Light Color and AtmosphericLight, Color, and Atmospheric Opticsp

Chapter 19Chapter 19April 28 and 30, 2009

ColorsColors• Sunlight contains all colors (visible spectrum)

Wavelength Ranges in µmUV-C: 0.25 - 0.29UV-B: 0.29 - 0.32UV-A: 0.32 - 0.38Visible: 0.38 - 0.75Near IR: 0.75 - 4Mid IR: 4 - 8Mid IR: 4 8Longwave IR: 8 - 100

ColorsColors• If object is white then all colors are

reflected• If object is red then only red is reflectedIf object is red then only red is reflected

and all others are absorbed

From Malm (1999). Introduction to Visibility

Blue Skies and Hazy daysBlue Skies and Hazy days• Selective scattering• Selective scattering

– Scattering depends on wavelength of light• Rayleigh scattering

– Very small particles gases– Very small particles, gases• Mie scattering

– Particles the size of wavelength of visible light• Blue hazeBlue haze • Some particles from plants: terpenes, α-

ipinene

Blue Ridge MountainsBlue Ridge Mountains

• Blue haze caused by scattering of blue light by fine particleslight by fine particles

Crepuscular Raysp y• Scattering of sunlight by dust and haze

produces white bands

Selective Scattering: SunsetSelective Scattering: Sunset• Notice the effects of scattering on color asNotice the effects of scattering on color as

sunlight penetrates into the atmosphere

Twinkling Twilight and the Green FlashTwinkling, Twilight, and the Green Flash

• Light that travels from a less dense to a• Light that travels from a less dense to a more dense medium loses speed and bends toward the normal, while light that enters a less dense medium increase speed and pbends away from the normal.

• Apparent position scintillation and green• Apparent position, scintillation, and green flash all depend of density variations in the t hatmosphere

• Scintillation also indicates the amount of turbulence in the atmosphere

Bending of Light Paths from AtmosphereBending of Light Paths from Atmosphere

Green Flash• Green flash at top may be seen under circumstances of

very cold (dense) air near the surface rapidly changing to warmer (less dense) air aloft

MiragesMirages• Mirages are caused by refraction of light g y g

due to strong density differences in atmospheric layers usually caused byatmospheric layers usually caused by strong temperature differences.

An object appears to be displaced from true position.

Inferior Image FormationInferior Image FormationI f i d t dInferior: down to ground

Superior Image FormationSuperior Image FormationSuperior: up into air

Example: Fata Morgana–Fairly uniform surface is transformed into one of vertical walls and columns with spirescolumns with spires–Usually in polar regions with warm air over a cold surface.

22° Halo22 Halo

Formation of 22° and 46° halosFormation of 22 and 46 halos

Effects ofEffects of Dispersion

SundogsSundogs

Sun PillarSun Pillar

Ice Crystal ShapesIce Crystal Shapes

http://www.atoptics.co.uk/halo/common.htm

RainbowsRainbows

• Face falling rain with sun at back, water drops disperse lightp p g

• Primary and secondary

Rainbow fromfrom

raindrops

Other Single Particle EffectsOther Single Particle Effects

Results from geometrical optics, snell’s law of refraction for drops of a few mm in diameter