APOD Aug4, 2004 - Solar Arcs and Halos - Beware of bad link

Comments and questions about the APOD on the main view screen.
Les Cowley

APOD Aug4, 2004 - Solar Arcs and Halos - Beware of bad link

Post by Les Cowley » Wed Aug 04, 2004 3:49 pm

The description has a link to the UIUC meteorolgy site
http://ww2010.atmos.uiuc.edu/(Gh)/guide ... lo/22.rxml

Here is the page concerned with my critique in brackets ...

22 Degree Halo
a ring of light 22 degrees from the sun or moon

A halo is a ring of light surrounding the sun or moon.

(Halos are not necessarily circular nor around the sun/moon.
Rings around the sun/moon could also be a diffraction corona)

Most halos appear as bright white rings

(most halos are not 'rings')

but in some instances, the dispersion of light as it passes
through ice crystals found in upper level cirrus clouds can
cause a halo to have color.

IMAGE

(this is a 22º halo fragment and poor upper tangent arc
and is thus misleading)

Halos form when light from the sun or moon is refracted by ice
crystals associated with thin, high-level clouds. A 22 degree
halo is a ring of light 22 degrees from the sun

(It is a broad circular halo with its inner edge 22º from
the sun)

and is the most common type of halo observed and
is formed by hexagonal ice crystals with diameters less
than 20.5 micrometers.

(No! Clear halos are formed by ice crystals larger
than 0.05 mm across.

...hexagonal prisms do not have a diameter.)

Light undergoes two refractions as it passes through an ice
crystal

(No! It undergoes many, 22º halos result from light escaping
after two.)

and the amount of bending that occurs depends upon the
ice crystal's diameter.

(No! The angles of refraction and total ray deviation are NOT
a function of crystal size)

A 22 degree halo develops when light enters one side of a
columnar ice crystal

( the crystals forming a 22º halo are not known
with certainty. They are not necessary columnar.

The diagram misleadingly shows a 'singly oriented
column' whereas the essence of a 22º halo is that it is
formed by pseudo randomly oriented crystals)

and exits through another side. The light is refracted when it
enters the ice crystal and once again when it leaves the ice
crystal.

(This 'explanation' does not mention the critical need for
pseudo random orientations. It implies that rays are only
deviated through 22º - in fact they are deviated through 22 to
50+º by the two refractions. The key fact that 22º is the
angle of minimum deviation for a ray passing through an ice
crystal's faces inclined at 60º is entirely missed.)

Please avoid the UIUC site on meteorological optics, almost
every page has errors - many serious.

I have no wish to belittle other websites. However, I do feel
strongly that students and the public should not be misled -
particularly by a university site and when when the site
developers concerned have had opportunities to correct
errors.

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RJN
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Post by RJN » Wed Aug 04, 2004 4:09 pm

OK. Thanks, Les. I de-linked it.
- RJN