5.8. Interaction of EMR with Atmosphere

When EMR strikes a material/object on the ground, it is called incident radiation. This incident radiation will first interact with atmosphere and then on the surface of Earth, and then again with atmosphere before it reaches the sensor. Four types of interactions will take place with the atmosphere and surface/objects; scattering, absorption, transmission and reflection.

This chunk introduces the concept of incident radiation, which refers to electromagnetic radiation (EMR) that reaches the Earth. When this radiation interacts with both the atmosphere and the surface of the Earth, it can do so in four distinct ways: scattering, absorption, transmission, and reflection. Each of these interactions can significantly affect the quality and usability of remotely sensed images. For example, scattering tends to blur objects and reduce image clarity.

Think of tossing a ball at a wall. The way the ball behaves when it hits the wall (bouncing back, getting absorbed, or passing through cracks) is similar to how EMR interacts with materials in nature.

It is the process whereby incident radiation bounces-off the surface in a single predictable direction. Reflection is caused by the surfaces that are smooth relative to the wavelengths of incident radiation. As per Snell’s law, the angle of reflection is always equal and opposite to the angle of incidence.

Reflection occurs when incoming electromagnetic radiation hits a smooth surface and bounces back in a predictable direction. For instance, if light hits a calm pond or a mirror, it reflects back. This reflection is governed by Snell’s law, which states that the angle at which the light hits the surface (angle of incidence) is equal to the angle at which it reflects away (angle of reflection). The amount of energy that reflects back depends on factors like surface material and atmospheric conditions.

Imagine looking into a mirror. The way you see your reflection back at you is much like how EMR reflects off smooth surfaces on the Earth, such as water or glass, allowing sensors to capture clear images.

It is the process by which small particles diffuse a portion of the incident radiation in all directions. Scattering occurs when incident radiation is dispersed or spread out unpredictably in different directions. Scattering of radiation by the constituent gases and aerosols in the atmosphere causes degradation of remotely sensed images.

Scattering happens when small particles in the atmosphere, such as dust or water droplets, cause the incident radiation to spread in multiple directions rather than reflecting back in a straight line. This can lead to loss of detail in remotely sensed images as blurring can obscure the features that the sensor is trying to detect. In practice, this results in images that can be less sharp and harder to interpret.

Consider a foggy day. Just as the fog scatters light, making it difficult to see clearly, scattering in the atmosphere creates similar challenges for satellites trying to capture clear images of the Earth.

Three types of scattering commonly take place in the atmosphere: Rayleigh scattering, Mie scattering and Non-selective scattering. These will depend on the wavelength of incident radiant energy, and the size of gas molecule, dust particle, and/or water vapor droplet interacting with the EMR.

There are three main types of scattering: Rayleigh scattering, Mie scattering, and non-selective scattering. Rayleigh scattering occurs when particles are much smaller than the wavelength of radiation (like gases in the atmosphere), and it causes shorter wavelengths (like blue light) to scatter more. Mie scattering occurs with particles similar in size to the wavelength and affects both longer and shorter wavelengths. Non-selective scattering involves larger particles that scatter all wavelengths equally. Each type of scattering impacts how energy is perceived by sensors.

Imagine throwing different-sized balls (particles) at a net (wavelength). Small balls slip through easily (Rayleigh), medium balls get stuck in the net more evenly (Mie), while large balls won't fit and just bounce around (non-selective). Each size interacts differently with light, affecting how we perceive colors in the sky.

Rayleigh scattering mainly consists of scattering from the gases present in the atmosphere. It is primarily caused by air particles i.e., O and N molecules. Rayleigh scattering takes place when the dimension of the particles present in the atmosphere is much smaller than the size of wavelength λ, and can be represented as: Rayleigh scattering ꭀ 1 / λ4.

Rayleigh scattering happens when gas molecules in the atmosphere scatter shorter wavelengths of light (like blue) more than longer wavelengths (like red). This is why the sky appears blue during the day—blue light is scattered in all directions while red light continues straight through. The intensity of this scattering is inversely related to the fourth power of the wavelength, meaning the shorter the wavelength, the stronger the scattering effect. This principle affects both how we see the sky and how satellite sensors gather data.

Think of a prism dispersing light into a rainbow. Just as the prism separates colors based on their wavelengths, Rayleigh scattering causes different colors of light to scatter differently in the atmosphere, making the sky appear blue instead of red during the day.

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