Ticker for September 20, 2002

                
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September 20, 2002 September 20, 2002 September 20, 2002 September 20, 2002



Somewhere Between the Rainbows

The Ticker photography department (amateurs, obviously) sent these
pictures, taken shortly after last Saturday's storms in Oklahoma City:




What's the big deal about a rainbow?  First of all, if you look
closely, you'll see two rainbows: the primary rainbow (between the
green arrows), and a secondary rainbow (between the blue).

And if you'll look even more closely, you might notice that the area
of the sky between these rainbow is darker than the area on either
side of them.  That area is called Alexander's Dark Band, and it's
a real phenomenon.

When sunlight is backscattered to the eye via small spherical objects
like cloud droplets, those rays that reflect twice within the droplet
(sun-bounce-bounce-eyeball) exit at angles up to about 42 degrees
from the line from source (the sun) to the target (the droplet), with
most of the light focused near this upper limit (this makes the bow).
Those rays that reflect thrice within the droplet (sun-bounce-bounce-
bounce-eyeball) exit at angles greater than (but focused at) about
50 degrees.  The area between these two arcs is devoid of such
backscattered illumination and is darker.

A more thorough treatment of the topic can be found on several
academic websites, such as UCARs:
http://www.unidata.ucar.edu/staff/blynds/rnbw.html

By the way, the Alexander of Alexander's Dark band is the Greek
philosopher Alexander of Aphrodisias, who described the phenomenon
in the early 3rd Century.  Unfortunately, because he spent most of
his time critiquing Aristotle, he never figured out if the dark band
was the place where troubles melt like lemondrops.