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Energy Interaction with Atmosphere and Earth Surface


In Remote Sensing, satellites record electromagnetic radiation (EMR) that is reflected or emitted from the Earth. Before reaching the sensor, radiation interacts with:

  1. The Atmosphere

  2. The Earth's Surface

These interactions control how satellite images look and how we interpret them.

I. Interaction of EMR with the Atmosphere

When solar radiation travels from the Sun to the Earth, four main processes occur:

1. Absorption

Definition:
Absorption occurs when atmospheric gases absorb radiation at specific wavelengths and convert it into heat.

Main absorbing gases:

  • Ozone (O₃) → absorbs Ultraviolet (UV)

  • Carbon dioxide (CO₂) → absorbs Thermal Infrared

  • Water vapour (H₂O) → absorbs Infrared

Concept: Atmospheric Windows
These are wavelength regions where absorption is very low, allowing radiation to pass through the atmosphere. Remote sensing depends on these windows.

For example, satellites like Landsat 8 use visible, near-infrared, and thermal bands located in atmospheric windows.

2. Transmission

Definition:
Transmission is the passage of radiation through the atmosphere without being absorbed or scattered.

Importance:
High transmission means better image quality.

Example:
Microwave signals from Sentinel-1 can pass through clouds because microwaves are less affected by atmospheric absorption.

3. Refraction

Definition:
Refraction is the bending of radiation when it passes through layers of air with different densities.

Cause:
Changes in temperature and air pressure.

Example:
Mirage seen on hot roads.

In remote sensing, refraction slightly changes the path of radiation.

4. Scattering

Definition:
Scattering is the redirection of radiation by atmospheric particles.

It reduces image clarity and causes haze.

There are three types:

(a) Rayleigh Scattering

  • Occurs when particles are much smaller than the wavelength (e.g., oxygen and nitrogen molecules).

  • Strongly affects short wavelengths (blue light).

  • Scattering is proportional to 1/λ⁴.

Effect:

  • Sky appears blue.

  • Sun appears red during sunrise and sunset.

(b) Mie Scattering

  • Occurs when particle size is similar to wavelength.

  • Caused by dust, smoke, and pollution.

  • Affects visible and near-infrared wavelengths.

Effect:

  • Causes haze.

  • Reduces image contrast.

(c) Non-Selective Scattering

  • Occurs when particles are much larger than wavelength.

  • Caused by water droplets and ice crystals.

  • Independent of wavelength.

Effect:

  • Clouds and fog appear white.

II. Interaction of EMR with the Earth's Surface

When radiation reaches the Earth's surface, three processes occur:

[
Incident Energy = Reflected + Absorbed + Transmitted
]

1. Reflection

Definition:
Reflection is the return of radiation from a surface.

Types:

  • Specular Reflection
    Occurs on smooth surfaces (e.g., calm water).
    Acts like a mirror.

  • Diffuse (Lambertian) Reflection
    Occurs on rough surfaces (e.g., soil, vegetation).
    Radiation is scattered in many directions.

Example:

  • Vegetation strongly reflects Near Infrared (NIR).

  • Water reflects very little NIR.

This principle is used in NDVI calculation.

2. Absorption

Definition:
Absorption occurs when surface materials absorb radiation and convert it into heat.

Later, this energy is re-emitted as thermal radiation.

Examples:

  • Vegetation absorbs red light for photosynthesis.

  • Water absorbs most Near Infrared.

  • Dark soils absorb more energy than light soils.

Thermal sensors on satellites like Landsat 8 measure emitted heat energy.

3. Transmission

Definition:
Transmission occurs when radiation passes through a material.

Examples:

  • Light penetrating clear water.

  • Microwave signals penetrating vegetation and soil.


ProcessWhere it OccursCauseExample
AbsorptionAtmosphereO₃, CO₂, H₂OUV blocked by ozone
TransmissionAtmosphereAtmospheric windowVisible light
RefractionAtmosphereDensity changeMirage
Rayleigh ScatteringAtmosphereSmall moleculesBlue sky
Mie ScatteringAtmosphereDust, smokeHaze
Non-selective ScatteringAtmosphereWater dropletsWhite clouds
ReflectionSurfaceSurface roughnessNDVI
AbsorptionSurfaceMaterial propertySoil heating
TransmissionSurfaceTransparencyLight in water

importance 

  • Explains why images appear hazy.

  • Helps in atmospheric correction.

  • Forms the basis of vegetation, water, and urban indices.

  • Essential for spectral signature analysis.

  • Important for MSc-level Remote Sensing examinations.


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