Naked Science Forum

Life Sciences => The Environment => Topic started by: AllenG on 29/07/2008 23:09:18

Title: What causes iridescent clouds?
Post by: AllenG on 29/07/2008 23:09:18: It formed and disappeared inside of about ten minutes.
29 July 2008 6:00 pm Georgia, USA

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.1.jpg&hash=0adc64cd244c7414f29dd9d4909bf564)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.2.jpg&hash=8035fb097ba46372cc03b81e4007a31a)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.3.jpg&hash=a2df119bd81c3f90ab65b78aa2c88575)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.4.jpg&hash=f27447c8bd32de04402865b12aba8e74)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.5.jpg&hash=e4d0991d087a25bad4b2f8c2c4e41ef4)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.6.jpg&hash=be413edac4cdb7571746a76ea815d371)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.7.jpg&hash=c55f80e19d8e99d002cc9159e2bb9940)

I've got crap on my lens, sorry about that.
Title: Re: What causes iridescent clouds?
Post by: RD on 30/07/2008 08:28:16: (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fawcg.com%2FAllen%2FImages%2FSunDog%2FSunDog.3.jpg&hash=a2df119bd81c3f90ab65b78aa2c88575)

Your photo (above) looks like iridescent cloud (below)

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source (http://www.atoptics.co.uk/droplets/irid1.htm)
Title: Re: What causes iridescent clouds?
Post by: AllenG on 30/07/2008 22:24:33: Iridescent cloud, so that is what it's called. I've been calling that phenomenon the incorrect name for years, oops.
I've seen chunks of rainbow in cloud borne ice before, but nothing this spectacular.
Title: Re: What causes iridescent clouds?
Post by: DoctorBeaver on 30/07/2008 22:43:36: Good photos.
Title: Re: What causes iridescent clouds?
Post by: AllenG on 31/07/2008 00:53:50: Thanks Doc.
Title: Re: What causes iridescent clouds?
Post by: paul.fr on 01/08/2008 18:12:46: Are you sure?
Where is the sun in relation to the illusion?
Title: Re: What causes iridescent clouds?
Post by: AllenG on 02/08/2008 00:36:21: Quote from: Paul. on 01/08/2008 18:12:46
Are you sure?
Where is the sun in relation to the illusion?
The cloud was to the north west, the sun just north of due west and about 15º above the horizon.
Title: What causes iridescent clouds?
Post by: chris on 02/08/2008 13:37:00: What's the science behind this phenomenon?
Title: What causes iridescent clouds?
Post by: LeeE on 02/08/2008 14:17:15: I believe it's due to refraction through ice crystals in the upper atmosphere.
Title: What causes iridescent clouds?
Post by: RD on 02/08/2008 14:37:36: looks like interference (http://www.itp.uni-hannover.de/~zawischa/ITP/multibeam.html), (e.g. soap-film, oil-on-water).

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Explanation: Why would a cloud appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts.
http://apod.nasa.gov/apod/ap071125.html
Title: What causes iridescent clouds?
Post by: paul.fr on 02/08/2008 19:52:16: Quote from: chris on 02/08/2008 13:37:00
What's the science behind this phenomenon?

That depends on what it is!
Sun dogs (mock suns or parhelion) appear as bright spots on either side of the sun, and area form of halo. They are caused by the refraction of sunlight by the ice crystals that are present in the cirriform clouds, cirrostratus being a main form.

They mostly occur when the sun is low in the sky and will appear to move closer to the sun as it sinks further (sets), the rainbow colours of the sun dog have the red spectrum colours on the inside (inner edges) and the blue on the outer.

Then it is all down to the way the ice crystals are orientated. Flat horizontal crystals will produce sun dogs, and random, mixed shapes of crystals will produce a (22 degree) halo.

The link above, provided by RD, gives this for iridescent clouds.

Quote
When parts of clouds are thin and have similar size droplets, diffraction can make them shine with colours like a corona. In fact, the colours are essentially corona fragments. The effect is called cloud iridescence or irisation, terms derived from Iris the Greek personification of the rainbow.

The usually delicate colours can be in almost random patches or bands at cloud edges. They are only organised into coronal rings when the droplet size is uniform right across the cloud. The bands and colours change or come and go as the cloud evolves. They occur most often in altocumulus, cirrocumulus and especially in lenticular clouds. Iridescence is seen mostly when part of a cloud is forming because then all the droplets have a similar history and consequently have a similar size.

Sometimes iridescence can be seen far from the sun but is most frequent near to it. As for coronas, search safely by hiding the sun behind a building and, even better, also viewing the reflection of the sky in water.

Very much rarer iridescence is that of nacreous or mother-of-pearl clouds. They can glow very brightly and are far higher than ordinary tropospheric clouds. Iridescence is also seen in rocket exhaust trails.