As the sun was above the horizon, this was irisation (also known as
iridescence) in thin altocumulus. This occurs when the water droplets
comprising the cloud have a size close to the wavelength of light, the
effect being caused by diffraction, and is in the same family as coronae.
Irisation is normally seen within about 30 degrees of the sun.

--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html

"Stephen Tonkin" wrote in message
...
I've just come back form an hour of kite-flying on the North Downs. For
most of the hour I was flying, there were a bright pair of sun dogs (now
I fly regularly again, I see lots of halo effects, etc.), but from 16:10
to 16:30 (UT) there was also what I can only describe as daytime
nacreous cloud. A large oval-ish (10* x 5*) patch of cirrus about 5*
above the Sun, which was itself settling into the unusually thick murk
above Ashford.

The iridescence, almost entirely cyan and magenta, was easy to see
without sunglasses, and was blindingly obvious with. Nacreous clouds,
AIUI, only occur when the Sun is below the horizon, so what was this I
saw?

And did anyone else notice this?

Best,
Stephen

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