Atmospheric Optics

Gabriel Lombardi

presented atCPS-AAPT Meeting

Spring 2003

A selection of phenomena

• rainbows• halos• sunsets• sun pillars• the green flash• noctilucent & iridescent clouds• contrails / rocket trails

Causes

• refraction (rainbows, halos, green flash)• diffraction (iridescent clouds)• Rayleigh scattering (sunsets)• Mie scattering (noctilucent clouds)• reflection (sun pillars)

What color is the Sun?

yellow... orange?

Chromaticity

• CIE chromaticity diagram

• 5000-6000Kblackbody is white

http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

The color of stars

http://www.physics.sfasu.edu/astro/color.html

http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

A real picture

The Sun is white!

Sunset colors

B:G:R

0.16 : 0.90 : 1.0 0.16 : 0.62 : 1.0 0.16 : 0.37: 1.0 0 : 0.28: 1.0

Rayleigh scattering

k

E

• polarized• spectrally weighted to the blue• scattering from density fluctuations

I ~ !"4

0.2 0.4 1.0R:G:B

0.0 0.0 1.0R:G:B

Rainbows

• secondary bow (colors reversed)• brighter inside the primary• polarization

Rainbow theory

Isaac Newton, Opticks

primary

secondary

from Sun

Rainbow geometry

10

15

20

25

30

35

40

45

0.2 0.4 0.6 0.8 1

distance from axis

angle

distance

220 Halos

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/halo.htmlFred Kinley, personal communication

Hexagonal ice crystals

n=1.31

600

Deviation by a prism

20

25

30

35

40

45

0 20 40 60 80 100

incident angle

apex: 600

index: 1.31

220

Sun pillars

• reflection from planar ice crystals

Iridescent clouds

• uniform drop size (Mie scattering)• thin cloud (single scattering)

http://antwrp.gsfc.nasa.gov/apod/ap030311.html

Rocket trails

• altitude/reddening• shear

http://www.freqofnature.com/photos/mmiii/

Noctilucent geometry

! =D

R= 2

2h

R

D = 2Rh h

Pinatubo sunset

http://www.liathach.supanet.com/volcansun.html

• still glowing one hour after sunset• h~20 km

Green flash

http://www.intersoft.it/galaxlux/GreenFlashGallery.htm http://www.isc.tamu.edu/~astro/research/sandiego.html

Green flash geometry

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html

Chinese lantern

Ed Ruf, personal communication