2.3 Interactions with Earth
Interactions with Earth's Surface
Once the electromagnetic radiation reaches Earth, the energy begins to interact with the atmosphere and surface objects in ways that must be understood by those who use remote sensing for analysis. Let’s first focus on Earth’s surface. When the electromagnetic radiation comes into contact with the Earth’s surface, land or water, it is called incident radiation. This is because three things can happen to the energy; it can be reflected, transmitted, or absorbed depending on the physical composition of the object, its biological and chemical makeup, the wavelengths and frequency of the energy striking it, and the angle at which the energy strikes the object.
Once the electromagnetic radiation reaches Earth, the energy begins to interact with the atmosphere and surface objects in ways that must be understood by those who use remote sensing for analysis. Let’s first focus on Earth’s surface. When the electromagnetic radiation comes into contact with the Earth’s surface, land or water, it is called incident radiation. This is because three things can happen to the energy; it can be reflected, transmitted, or absorbed depending on the physical composition of the object, its biological and chemical makeup, the wavelengths and frequency of the energy striking it, and the angle at which the energy strikes the object.
When the electromagnetic energy is reflected off an object, it bounces back into the atmosphere. The type of reflectance is determined by the ratio between how much energy strikes the object versus how much bounces back into the atmosphere. Reflection can be broken down further into specific types based on how the energy strikes the object. If an object is smooth, such as a very calm lake or ice like a glacier, nearly all of the energy that strikes the object is reflected away at the same angle it was received, but in the opposite direction. This type of reflection is called specular reflection. Now if the object is very rough and rugged, the energy striking the object will get reflected in multiple directions, called diffused reflection. The type of these two diffusion is also depending on the wavelengths of the incoming incident radiation. If the wavelength of the incoming energy is larger than the individual particles it strikes, the reflection will be specular. If the wavelength energy is smaller than the individual particles it strikes, the reflection will be diffused.
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Transmission is the term used to determine if electromagnetic energy can move through an object, such as water. Transmission is highly dependent on the wavelength of the energy reaching an object. The third type of incident radiation is absorption, which is the ratio between how much energy is absorbed by an object and the incident radiation. Ultimately, incident radiation is a mathematical equation expressed as: Incident radiation = reflected radiation + transmitted radiation + absorbed radiation
Interactions with Earth’s Atmosphere So when the electromagnetic radiation comes to Earth, most of the short wave energy gets absorbed before entering the lower atmosphere, for example the ozone layer absorbing the ultraviolet radiation. Once it reaches Earth’s surface, it can be reflected, transmitted, or absorbed by objects on Earth’s surface. The third step is that the reflected energy must pass through the atmosphere again before it can reach the orbiting satellites or other remote sensing sensors. The amount of energy that reaches the remote sensors is again dependent on several variables, which includes the composition of the atmosphere, wavelengths of the incident radiation, and the length the reflected energy must travel to reach the sensors. |
We have all experienced scattering of
incident radiation when you shut off the lights during the daytime and still
have light in the room. Atmospheric Scattering
is caused when energy strikes atmospheric particles (e.g. atmospheric particles
and aerosols like smoke, ash, salt, or condensed water) and is redirected in
multiple directions away from its original intended direction. Rayleigh scattering occurs when the atmospheric
particles are smaller than the wavelength of the incident radiation. Another
type of scattering is called Mie scattering,
which occurs when the atmospheric particles are about the same size as the wavelengths
of the incident radiation impacting it. Atmospheric particles of this size
include smoke, ash, pollen, dust, and water droplets. The third type is called non-selective scattering tends to occur
in the lower atmosphere where particles tend to be larger than the impacting
wavelengths.
Another important process we must consider is atmospheric absorption, a process where electromagnetic radiation is selective absorbed dependent on atmospheric molecules and the wavelengths of radiation. The major atmospheric players here are ozone, carbon dioxide, and water vapor. These three molecules, along with others, are often called selective absorbers. A selective absorber are molecules that will “selective absorb” specific wavelengths at a specific absorption percent. Ozone tends to absorb short wave radiation, specifically ultraviolet radiation, while carbon dioxide selective absorbs long wave radiation in the range of 13 to 17 mm. Water vapor, the strongest greenhouse gas, can absorb energy with wavelengths ranging from 5.5 and 27 mm. It is considered the strongest because of its ability to absorb such a wide range of long wave radiation.
Another important process we must consider is atmospheric absorption, a process where electromagnetic radiation is selective absorbed dependent on atmospheric molecules and the wavelengths of radiation. The major atmospheric players here are ozone, carbon dioxide, and water vapor. These three molecules, along with others, are often called selective absorbers. A selective absorber are molecules that will “selective absorb” specific wavelengths at a specific absorption percent. Ozone tends to absorb short wave radiation, specifically ultraviolet radiation, while carbon dioxide selective absorbs long wave radiation in the range of 13 to 17 mm. Water vapor, the strongest greenhouse gas, can absorb energy with wavelengths ranging from 5.5 and 27 mm. It is considered the strongest because of its ability to absorb such a wide range of long wave radiation.
Atmospheric molecules, however, cannot absorb all the energy reflected from Earth’s surface. Certain portions of the electromagnetic spectrum can successfully pass through the entire atmosphere and reach the sensors of orbiting satellites. These portions of the atmosphere where radiation can pass through and into space are called atmospheric windows. Specifically, these “windows” include the entire visible spectrum (0.4 to 2.5 mm), mid-infrared spectrum (3 to 5 mm), the thermal spectrum (8-14 mm), and microwave radiation (10 to 30 mm). It is in these specific atmospheric windows that we can analyze the Earth using remote sensors. |
Spectral Signatures
Let’s refer back to when we were discussing the interactions of electromagnetic energy with the Earth’s surface. It probably makes sense that different objects have different reflectance and absorption values. A white, smooth glacier would reflect electromagnetic energy quite differently than a forest, farmland, or rock outcrop. Even more, as was discussed earlier, unhealthy vegetation will appear different in the visible spectrum and infrared spectrum. Seasonality will also play a role in how objects on Earth will appear in each spectrum. That’s why it’s important to try to compare imagery in the same season if you are analyzing glaciers, vegetation, or even land use patterns. When we graph the spectral reflectance of an object based on wavelength and percentage of reflectance, we begin to generate a spectral signature. A spectral signature is the percentage of total incident energy that is reflected by an object. Think of it like a spectral fingerprint of an object that allows for accurate identification. If we know what object we are looking for and what its spectral signature should be, we can properly identify that object compared to the spectral signatures of its surroundings. An unhealthy portion of a forest will have a different spectral signature compared to the signature of the surrounding healthy forest. That could aid the forest agencies in identifying potential problems such as infestation, disease, or environmental pollution. |