1. Part 7. Special Topics and Appendices Chapter 17 Atmospheric Optics

2. Refraction causes a distortion of images Refraction is the change in the propagation direction of light due to a change in the velocity of the light. The thicker the atmosphere, the slower the speed of light through the atmosphere. The light ray turns downward due to the thicker atmosphere at lower elevations

3. Refraction and the Setting or Rising Sun Refraction greatest with low solar angles because the radiation is passing through the thicker lower atmosphere at a great angle from the vertical The real sunset/sunrise position is different from the one that is visible; refraction also affects the apparent shape of the sun The color of the sun at sunrise/sunset is affected by Rayleigh and Mie scattering

4. Effects of refraction on the solar disk at sunrise or sunset The sun is actually below the horizon when we first see it at sunrise or last see it at sunset!

5. Mirages Mirages arise due to refraction from steep near- surface temperature gradients causing density changes in the air Distant objects appear lower than they actually are Further objects appear distorted Very far objects disappear Inferior mirages –A wavy inverted image of the actual image Superior mirage –Images displaced upward

6. Temperature effects and mirages Person at A is fully seen. Only top half of person at B is seen. The person at C is not seen.

7. An inferior duplicate mirage In the inferior mirage, the tree appears to be upside down and below the Earth’s surface. It will appear to be moving like the surface of water on a pond.

8. Rainbows Refraction and reflection of sunlight entering a raindrop Droplets in front, sun behind the viewer Primary rainbows –Brightest and most common –Shortest wavelengths occupy innermost portion of the rainbow –Secondary rainbow surrounds primary –A partial rainbow may appear

9. Rainbow formation The sun is directly behind the viewer, who is looking at the drops Secondary rainbows form due to light that reflects twice within the drops

10. A secondary bow requires two reflections

11. Halos, Sundogs, and Sun Pillars From cirrostratus clouds Ice crystals refract light Halos form around the Sun or Moon –Radii of 22 o or 46 o Sundogs –Whitish spots in the sky –They occur when ice crystals of 30  m align horizontally

12. Halo formation

13. Sundog formation A sundog is a false image of the sun at 22° from the sun’s true position

14. Sun pillars Ice crystals reflect sunlight off their tops and bottoms Coronas and glories The form from light bending around water droplets due to diffraction (light changing direction around an object) A corona is a circular illumination surrounding the Moon or Sun due to clouds with uniform droplet sizes Circular illumination surrounding the Moon or Sun due to clouds with uniform droplet sizes A glory is a rainbow seen on clouds from above (such as while flying in aircraft)

15. Refraction, reflection and diffraction associated with a glory

16. End of Chapter 17 Understanding Weather and Climate 4th Edition Edward Aguado and James E. Burt