In this article we will discuss about how interaction of electromagnetic radiation with atmosphere takes place.
Electromagnetic radiation (EMR) plays an important role in the process of remote sensing. This electromagnetic radiation travels through the atmosphere before falling onto the earth surface objects. While travelling, various particles and gaseous molecules present in the atmosphere interacts with electromagnetic radiation. This interaction of EMR with atmosphere depends on various factors such as wavelength of energy, size of atmospheric particles, nature of particles etc.
Mainly 2 types of interaction happens between electromagnetic radiation and atmosphere.
The unpredictable diffusion of energy into multiple directions by the interacting particles is called scattering. Depending on wavelength of energy and particle size, the scattering behavior changes. Hence, by comparing the particle dimension and wavelength of radiation, scattering is classified into following types.
Rayleigh scattering takes place when energy interacts with gaseous molecules, very small atmospheric particles which are smaller than the wavelength of radiation. The effect of rayleigh scattering is inversely proportional to the 4th power of wavelength of interacting energy.
Due to this characteristic, shorter wavelengths have a stronger tendency to scatter compared with longer wavelengths. For example, compared with EMR having 1 micrometer wavelength, 0.5 micrometer wavelength energy scatters 16 times more. In simple, when the size of particles is very much smaller than wavelength of energy, Rayleigh scattering take place.
Rayleigh scattering results in blue color to the sky. While the EMR is passing through the atmosphere, Blue portion of electromagnetic spectrum scatters more in the atmosphere, hence we see the sky in blue color. However, during the evening and morning times sky is seen in orange or red color. The reason is the radiation has to travel longer distance to reach the earth surface during mornings and evenings. As a result scattering of blue is complete in the top layers of atmosphere and less scattered orange and red portions of EMR are seen to the observer.
You may watch this video to understand why sky is blue:
Mie scattering takes place when size of atmospheric particles are almost equal to wavelength of electromagnetic radiation. Tiny dust particles and Water vapour molecules causes mie scattering. In mie scattering slightly larger wavelengths are reflected more. Though rayleigh scattering takes place in almost all atmospheric conditions, mie scattering predominantly takes place in overcast conditions.
When size of atmospheric particles are much larger than wavelength of interacting energy, non-selective scattering takes place. Large dust particles, water droplets sizing 5 to 100 micrometers are responsible for this type of scattering. All visible and infrared radiation tends to be scattered uniformly in this case. Hence, this scattering is called non-selective (same for all wavelengths). Where as Rayleigh and Mie scattering can be grouped as selective scattering as scattering takes place predominantly at selected portions of EMR only.
Fog and snow appears in white color as a result of non-selective scattering. All portions of visible spectrum i.e., blue, green and red are scattered uniformly hence we see white color.
Second type of interaction that takes between atmosphere and energy is absorption. Gaseous molecules, water vapor present in the atmosphere absorbs certain portions of electromagnetic energy. As a result, at some wavelengths electromagnetic energy doesn’t reach the earth surface and not available for remote sensing.
Hence, remote sensing is limited only to those portions where energy is transmitted onto the earth surface. These regions are denoted as atmospheric windows.
Atmospheric windows are the wavelength regions in the electromagnetic spectrum, where atmosphere is completely transparent to the energy. Remote sensing is possible and limited to only these regions. Hence, most of the satellite sensors are designed to operate in this region only.
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