Meteorology 125
Green Flash and other
Atmospheric Optics
Henry Bartholomew
February 14, 2011
Green Flash
• What is the Green Flash?
– An afterimage from staring at the sun?
– A green spot that forms above the sun?
– The top of the sun appearing green to our
eyes?
Green Flash
Green Flash
• The green flash is not an after image
– It appears at both sunrise and sunset!
Physics behind the green flash
• As light from the sun travels into and through
our atmosphere, refraction occurs
• Apparent sunset occurs a few minutes after the
sun has dipped below the horizon
Physics behind the green flash
• Refraction amount is dependent on wavelength
– And therefore its…
• COLOR
• Blue light is refracted the most, red least
• Highest red image + lowest blue image create
white sun with top blue rim, bottom red rim
Physics behind the green flash
• When the sun is setting, eventually all
wavelengths longer than green can be blocked
– Only green visible
• Green Flash mainly seen by observer
• I-Mir (Inferior Mirage) Green Flash
– Lasts 1-2 seconds
– You might ask: Wait, what about blue? Blue light is
shorter wavelength and refracts most, shouldn’t we
see blue just prior to the sun completely dipping
below the horizon?
• Ideas?
Why we don’t see a blue flash
• Think about why the sky is blue…
• Atmosphere scatters blue light more than
any other wavelength (color)
– Rayleigh Scattering
• Under very clear conditions, however, blue
flash can be seen!
Blue Flash
Green Flash
• Best places to view
– Clear view of the horizon
• Ocean
• Mountain tops
Mock Mirage Green Flash
Mock Mirage Green Flash
• Piece of sun breaks off on top and is
green for 1-2 seconds
• Temperature inversion must be present,
and observer must be above it
– Doesn’t have to be large
Other Green Flashes (Rarely seen)
• Sub-duct flash
– Observer below large inversion
– Lasts up to 15 seconds!
Sub-Duct Flash
Green Ray
• Green light spurts upward from sun
• Lasts several seconds
Green Flash over Golden Gate
http://www.atoptics.co.uk/atoptics/gfvid1.htm
Green flash over St. Kitts
http://www.youtube.com/watch?v=jYkwmW0rGrM
Rainbows
• We have learned about primary and
briefly, secondary rainbows
– Is there a such thing as a triple rainbow?
• Thoughts?
• In order to discuss, we need to first
review how a rainbow forms
One possible explanation
http://www.ebaumsworld.com/pictures/view/947432/
Colin McKellar
Rainbow Formation
• Light from the sun enters raindrop
• Because water is different medium, light
bends and therefore travels slower
through drop
• Different wavelengths refract different
degrees
• Light then reflects inside raindrop, exits
and then refracts more
Rainbow Formation
http://svr225.stepx.com:3388/rainbow/file/69807.png
http://www.rebeccapaton.net/rainbows/formatn.htm
Secondary Rainbow
Secondary Rainbow
• Instead of reflecting once, light entering
raindrop undergoes two reflections
– Result
• Colors are now flipped, so blue is on top and red is on
bottom
• Due to longer time in raindrop, there is more time of
refraction, so colors are spread out over a greater area, and
the rainbow is broader
• Two reflections means more light is lost, as some travels
through out of raindrop instead of being reflected
– Reflection produces a rainbow with 43% brightness intensity of
primary
 Broadening lowers surface intensity even more – hence,
why we don’t see double rainbows more often
Tertiary Rainbow
• It is possible for light to be reflected 3
times inside of a droplet
• The 3rd rainbow has about 24% brightness
of primary bow
– However, “triple rainbows” are almost
impossible to see by observers
• Numerous reasons why
Triple Rainbow
• First, instead of being opposite of the sun,
it is on the same side!
– Its light is overtaken by the 0th degree
rainbow from the sun – basically, a glare of
the sun through raindrops
• Much broader, due to longer time in
raindrop and hence more refraction and
spreading out of colors
– Less intensity per unit area
Even Higher Orders
• 4th order rainbow: Above 3rd order
rainbow, with colors reversed
– 15% intensity of primary rainbow
• 5th and 6th order rainbows: On opposite
side of sun
– Much broader than lower order bows
Higher Order Rainbows
http://www.atoptics.co.uk/rainbows/orders.htm
New topic
• Look to the right of the title!
Clouds
• Tend to think clouds only occur in troposphere
(except overshooting tops)
• Actually, there are clouds that can not only form
in the stratosphere, but even the mesosphere!
• We’ll look at the lower ones first, called polar
stratospheric clouds (PCS)
– One type is nacreous clouds
Nacreous clouds
Very bright: Why?
http://www.atoptics.co.uk/highsky/nacr1.htm
Physics behind PCS
• Form at very high altitude
in the stratosphere, at
heights of 15-25 km
– Lower part – why?
• Composed of ice crystals
diameter 10 µm
• Colors due to diffraction
and interference
• Form at temperatures
below -78ºC (-108ºF)
http://www.pahof.de/28457.html
Two types of PCS
• Nacreous, which form at temperatures below 85ºC
• Type I: Less dramatic, slightly warmer (-78ºC)
– Ia: Chemical compounds of H2O and nitric acid
– Ib: Spherical droplets of nitric and sulphuric acid
– Ic: Particles of metastable nitric acid - H2O
Noctilucent Clouds
• Literally, the highest clouds in our atmosphere
• Form high in the mesosphere 80 km (50 miles)
above the surface, 10 times as high as cirrus
clouds!
• Usually white and blue wispy streak
• Extremely rare to see!
Physics behind Noctilucent Clouds
• Usually seen in polar regions during twilight, in summer
months, and shortly before sunrise and after sunset
– Why?
• There is very little H2Ov at this level, but at extremely
cold temperatures of probably around -160ºC (-256ºF!!),
the tiny amount available can freeze onto nuclei
– One possible supply is meteoric dust
• There are still a lot of uncertainties behind these clouds,
but they have increased over the past several years
– Could be related to climate change
Discussion of noctilucent clouds
wouldn’t be complete without
pictures!
http://www.thelivingmoon.com/43ancients/02files/Noctilucent_Cloud_Images_01.html
Noctilucent clouds
http://www.atoptics.co.uk/highsky/nlc1.htm
NASA video
Interested in even more
atmospheric optics?
• There are numerous other features!
• Circumzenithal arcs, circumhorizontal arcs,
moonbows, coronas, supernumerary rainbows,
aurora borealis/australis, and more!
• Check out this awesome site
– http://www.atoptics.co.uk/
Thanks for listening!
• Any further questions?