1. Atmospheric Optics - I

2. First midterm exam is next Wednesday!
The exam will be in-class, during our regular lecture on Wednesday February 13 at 3pm
The exam will be CLOSED BOOK
No textbooks
No calculators
No cheat-sheets
Alternate seating
The grades will be posted on WebCT
The exam covers Chapters 1,2,3,4,5, and 19.
For a list of sections which are not covered, see the Class Notes on the class web page

3. 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 bird’s 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 astronaut’s

4. Color
The human eye is sensitive to the visible part of the electromagnetic spectrum (l= 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).

5. 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: R>>l
Rain drops (R~ mm)
All wavelengths equally scattered
Optical effects: white clouds
Mie scattering: R~l
Aerosols (R~ mm)
Red scattered better than blue
Blue moon, blue sun
Rayleigh scattering: R<<l
Air molecules (R~ mm)
Blues scattered better than red
Blue sky, blue mountains, red sunsets l R l R l R

7. White Clouds

8. 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.

10. 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.

11. Blue Mountains

12. Crepuscular Rays Light scattered underneath low clouds
“the sun is drawing up water”, “Jacob’s ladder”

13. Recap: blue sky and white clouds

14. 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.

15. Extraterrestrial skies
Moon sky
Mars at noon
Mars at sunset
Venus sky
Titan

16. The Color of the Sun

17. The Color of the Sun

18. 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).

19. 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.

20. Reflection and Refraction of Light
The speed of light in vacuum is c=300,000 km/s
Snell’s 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.

21. 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.

22. The Timing of the Sunset & Sunrise
We see the sun before it actually rises above the horizon and after it sets below the horizon.

23. Twilight

24. Flattening of the Sun’s Disk at Sunset
Green flash

25. Flattening of the Moon
Refraction by the Earth’s atmosphere (image from ISS)