Part 7. Special Topics and Appendices

Chapter 17

Atmospheric Optics

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

Refraction and the Setting or Rising SunRefraction 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

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!

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

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.

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.

RainbowsRefraction 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

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

A secondary bow requires two reflections

Halos, Sundogs, and Sun Pillars• From cirrostratus clouds• Ice crystals refract light • Halos form around the Sun or Moon

– Radii of 22o or 46o

• Sundogs– Whitish spots in the sky– They occur when ice crystals of 30 m

align horizontally

Halo formation

Sundog formation

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

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)

Refraction, reflection and diffraction associated with a glory

End of Chapter 17

Understanding Weather and Climate

4th Edition

Edward Aguado and James E. Burt