Atmospheric absorption and scattering
Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"
Meaning
On its way back up to the satellite, reflected energy has to pass through the air. Some of it is lost. Carbon dioxide, hydrogen and water vapour absorb (soak up) energy in the middle infrared. Dust scatters (bounces in all directions) blue light. This lost energy never reaches the sensor, so it is not recorded. This is why sensors are built to work in "atmospheric windows". These are the wavelengths that pass through the air easily.
Example
In the dusty pre-monsoon months over the Thar Desert in Rajasthan, dust scatters blue light. So images in the blue band look hazy and low in contrast. Bands such as near infrared, which pass through the air more easily, give a clearer picture of the ground.
Don't confuse with
- Absorptance: this is the share of energy that the ground surface itself absorbs. Atmospheric absorption happens in the air, between the surface and the sensor.
Related concepts
- Electromagnetic radiation
- Electromagnetic spectrum
- Spectral band
- Near infrared
- Thermal infrared remote sensing
- Reflectance
- Absorptance
- Spectral signature
- NDVI