Skip to main content

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.


Comments

Post a Comment

Popular posts from this blog

History of GIS

1. 1832 - Early Spatial Analysis in Epidemiology:    - Charles Picquet creates a map in Paris detailing cholera deaths per 1,000 inhabitants.    - Utilizes halftone color gradients for visual representation. 2. 1854 - John Snow's Cholera Outbreak Analysis:    - Epidemiologist John Snow identifies cholera outbreak source in London using spatial analysis.    - Maps casualties' residences and nearby water sources to pinpoint the outbreak's origin. 3. Early 20th Century - Photozincography and Layered Mapping:    - Photozincography development allows maps to be split into layers for vegetation, water, etc.    - Introduction of layers, later a key feature in GIS, for separate printing plates. 4. Mid-20th Century - Computer Facilitation of Cartography:    - Waldo Tobler's 1959 publication details using computers for cartography.    - Computer hardware development, driven by nuclear weapon research, leads to broader mapping applications by early 1960s. 5. 1960 - Canada Geograph...
Post a Comment
Read more

Accuracy Assessment

Accuracy assessment is the process of checking how correct your classified satellite image is . 👉 After supervised classification, the satellite image is divided into classes like: Water Forest Agriculture Built-up land Barren land But classification is done using computer algorithms, so some areas may be wrongly classified . 👉 Accuracy assessment helps to answer this question: ✔ "How much of my classified map is correct compared to real ground conditions?"  Goal The main goal is to: Measure reliability of classified maps Identify classification errors Improve classification results Provide scientific validity to research 👉 Without accuracy assessment, a classified map is not considered scientifically reliable . Reference Data (Ground Truth Data) Reference data is real-world information used to check classification accuracy. It can be collected from: ✔ Field survey using GPS ✔ High-resolution satellite images (Google Earth etc.) ✔ Existing maps or survey reports 🧭 Exampl...
Post a Comment
Read more

Supervised Classification

Image Classification in Remote Sensing Image classification in remote sensing involves categorizing pixels in an image into thematic classes to produce a map. This process is essential for land use and land cover mapping, environmental studies, and resource management. The two primary methods for classification are Supervised and Unsupervised Classification . Here's a breakdown of these methods and the key stages of image classification. 1. Types of Classification Supervised Classification In supervised classification, the analyst manually defines classes of interest (known as information classes ), such as "water," "urban," or "vegetation," and identifies training areas —sections of the image that are representative of these classes. Using these training areas, the algorithm learns the spectral characteristics of each class and applies them to classify the entire image. When to Use Supervised Classification:   - You have prior knowledge about the c...
Post a Comment
Read more

Development and scope of Environmental Geography and Recent concepts in environmental Geography

Environmental Geography studies the relationship between humans and nature in a spatial (place-based) way. It combines Physical Geography (natural processes) and Human Geography (human activities). A. Early Stage 🔹 Environmental Determinism Concept: Nature controls human life. Meaning: Climate, landforms, and soil decide how people live. Example: People in deserts (like Sahara Desert) live differently from people in fertile river valleys. 🔹 Possibilism Concept: Humans can modify nature. Meaning: Environment gives options, but humans make choices. Example: In dry areas like Rajasthan, people use irrigation to grow crops. 👉 In this stage, geography was mostly descriptive (explaining what exists). B. Evolution Stage (Mid-20th Century) Environmental problems increased due to: Industrialization Urbanization Deforestation Pollution Geographers started studying: Environmental degradation Resource management Human impact on ecosystems The field became analytical and problem-solving...
Post a Comment
Read more

GIS: Real World and Representations - Modeling and Maps

Geographic Information Systems (GIS) serve as a bridge between the real world and digital representations of geographic phenomena. These representations allow users to store, analyze, and visualize spatial data for informed decision-making. Two key aspects of GIS in this context are modeling and maps , both of which are used to represent real-world geographic features and phenomena in a structured, analyzable format. Let's delve into these concepts, terminologies, and examples in detail. 1. Real World and Representations in GIS Concept: The real world comprises physical, tangible phenomena, such as landforms, rivers, cities, and infrastructure, as well as more abstract elements like weather patterns, population densities, and traffic flow. GIS allows us to represent these real-world phenomena digitally, enabling spatial analysis, decision-making, and visualization. The representation of the real world in GIS is achieved through various models and maps , which simplify...
Post a Comment
Read more