Atmospheric Optics
Water Drop ! Effects: ! Plot shows what! direction light! likes to scatter!
Coronas (around Sun,moon)
0 deg. = in line with! light beam! 180 deg. = Opposite of! light beam!
Rainbows (Sun,moon behind you)
Glories (Sun,moon behind you)
Rainbows
Primary band @ 420; blue inside, red outside Secondary band @ 510; red inside, blue outside Alexander’s dark band in between (dark background best viewing)
Brighter inside; supernumeraries
Rainbows through large rain drops have such large size parameters, they can be explained through geometric optics/ ray tracing as well as Mie calculations.
Rainbow intensity varies According to: 1. Size of drop 2. Strength and hue of ambient light 3. Index of Refraction
Larger drops create more Intense rainbows Less wave interference, less Overlap of colors
In dry environments all the small drops evaporate, leaving only the larger drops
Rainbows are 420 in radius, centered on the point directly opposite the sun. when the sun is 420 high, only the uppermost portion is visible
Marine rainbows less spectacular
Rainbows vary with hue of incident light. Rainbows near sunset/sunrise more reddish - blue light attenuated out by aerosols,haze (recall sun behind you, sunlight has to travel)
Even more obvious….
Moonbow. Like a rainbow but the moon is the light source
Probably enhanced photographically - not enough light to see much color w/ eye
Bows caused by small drops: more diffuse, diffraction effect Lunar fogbow
Daytime fogbow
Arctic fogbow
SHEBA cloud bow Probably super-cooled water drops. Note some color
RICO bow. Drizzle ? Large drops ? Supposedly at r< 25 micron all color disappears
I don’t see a bow on this plot anymore at X=100, R=X*wavelength/(2*pi) = 8 micron & supposedly no color at X< 300 (r=25 micron)
The 42 degree angle is set partly by the index of refraction, or how light propagates through a medium. This index differs for fresh water and sea water. Seawater has a slightly smaller rainbow angle than fresh water.
Reflection Rainbow Produced after sunlight Has reflected off of the Calm water surface In effect like a second Sun located below The horizon.
Twinned rainbows. Thought to be caused by non-spherical raindrops.
Coronas Forward-scattering; Seen around Sun & moon; Common Note dependence on Particle size. Aerosol won’t do it; big drops/ particles create a very thin (if intense) corona. at x=30 (r=8 micron) get good patterns
Coronas: seen around moon, sun, when a thin haze/cloud layer Exists between you and the light source
solar
Bright white inner circle + circles of different colors
Lunar corona easier to see & you don’t’ need such small particles (can also be ice)
Can think of it as constructive/destructive wave Interference
Effect of droplet sizes on coronas:
Monodispere droplet Population => more Obvious pattern
Coronas are fairly common; often altocumulus clouds
Iridescent water clouds are very similar to coronas. Most obvious when droplet distribution is narrow and Droplets are small - a young cloud, or lenticular cloud
Glories !
BackscatteringSun,moon behind you
Glories 0 deg. = in line with! light beam! 180 deg. = Opposite of! light beam!
(Sun,moon behind you)
Since light source must be behind you & above the horizon, glories are often seen either from hill/mountain sides, or from an airplane
Airplane glories very common - look for one !
Explanation requires both Ray tracing + Surface waves
In this case, droplet diameter = 9.6 micron
Effect of droplet size on glories. Similar to corona but less of the Intense white inner circle Dropsize distribution
Pattern more pronounced for Monodisperse drop population
Drop diameter
and smaller drops
Nature really likes to light up the “anti-solar” point Besides glories, which are imaged by cloud drops: 1. “heiligenschein” or holy light. similar to a glory but caused by dew drops resting on greenery 2. “opposition effect”. requires no drops or water, caused simply by a lack of shadows
Heiligenschein “holy light”
Wet grass
The mechanism
Dew drops act as lenses, some preference for direct backscattering
Opposition Effect: at the angle at which light is directly backscattered, no shadows can be seen. Thus an apparent brightening.
Coral Gables, April 12, 2006. Dry grass.
Really obvious on the moon
Astronaut taking picture
Moon’s brightness goes up significantly when it is full due to this effect => responsible for its naming
http://www.atoptics.co.uk
That’s it for now…
