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Hi Les, others,
here are some additional things that can be done to study the twinned bow
phenomenon:
- use polarizer in conjunction with the telephoto lens to make close-up
photos of any twinned bows (the bows are tangentially polarized so the
polarizer doesn't work too well for wide-angle photos). The polarizer
decreases the background brightness (it acts as a ND filter for
unpolarized
light) while the rainbow light is transmitted almost unattenuated, if the
filter is turned the correct way. The result is a rainbow that is about 2
stops brighter than the background.
- when seeing such a bow, download and save precipitation radar images of
the local weather office. Maybe it can be figured out in which direction
the bow was and in what range of distances, and correlate this with the
precipitation intensities.
- when seeing a rainbow in a storm that is NOT twinned, try to study its
roundness anyway. This cannot be done by photos, since wide-angle lenses
will distort the image. If we can discover rainbows that are sometimes
not
round, we are one step closer to accepting that the twinned bow is due to
a
distinct mix of oblate and round droplets.
- make stereo photos to study the 3D pattern of the twinned bow; see
remark
below
Remark: some may doubt the possibility of two quite distinct sizes and
shapes of droplets occuring in storms. One should realize that the
rainbow
is a 3D cone of light with its apex at your eye; if rainfall is
everywhere. That means that the twinned bow may well be caused by drizzle
very near you and a heavy downpour with large oblate droplets in the
distance. Thus, the mix of droplets does not even have to be at the same
position in space!
Actually, I find this explanation very pleasing, since rainbows are
usually
seen at the back of receding thunderstorms. When a thunderstorm passes,
you usually first get a heavy downpour of big oblate droplets, followed by
a "drizzle" of smaller, rounder droplets while the trailing stratiform
region is passing over.
Harald
At 16:14 2003-05-21 +0100, you wrote:
>Dear Meteoptics Watchers,
>
>May I make a plea for more images of "twinned" primary rainbows
>and indeed any other anomalous bows. The twinned bows
>appear to be a phenomena that has not been very widely observed
>or reported.
>
>If possible use two lens focal lengths (short and long) so that the
>separation of the bows can be estimated from the bow curvature on
>the shorter focus shots whilst the longer focus lens will hopefully show
>the colour distribution nicely.
>
>Mie and Airy scattering programs can nowadays easily simulate
>primary bow and supernumerary colours well. It will be interesting
>to match new "twinned" bow images against their predictions and
>those of other models that employ mixtures of non spherical/spherical
>raindrops or even storms with mixed drops of water and ice. The test
>of all these hypotheses is more observations!
>
>Les
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