2. Interactions of EM Radiation with the
Atmosphere
The most important source of energy is the Sun. Before the
Sun’s radiation reaches the Earth's surface it has to travel
through some distance of the Earth's atmosphere.
The composition of the atmosphere is thus of importance in
remote sensing because EMR must pass through it in order to
reach the Earth’s surface.
3. Interactions of EM Radiation with the
Atmosphere
The atmosphere also
contains particles with a
range of sizes and sources
which are of great importance
in remote sensing.
Composition of the atmosphere
Component Percentage
N2 78.08
O2 20.94
Ar 0.93
CO2 0.0314
O3 0.00000004
4. Interactions of EM Radiation with the Atmosphere
EMR interacts with particles and gases in the
atmosphere. Three processes serve to attenuate
the signal we are trying to detect
1.Scattering: Redirection of EMR from its original path
2.Absorption: Retention of EMR by molecules in the
atmosphere
3. Refraction: Passing of EMR through the atmosphere
5. Scattering
• Scattering of electromagnetic radiation is
caused by the interaction of radiation with
matter resulting in the reradiation of part of
the energy to other directions not along the
path of the incident radiation.
• Scattering effectively removes energy from
the incident beam. Unlike absorption, this
energy is not lost, but is redistributed to other
directions.
6. • Scattering - Scattering is the redirection of EM
energy by particles suspended in the
atmosphere.
• It is dependant upon the number of particles
present in the atmosphere
• the size of the particles
• the wavemlength of incoming radiation and
• the depth of atmosphere that the radiation
must travel through.
10. Rayleigh Scattering
• Rayleigh scattering , named after the British
physicist Lord Rayleigh.
• Rayleigh scattering occurs when the size of the
particle responsible for the scattering event is
much smaller than the wavelength of the
incident light.
• wave length dependant, favoring short
wavelengths, and is responsible for our sky
appearing blue.
11. Rayleigh scattering
• Scatters light in all directions
• Most effective at short
wavelengths(blue,voilet) hence blue sky
12. Rayleigh scattering
Diameter of particles << wavelength of EMR (small
specks of dust or N2 and O2)
Rayleigh scattering causes shorter wavelengths of energy
to be scattered much more than longer wavelengths.
Rayleigh scattering is the dominant scattering mechanism
in the upper atmosphere.
The fact that the sky appears "blue" during the day
is because of this phenomenon.
13. Blue light from the sun is scattered more than red so the sky is blue during the day. At
sunset the angle of light is very low and the blue light is scattered away completely
leaving only the less scattered red.
14. Why sky is blue?
Figure showing the greater proportion of blue
light scattered by the atmosphere relative to
red light.
15. Why is the sky blue?
• As sunlight moves through the atmosphere,
longer wavelengths(eg.red) pass straight
through .
• However, shorter wavelengths(eg.blue)
interact with gas molecules and scatter in the
atmosphere
16. Mie scattering
Diameter of particles = wavelength of EMR (Dust,
smoke and water vapor)
Dust, smoke and water vapour are common causes of
Mie scattering which tends to affect longer wavelengths
than those affected by Rayleigh scattering.
Mie scattering occurs mostly in the lower portions of the
atmosphere where larger particles are more abundant,
and dominates when cloud conditions are overcast.
17.
18. Nonselective scattering
Diameter of particles >> wavelength of EMR
(Water droplets and large dust particles)
This occurs when the particles are much larger than the
wavelength of the radiation. Water droplets and large
dust particles can cause this type of scattering.
Nonselective scattering gets its name from the fact that
all wavelengths are scattered about equally.
