|
meteoptic-l [ät] ursa.fi
message archive
This is meteoptic-l [ät] ursa.fi message archive.
Note, your can reply the messages on this page only if your
are already subscribed the list.
» To the end of the list/message
Paul Schlyter: "Re: DD meteoptics" (Feb 2, 10:48):
> 3. "Spectre of Brocken", "Glory", "Anti-Corona": these are different
> names of the same phenomenon, which is due to diffraction in small
> water droplets floating freely in the air. It's typically seen on a
> cloud if you're above the cloud. This phenomenon, which is quite
> complex, is probably related to the rainbow. We all know the
> appearance and the cause of the ordinary rainbow, right? If the
> water drops get smaller, diffraction of light will cause visible
> effects. First one will see extra (or supernumerary) rainbows,
> immediately inside the primary raindow, with colors like
> red-blu-red-blue/etc up to three or even 4-5 times. If the water
> droplets are even smaller, the primary rainbow vanishes and only the
> supernumerary bows are seen, but their colors are now so well mixed
> that we only see them as white -- this is the "fog-bow". The fog-bow
> is a few degrees smaller than the normal primary rainbow. Now, if
> the water droplets get even smaller than that, the fogbow will become
> smaller still, and fainter -- until it all "converges" into a spot a
> few degrees in diameter, directly opposite the Sun. Since all the
> light now is concentrated in a fairly small spot, this "degenerated
> fog-bow" once again becomes visible, as the anti-corona, or glory.
>
This theory may not be quite correct. It cannot explain the bright
colours usually seen in glories, for example. I think it was Van De
Hulst who showed in his famous book (scattering light by small
particles, 1957, or something like that) that glories cannot be
explained as a phenomenom of geometric optics, but instead, one must
consider a phenomenom called surface waves as well. I don't want to go
into details which I don't remember (or even quite understand), but
those who wish can check it out from the forementioned book, or
Greenlers book, for example.
About diffraction. I think diffraction cannot cause backscattering, even
for rayleigh scatterers all the diffracted light is concentrated to the
forward half of the space. Thus, diffraction can only play the part in
glory if multiple scattering takes place. This in turn is quite likely
as the clouds where glories are seen are usually optically thick. Were
you referring to multiple scattering or did I miss a point somewhere?
I wish these light scattering theories would be simpler, but they aren't.
I guess we just have to live with that. :-(
|