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> This phenomenon, often called opalescent or irridescent clouds,
> occurs very frequently in western Canada and United States. I have
> seen many beautiful displays in Central Alberta in all seasons, and
> at any time from about an hour after sunrise to about an hour
> before sunset.
There are two phenomena, which look very similar, but there
are some physical differencies:
- Iridescent clouds
These are ordinary tropospheric clouds, eg. altocumulus or
cumulus. Iridescence are seen very often, usually very near
the Sun (or sometimes the Moon). These clouds could be seen
coloured also in daytimes.
2) Mother-of-pearl (nacreous) clouds
Strastospheric cloud in about 17-30 km altitude. These
clouds are in their best in times of sunset or sunrise.
Nacreous clouds are also seen in daytimes, but I think
those have much pale colours.
Whats common with these? Both have same kind 'pearlish'
colors: pink, light green, indigo, .. This is due to
similar optical mechanism. For both homogenous population
of droplets are needed. In case of m-o-p clouds supercooled
water droplets are assumed.
How to distinguish these? Mother-of-pearl clouds are seen
mainly in sunset and sunrise. They are visible even the Sun
in several degrees below the horizon. Nacreous clouds are
also seen only in winter time.
-VeikkoM-
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