As long as this list has grammar and punctuation pedants, we might as well
have an atmospheric optics one as well :)
Moose's photo is an example of cloud iridescence, which is caused by
diffraction (not refraction) of light by small cloud droplets or crystals.
Halos are caused by refraction through hexagonal ice crystals. There are
many different kinds of halos and complex displays can be quite
spectacular. One of the more common ones is the 22 degree halo, and sundogs
are bright spots in this halo caused by alignment of the crystals.
The rainbow is pretty straightforward refraction and reflection, with the
primary bow caused by one bounce off the back of the droplet and the
secondary bow caused by two bounces. Supernumerary bows, one of which is
visible in John's photo inside of the purple band, are interference effects
and can't be described with geometric optics.
The glory in Tina's photos is also more complicated and can't be described
by geometric optics alone either. It's also harder to see since the
observer has to be above the clouds, as Tina was in Machu Picchu.
A corona is another diffraction phenomena, and is an Airy disk seen around
the light source, like the sun or moon.
There's lots of good web sites on atmospheric optics around for anyone who
is interested.
Mark
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