Atmospheric Optics - I

1
Atmospheric Optics - I
2
Recap

• Condensation above the Earth surface produces
  clouds.
• Clouds are divided into 4 main groups
• High
• Middle
• Low
• Clouds with vertical development
• Satellites give a birds eye view of clouds.
• Geostationary satellites
• Polar-orbiting satellites
• Infrared images can provide a three-dimensional
  view of the clouds
• Height of the cloud
• Temperature of the cloud

an astronauts
3
Color

• The human eye is sensitive to the visible part of
  the electromagnetic spectrum (l0.4-0.7 mm)
• Intensity - light or dark
• Wavelength color.
• We see the objects because they emit and/or
  interact with light (reflection, transmission,
  absorption, refraction, scattering, diffraction)
• Perception of color
• Each color corresponds to a particular
• wavelength.
• White all wavelengths are present with
• equal intensity
• Black no light is emitted and/or reflected
• from the object.
• Color of emitted light and temperature
• The sun appears white.
• Colder stars look redder (lmax is longer than
  lmax of the sun).
• Hotter stars look bluer (lmax is shorter than
  lmax of the sun).

4
Scattering of light

• The scattering of light in the atmosphere depends
  on the size of the scattering particles, R, and
  on the wavelength, l, of the scattered light.
• Geometric scattering Rgtgtl
• Rain drops (R10-100 mm)
• All wavelengths equally scattered
• Optical effects white clouds
• Mie scattering Rl
• Aerosols (R0.01-1 mm)
• Red scattered better than blue
• Blue moon, blue sun
• Rayleigh scattering Rltltl
• Air molecules (R0.0001-0.001 mm)
• Blues scattered better than red
• Blue sky, blue mountains, red sunsets

l
R
l
R
l
R
5
(No Transcript)
6
White Clouds
7
White clouds

• Description the clouds appear white or grey.
• Physical process geometric scattering by rain
  drops.
• Explanation
• Visible light at all l is scattered in all
  directions.
• Clouds are optically thick with respect to light
  scattering but they do not absorb light well.
• The thicker the cloud is the more light is
  scattered backwards and less solar light reaches
  the bottom of the cloud. Therefore thicker clouds
  appear darker.
• At the bottom of very thick clouds the raindrops
  are even larger and absorb sun light better.
  These clouds look even darker.
• Conclusion By the appearance of the cloud one
  can judge whether it will rain and how strong the
  storm will be.

8
(No Transcript)
9
Blue Sky

• Description on clear days the sky is blue and on
  hazy days the sky is white.
• Physical process Rayleigh scattering by air
  molecules.

• Explanation shorter l (violet, blue) are
  scattered more efficiently by air molecules than
  longer l (red).
• Conclusion Clear sky, distant objects have a
  bluish hue.

10
Blue Mountains
11
Crepuscular Rays

• Light scattered underneath low clouds
• the sun is drawing up water, Jacobs ladder

12
Recap blue sky and white clouds
13
White Sky

• Description on hazy days the sky is white.
• Physical process geometric scattering by dust
  and salt particles.
• Explanation all l are scattered equally well.
• Conclusion By the color of the sky we can judge
  the quality of the air, the dust load or the
  humidity of the air.

14
Extraterrestrial skies
Mars at noon
Moon sky
Mars at sunset
Venus sky
Titan
15
The Color of the Sun
16
The Color of the Sun
17
The Color of The Sun

• Description at sunrise and sunset the sun is
  yellow, orange or red
• Physical process Rayleigh scattering by air
  molecules and fine dust particles.
• Explanation on clear days only the blue light is
  scattered away, on hazy days the yellow and the
  orange wavelengths are also scattered and only
  the red remains in the direct solar light.
• Conclusion Red sunsets suggest that there is
  dust in the air (pollution, haze over the ocean,
  volcanic activity, dust storms).

18
Blue Moon

• Description the moon appears blue.
• Physical process Mie scattering by dust
  particles.
• Explanation When the size of the dust particles
  is approximately equal to the visible wavelengths
  the red light is scattered better than the blue
  light.
• Conclusion one can guess what is the size of the
  particles in the air.

19
Reflection and Refraction of Light

• The speed of light in vacuum is c300,000 km/s
• Snells law The angle of incidence is equal to
  the angle of reflection.
• Light that enters a more-dense medium slows down
  and bends toward the normal.
• Light that exits a more-dense medium speeds up
  and bends away from the normal.

20
True and apparent position of objects

• Due to the refraction of light the objects on the
  sky appear higher than they actually are.
• Star location and scintillations
• Timing of the sunset and the sunrise
• The sun on the horizon looks flattened
• Twilight.

21
The Timing of the Sunset Sunrise
We see the sun before it actually rises above the
horizon and after it sets below the horizon.
22
Twilight
23
Flattening of the Suns Disk at Sunset
Green flash
24
Flattening of the Moon
Refraction by the Earths atmosphere (image from
ISS)
http//www.sundog.clara.co.uk/atoptics/phenom.htm