Atmospheric Scattering in remote Sensing
- 1. Energy Interactions in the Atmosphere
- 2. Composition of the Atmosphere Atmosphere : Gaseous envelop that surrounds the Earth’s surface Much of the gases are concentrated within the lower 100km of the atmosphere Only 3x10-5 percent of the gases are found above 100 km (Gibbson, 2000)
- 3. Energy Interactions • From the source to the sensor, the radiation passes through the atmosphere • Path length: The distance traveled by the radiation through the atmosphere - Varies depending on the remote sensing techniques and sources - Space photography using solar energy • Path length = 2x Thickness of the earth’s atmosphere - Airborne thermal sensors using emitted energy from the objects on the earth • Path length = One way distance from the earth’s surface to the sensor
- 4. Energy Interactions… • The intensity and the spectral composition of the incident radiation are altered by the atmospheric effects • Atmospheric interaction depends on the - Properties of the radiation such as magnitude and wavelength - Atmospheric conditions - Path length • Interaction with the atmospheric particles - Scattering - Absorption
- 5. Energy Interaction in the Atmosphere • Atmosphere as being between the sun and the ground and between the ground and the remote sensor. • Three fundamental interactions are possible. • Absorption • Transmission • Scattering
- 6. Energy Interaction in the Atmosphere
- 7. Energy Interaction in the Atmosphere:Absorption • Absorption : Process in which the incident energy is retained by particles in the atmosphere • Energy is transformed into other forms • Unlike scattering, atmospheric absorption causes an effective loss of energy • Absorption depends on – Wavelength of the energy – Atmospheric composition – Arrangement of the gaseous molecules and their energy level • The absorbing medium will not only absorb a portion of the total energy, but will also reflect, refract or scatter the energy. The absorbed energy may also be transmitted back to the atmosphere.
- 8. Absorption The most efficient absorbers of solar radiation are Water vapour, carbon dioxide, and ozone Gaseous components are selective absorbers of the electromagnetic radiation Absorb electromagnetic energy in specific wavelength bands Depends on the arrangement of the gaseous molecules and their energy levels Atmospheric window The ranges of wavelength that are partially or wholly transmitted through the atmosphere Remote sensing data acquisition is limited through these atmospheric windows
- 9. • Wavelengths shorted than 0.1 μm – Absorbed by Nitrogen and other gaseous components • Wavelengths shorter than 0.3μm (X-rays, Gamma rays and part of ultraviolet rays) – Mostly absorbed by the ozone (O3) • Visible part of the spectrum – Little absorption occurs • Oxygen in the atmosphere causes absorption centered at 6.3μm. • Infrared (IR) radiation – Mainly absorbed by water vapour and carbon dioxide molecules • Far infrared region – Mostly absorbed by the atmosphere • Microwave region – Absorption is almost nil Atmospheric window
- 10. Scattering • Process by which small particles in the atmosphere diffuse a portion of the incident radiation in all directions • There is no energy transformation during scattering • Spatial distribution of the energy is altered • Types of scattering Rayleigh scattering Mie scattering Non-selective scattering http://www.geog.ucsb.edu/~joel/g110_w08/lecture_note s/radiation_atmosphere/radiation_atmosphere.html
- 11. Energy Interaction in the Atmosphere: Scattering • Atmospheric Scattering is the unpredictable diffusion of radiation by particles in the atmosphere. The amount of scattering depends on several factors. • Wavelength of the radiation • Amount of particles and gasses • Distance the radiation pass through the atmosphere • Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause EM radiation to be redirected from its original path. • Affects shorter wavelengths. • In the absence of particles and scattering the sky would appear black. • At sunrise and sunset the sunlight travels a longer distance through the atmosphere
- 12. Types of Scattering Rayleigh Scattering (blue light) Nonselective Scattering (all light) Mie Scattering (red and infrared)
- 13. Rayleigh Scattering • When Particles are smaller than coming wavelength. Can be small particles of dust, nitrogen or oxygen particles. • Also known as selective scattering or molecular scattering • Dependent on the wavelength • Occurs when particles are much smaller than the wavelengths of the radiation Particle size less than (1/10)th of the wavelength • Intensity of the scattered light is inversely proportional to the fourth power of wavelength Shorter wavelengths are scattered more than longer wavelengths
- 14. Rayleigh Scattering of the Visible Part of the EM Energy • Scattering of the visible bands is caused mainly by the molecules of Oxygen and Nitrogen Blue (shorter wavelength) is scattered more - Blue light is scattered around four times the red light - UV light is scattered about 16 times the red light - A "blue" sky is a manifestation of Rayleigh scatter Orange or red colour during sunrise and sunset - Sun rays have to travel a longer path - Complete scattering (and absorption) of shorter wavelength radiations - Only the longer wavelength (orange and red) which are less scattered are visible Other examples - The haze in imagery - Bluish-grey cast in a color image when taken from high altitude
- 15. Mie Scattering • Occurs when the wavelengths of the energy is almost equal to the diameter of the atmospheric particles • The most important cause of Mie scattering are aerosols(mixture of gases, water vapor and dust) • It is generally restricted to lower atmosphere where larger particles are more abundant and dominates under overcast cloud conditions. • It influence spectral region from near-ultraviolet up to including the near infrared . • Longer wavelengths also get scattered compared to Rayleigh scatter • Intensity of the scattered light varies approximately as the inverse of the wavelength
- 16. Non Selective Scattering • When particle size is much larger than the radiation wavelength. like water droplets and larger dust particles. • It is independent of wavelength , with all wavelength scattered about equallly. Particles such as pollen, cloud droplets, ice crystals and raindrops can cause non-selective scattering of the visible light. This type of scattering causes Fog and cloud to appear white to our eyes because blue, green and red are scattered approx. in equal quantities.
- 17. Sensor Selection for Remote Sensing Criteria for sensor selection The spectral sensitivity of the available sensors The available atmospheric windows in the spectral range(s) considered. The source, magnitude, and spectral composition of the energy available in the particular range. Energy interactions with the features under investigation in the selected bands
- 18. Thanks
Editor's Notes
- #8: Ozone, carbon dioxide, and water vapor are the three main atmospheric constituents which absorb radiation. Ozone serves to absorb the harmful (to most living things) ultraviolet radiation for the sun. Without this protective layer in the atmosphere our skin would burn when exposed to sunlight. Carbon dioxide referred to as a greenhouse gas. This is because it tends to absorb radiation strongly in the far infrared portion of the spectrum - that area associated with thermal heating - which serves to trap this heat inside the atmosphere. Water vapour in the atmosphere absorbs much of the incoming longwave infrared and shortwave microwave radiation (between 22μm and 1m). The presence of water vapour in the lower atmosphere varies greatly from location to location and at different times of the year. For example, the air mass above a desert would have very little water vapour to absorb energy, while the tropics would have high concentrations of water vapour (i.e. high humidity).