by either molecules or atoms. It is not possible to predict the direction in which a particular molecule or atom will produce a photon, thus scattering. The amount of energy needed to excite an atom is associated with high frequency and short wavelength radiation (Clays and Andre, 1992, 3286). The amount of scattering demonstrates an inverse relationship with the fourth power of the radiation’s wavelength. It is important to note that the type of scattering depends on the wavelength of the incident radiation energy and the diameter of the dust particle, gas molecule or water vapor droplet encountered.
Rayleigh scattering is responsible for the blue sky. As compared to the longer orange and red wavelengths, the short blue and violet wavelengths are more proficiently scattered. The existence of the blue sky is as a result of privileged scattering of the short blue and violet wavelength sunlight. Additionally, Rayleigh scattering is accountable for red sunset. The earth’s atmosphere is made up of a slender covering of bound gravitational gas enclosing the earth. As a consequence, sunlight is forced to pass through a longer slope of air through sunset and sunrise as compared to noon. Given that the blue and violet wavelengths are scattered more on their longer path via the air as compared noon when the sun is overhead, only the residue is visible. That is, only the wavelengths of the sunlight (red and orange) that are barely scattered away are observable.
Mie scattering
For a Mie scattering to occur, the spherical particle must be present in the atmosphere. Also, the wavelength of radiation under consideration must be approximately equivalent to the diameter of the spherical particles in the atmosphere. The main scattering agents for visible light include dust, water vapor and other with a diameter ranging from a few tenths of a micrometer to numerous micrometers (Bates, 1984, 786). As compared to Rayleigh scattering, the amount of Mie scattering is larger, and the wavelengths are longer. The nature of the sunsets and sunrises are accounted for by pollution. The amount of dust and smoke particles in the earth’s atmosphere determines the amount of blue and violet light that is scattered away. A higher amount of smoke and dust particle in the earth’s atmosphere results in increased scattering of more blue and violet wavelengths as compared to red and orange wavelengths. As a result, only the red and orange wavelengths reach our eyes.
Non-selective scattering
For Non-selective scattering to occur, the diameter of the particles in the atmosphere must be bigger several times than the radiation being transmitted. As opposed to both Mie and Rayleigh scattering, non-selective scattering does not select wavelengths to scatter (Kumar, 2002, 45). It scatters all wavelengths as long as they meet the criterion for scattering as described in the process. The clouds appear white as a result of non-selective scattering. The droplets of water in the atmosphere scatter the all the wavelengths of visible light without selecting any special wavelength.
Absorption
Apart from reflection and scattering, absorption is another outcome of the interaction between electromagnetic energy, the surface of the earth and the atmosphere. It is the process through which radiant power is absorbed and transformed into other types of energy.
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