Having seen this, I'm surprised I've not seen one before, but still it
was very unfamiliar and I'm still not sure of the mechanism and precise
conditions that produces these.

While driving up the M5 and then M6 through Birmingham on Friday, for
some scores of miles, a very prominent whitish-but-faintly-coloured arc
in the same position in the sky as would be a rainbow, but giving a
strong sense that it was very much behind some mid-height rafts of cloud
present in the sky, very noticeable even at first glance, substantial,
and yet unfamiliar and probably unregarded by most people on the ground
beneath it ... the outer edge faintly tinged with red, but the blue
seeming to drop into the blue background of the sky itself ... has
anyone else come across one of these?

Having seen this, I'm surprised I've not seen one before, but still it was
very unfamiliar and I'm still not sure of the mechanism and precise
conditions that produces these.

While driving up the M5 and then M6 through Birmingham on Friday, for some
scores of miles, a very prominent whitish-but-faintly-coloured arc in the
same position in the sky as would be a rainbow, but giving a strong sense
that it was very much behind some mid-height rafts of cloud present in the
sky, very noticeable even at first glance, substantial, and yet unfamiliar
and probably unregarded by most people on the ground beneath it ... the
outer edge faintly tinged with red, but the blue seeming to drop into the
blue background of the sky itself ... has anyone else come across one of
these?

Cloudbows and fogbows form in the same way as rainbows.
The difference is that the drops forming them are smaller and
diffraction then broadens the bow and dilutes the colours.

Image he
http://www.sundog.clara.co.uk/rainbows/cldbow.htm
Some pages about them
http://www.sundog.clara.co.uk/droplets/fogbow.htm

Les Cowley
Atmospheric Optics:
http://www.sundog.clara.co.uk/atoptics/phenom.htm

Les wrote:

Les Cowley
Atmospheric Optics:
http://www.sundog.clara.co.uk/atoptics/phenom.htm

What a fantastic website! Beautiful.

Les wrote:
Having seen this, I'm surprised I've not seen one before, but still it was
very unfamiliar and I'm still not sure of the mechanism and precise
conditions that produces these.

While driving up the M5 and then M6 through Birmingham on Friday, for some
scores of miles, a very prominent whitish-but-faintly-coloured arc in the
same position in the sky as would be a rainbow, but giving a strong sense
that it was very much behind some mid-height rafts of cloud present in the
sky, very noticeable even at first glance, substantial, and yet unfamiliar
and probably unregarded by most people on the ground beneath it ... the
outer edge faintly tinged with red, but the blue seeming to drop into the
blue background of the sky itself ... has anyone else come across one of
these?

Cloudbows and fogbows form in the same way as rainbows.
The difference is that the drops forming them are smaller and
diffraction then broadens the bow and dilutes the colours.

Image he
http://www.sundog.clara.co.uk/rainbows/cldbow.htm
Some pages about them
http://www.sundog.clara.co.uk/droplets/fogbow.htm

Thanks. Presumably if the droplets of water are less than a certain size
light can't get into them to be refracted/reflected - hence we don't see
rainbows in every well positioned cloud.

The web site photo is very similar to the sky I saw, though last
Friday's bow's colouring was even more subtle if anything. The clouds
producing the bow are visible in the photo as they were last Friday, and
presumably the conditions needed for this are comparatively rare. This
doesn't seem to involve droplets of a size that would fall at any great
rate - perhaps the atmosphere is balanced just so to produce these
rather translucent sheets of thin cloud that stretch for very many miles.

Thanks. Presumably if the droplets of water are less than a certain size
light can't get into them to be refracted/reflected - hence we don't see
rainbows in every well positioned cloud.

To form a bow the droplet or raindrop collection has to be
sufficiently spread out that most of the sun rays only interact
with a single drop. Ordinary clouds are too dense or extensive
and light is scattered by many drops. The latter is why sunlit
clouds are white.

The web site photo is very similar to the sky I saw, though last Friday's
bow's colouring was even more subtle if anything. The clouds producing the
bow are visible in the photo as they were last Friday, and presumably the
conditions needed for this are comparatively rare. This doesn't seem to
involve droplets of a size that would fall at any great rate - perhaps the
atmosphere is balanced just so to produce these rather translucent sheets
of thin cloud that stretch for very many miles.

Smaller drops produce broader and less colourful bows.
http://www.sundog.clara.co.uk/droplets/fogdrpsz.htm
But most cloudbows will be made by drops at the larger end
of those found in clouds - very fine drizzle, virga?

Les Cowley
Atmospheric Optics:
http://www.sundog.clara.co.uk/atoptics/phenom.htm