All remote sensing radiation passes through the atmosphere both before and after it interacts with the Earth’s surface.
The atmosphere changes the frequency, intensity, and direction of the radiation. Of course, atmospheric effects on remotely sensed imagery are complex and varied in reality.
Radiance measured by an air- or space-borne sensor is influenced by a variety of factors, not just diffuse sky irradiance between the sensor and the target on Earth’s surface.
The atmosphere primarily affects visible and infrared wavelengths, with only minor effects on microwave wavelengths.
Radiation used for remote sensing must travel through the Earth’s atmosphere before reaching the Earth’s surface.
The incoming light and radiation can be affected by particles and gases in the atmosphere. These effects are caused by scattering and absorption mechanisms.
Three physical processes are involved:
scattering
A change in the direction of motion of a particle caused by a collision with another particle in physics. A collision can occur between particles that repel one another, according to physics.
in remote sensing, the redirection of electromagnetic radiation by particles suspended in the atmosphere is known as scattering.
The amount of scattering that occurs is determined by the size of the atmospheric particles, their abundance, the wavelength of the electromagnetic radiation, and the depth of the atmosphere.
happens when atmospheric particles or big gas molecules contact with electromagnetic radiation, causing it to be deflected from its intended course.
In remote sensing, there are four major forms of scattering:
a. Rayleigh scattering:
Lord Rayleigh discovered it in the 1890s, for which he got the Nobel Prize in Physics in 1904. Gases in the high atmosphere, especially particles smaller than the wavelength of the radiation, create this form of scattering.
The degree of scattering is inversely proportional to the fourth power of wavelength .
The blue color of the sky is caused by Rayleigh scattering is more effective at short wavelengths (the blue end of the visible spectrum).
As a result, the light dispersed down to the ground at a great angle with regard to the direction of the sun’s light is largely in the blue end of the spectrum.
Rayleigh scattering is elastic scattering because the photon energy of the scattered photons are not altered.
b. Mie Scattering
When the dipole size increases, quasi-static approximation is insufficient to approximate the solution.
When the diameters of air particles are similar to or bigger than the wavelengths of the dispersed light, Mie scattering occurs.
Mie scattering is commonly caused by dust, pollen, smoke, and small water droplets that create clouds.
Because Mie scattering is significantly greater than Rayleigh scattering, it might be a source of interference for this weaker light scattering mechanism.
Mie scattering, unlike Rayleigh scattering, is substantially less wavelength dependant.
The scattered intensity has a considerable angular dependency, especially for tiny particles, which must be addressed for effective Mie imaging investigations.
c. Non-selective scattering
is created by particles that are several orders of magnitude bigger than the incoming radiation (e.g. water droplets and large dust particles).
It usually takes place at a lower altitude. Non-selective scattering does not rely on wavelength; all wavelengths are dispersed equally.
The principal cause of haze in remotely sensed pictures is this form of dispersion.
The most obvious example of non-selective dispersion is the perception of clouds as white entities. A cloud is made up of water droplets, and since they scatter light of all wavelengths equally, a cloud appears white.
Clouds cannot be “seen through” by a distant sensor such as our eye. Clouds also have a limiting influence on optical remote sensing.
d. Raman scattering
This type of scattering happens when a photon collides with molecules, resulting in an energy loss or gain.
Such scattering can change the wavelength of radiation and is caused by air particles of any size or substance.
concurrently with the temperature of the gas The disadvantage of Raman scattering in the gas phase is that the signal is weaker, nearly six orders of magnitude weaker than Mie scattering.
Absorption
The process through which radiant energy is held by substances in the atmosphere is known as atmospheric absorption.
The atmosphere’s many distinct gases and particles absorb radiation at both longer and shorter wavelengths than visible light.
Unlike dispersion, this process involves a loss of energy. This energy is emitted by the atmosphere at longer wavelengths.
The process by which “incident radiant energy is retained by a material” is known as absorption. The material in this situation is the atmosphere.
When energy is absorbed by the atmosphere, it undergoes an irreversible change from radiation to another type of energy.
This energy is converted based on the type of the absorbing media.
The absorbent media is capable of much more. Only a percentage of the total energy will be absorbed by the medium.
The remaining energy will be reflected, refracted, or dispersed. The absorbed energy can also be transferred back into the atmosphere.
Because of the many gases and particles present, the atmosphere absorbs and transmits electromagnetic radiation of a wide range of wavelengths. The wavelengths that travel through the atmosphere unabsorbed are referred to as “atmospheric windows.”
The absorption of electromagnetic radiation by the atmosphere benefits the planet in two ways. For starters, absorption protects individuals by preventing high-energy radiation from reaching the surface, so limiting our exposure to hazardous radiation. The atmosphere absorbs the majority of the energy from the ultraviolet to the X-ray range.
The second method absorption benefits the planet is as a heat source. A vertical cross section of the whole atmosphere would reveal that temperature normally increases with height. This rise in temperature is produced by an increase in electromagnetic radiation absorption with height, which is caused by increased concentrations of high-energy wavelength absorbing gases present at higher air altitudes.
Refraction
When light passes through different levels of the atmosphere, refraction occurs. Light bends towards the normal as it travels through deeper and denser layers of the atmosphere.
the deviation of a light or other electromagnetic wave from a straight line as it travels through the atmosphere as a function of height due to variations in air density. With increasing density, the velocity of light through air decreases (the refractive index increases). Mirages are caused by atmospheric refraction near the ground. Without involving mirages, such refraction can also raise or lower, stretch or shorten the pictures of distant objects.
